abb - technical paper 1
TRANSCRIPT
CHHOS / AR 3257.99E (D)
Printed in Switzerland (99-09-1000 D/E)
ABB High Voltage Technologies Ltd.Division Surge ArrestersJurastrasse 45CH-5430 Wettingen 1Switzerland
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N Dimensioning, testing and application
of metal oxide surge arresters
in medium voltage networks
1
The first edition of our directions for dimensioning,testing and application of metal oxide surge arrestersin medium voltage networks, which appeared in 1994,was received very positively. We were pleased to getsuch a reception, which attested our belief that com-petent consulting with regard to the application of ourproducts is as important as the quality of the productsitself.
The technological progress made it now necessary to revise and renew the present booklet for the thirdedition.The dimensioning and the theoretical basis for the optimal application of the surge arresters are not changedand therefore they were taken as such in the new edition. Mr. René Rudolph, who was at the time of the firstedition responsible for the consulting concerning application in the surge arrester division, took on the taskof revising the tables. That was necessary because of the improvement of the technical data of the surgearresters and the enlargement of our product range. We would like to thank Mr. René Rudolph for his efforts.Mr. Bernhard Richter, who is now responsible for engineering and application of the overvoltage protectivedevices in the surge arrester division of ABB High Voltage Technologies Ltd, took on gladly the task of thegeneral revision of this booklet.Mr. Richter is a member in different working groups of IEC SC 37 A and IEC TC 81, and his activity fieldincludes, besides the development and testing, mainly the application of the surge arresters.
The silicon technology, which is used in medium voltage by ABB High Voltage Technologies Ltd, and thefurther developing of the metal oxide material opens new application spheres. All these are taken intoaccount in the new edition.
We hope, that you as a reader will be satisfied with our new revised edition and you will find it useful for yourpurpose. We welcome amendments and suggestions in order to meet all possible customer needs.
ABB High Voltage Technologies LtdWettingen, July 1999
First published: November 19942.revised edition: September 19953.revised edition: July 1999
All rights reserved.Neither the booklet or parts of it may be either copied or reproduced,transmitted in any way or translated info other languages withoutthe prior permission of ABB High Voltage Technologies Ltd.
Division Surge Arresters, Wettingen, Switzerland
ABB High Voltage Technologogies Ltd
Forewordin s
in V
in V
in V
in V
in V
in V
in V
in V
in V
in V
in
in
in V
in m/s
Ω
Ω
ts
U
Uc
UE
UK
Um
Up
Ur
Uref
UT
UTOV
Z
ZK
L
u(t)
v
δ
ω
time interval
peak value of the overvoltage of a travelling wave (mainly given in kV)
maximum continuous operating voltage of the arrester (mainly given in kV, rms)
maximum overvoltage at the end of an open line (mainly given in kV, peak value)
maximum overvoltage at cable end (mainly given in kV, peak value)
maximum voltage between phases (mainly given in kV, rms)
protection level of the arrester at I (mainly given in kV, peak value)
rated voltage (mainly given in kV, rms)
reference voltage (mainly given in kV, rms)
overvoltage at the transformer (mainly given in kV, peak value)
power frequency overvoltage of a limited duration (mainly given in kV, rms)
time function of a lightning overvoltage
speed of the travelling wave, v = 300 m / µs in the air
surge impedance of a distribution line conductor Z = 300........450
surge impendance of a cable conductor Z = 20 ...... 60
load rejection factor of a generator
angular frequency of the power frequency, at 50 Hz is = 314
n
K
Ω
Ω
ω
26
1 1
S S
conductor length
Basic Impulse Insulation Level (peak value)
conductor length
capacitance (given in nF or uF)
earth fault factor, C x U / 3 is the maximum voltage
between phase and earth in the case of an earth fault
section length of an overhead line before the substation
energy absorbed by the arrester (mainly given in kJ or kJ / kV )
discharge energy absorbed by the arrester (mainly given in kJ)
long duration current impulse
nominal discharge current (mainly given in kA, peak value)
50 Hz fault current (mainly given in kA, rms- value)
reference current (mainly given in mA, peak value)
peak current of the stroke current (mainly given in kA, peak value)
time function of the stroke current
corona damping constant
inductance of a line trap
protective distance
cable length
Maximum Continuous Operating Voltage = U (mainly given in kV, rms- value)
power losses of the arrester in the case of U
per unit, 1 p.u. = 2 x Um / 3
heat flow from the active arrester parts to the external environment (cooling)
maximum steepness of a voltage increase (mainly given in kV / µs)
time function of the steepness of a voltage increase (mainly given in kV / µs)
steepness of the lightning overvoltage at the point of the stroke (mainly given in kV / µs)
three-phase reactive power of a capacitor bank
withstand strength against temporary overvoltages U = T x U
temperature
time
e m
Uc
c
c
TOV c
√
√ √
in m
in kV
in m
in F
in m
in J
in A
in A
in A
in H
in J
in A
in A
in A
in m
in m
in V
in W
in W
in V /s
in V / s
in V / s
in Var
in C
in s
°
a
BIL
b
C
C
d
E
I
i
i(t)
K
L
e
Ec
In
IK
Iref
L
LK
MCOV
P
p.u.
Q
S
S(t)
So
SK
T
T
t
25
Index of symbols used
10.2
10.3
10.4
10.5
10.6
10.7
10.8
10.9
10.10
11
12
13
Overvoltage protection in cable sections
Cable sheath protection
Transformers at the end of cables
Transformer connected to a lightning endangered line onone side only
Arresters in metal enclosed MV-substation
Generator connected to a lightning endangered MV-line
Arrester protection for motors
Overvoltage protection in locomotives
Arresters parallel to a capacitor battery
Line traps (parallel protection)
Arresters for d.c.voltage
Consulting concerning questions on the use of arresters
Conclusions
Index of symbols used
Bibliography
8.4
8.5
8.6
8.7
9.1
9.2
9.3
9.4
10.1
9
Introduction
Surge arrrester technology
Metal oxide resistors as arrester elements
Medium voltage arresters from ABB
Special operating conditions
MO-arresters and spark-gap arresters
Construction of the arrester
Energy absorption capability and cool-down time
Nominal discharge current and energyabsorption capability
Network short circuit power
Elevated ambient temperature
Mechanical stability
Air pollution
Altitude adjustment for arrester housing
Insulation made of silicone rubber
1
2.1
4.1
4.2
4.3
4.4
5.1
5.2
5.3
5.4
5.5
2
3
4
5
Surge arrester protection level
Questions of stability of MO-surge arresters
Temporary overvoltages
Type tests
Routine tests
Acceptance tests
Special tests
Networks with earth fault compensation orwith a high-ohmic insulated neutral system
Networks with high-ohmic insulated neutralsystem and automatic earth fault clearing
Protection characteristics of the surge arrester,stability
Tests
Selection of surge arresters and determinationof Uc
6.1
6.2
6.3
7.1
7.2
7.3
7.4
6
7
8
Networks with solidly earthed neutral systems (Ce 1.4)
Networks with low-ohmic neutral transformer earthingwhich do not uniformly have Ce 1.4
Low-ohmic neutral earthing networks and Ce > 1.4
Arresters between phases (Neptune design)
Operating voltage with harmonic oscillation
Theoretical projection for the protective distance L
Expected steepness S from lightning overvoltages inMV-substations
Influences on the protective distance through electricalequipment, the types of the arresters and the arrangementof the arresters
Fault hazards in electrical equipment and their distancefrom the surge arrester
<
<
Protective distance of the surge arrester
8.2
8.1
8.3
2
Contents
Some special cases10
1 Introduction
Overvoltages in electrical supply networks result from the effectsof lightning strokes and switching actions and cannot be avoided.They endanger the electrical equipment because, due toeconomical reasons, the insulation cannot be designed for allpossible cases. A more economical and safe on-line network callsfor extensive protection of the electrical equipment againstunacceptable overvoltage loads. This applies to high voltage aswell as medium and low voltage networks.Overvoltage protection can be basically achieved in two ways:
intercept lightning.
•
•
•
•
•
Avoiding lightning overvoltage at the point of origin, for instancethrough shilding earth wires in front of the substation that
Limit overvoltage near the electrical equipment, for instancethrough surge arresters in the vicinity of the electrical equipment.
In high voltage networks both methods of protection are usual.The earth wire protection in medium voltage networks is generallynot very effective. Due to the small distance between the earth wireand the line wires, a direct lightning stroke hits usally the line wiresas well. In addition, induced overvoltages in the line wires (indirecteffects of the lightning strokes) cannot be avoided by the earthwires.
The magnitude of the overvoltage is usually given in p.u.(per unit). It is defined as
1 p.u. = 2 x U / 3 ,
the U means the maximum r.m.s voltage value between thephases in an undisturbed network operation [1].
Three types of overvoltages are essentially distinguished [2]:
occur for instance during load rejectionor because of earth connection faults. The duration of suchoperating frequency oscillating overvoltage lies between 0.1seconds and several hours. In general the surge does not exeed 3p.u. and therefore it is of no danger to the network operation,however it is decisive for the dimensioning of the arrester.
occur during switching actions andconsist mostly of heavily damped oscillations with frequencies upto several kHz and a magnitude up to 3 p.u.A special case is switching in inductive electrical circuits. Herethe front time of the overvoltage lies between 0.1 and 10 µs andthe peak value can reach 4 p.u.. Fast-front overvoltages are alsopossible when lines or cables are switched on.The peak value liesthen below 2.2 p.u. and does not endanger the network system.
originate in atmospheric discharges.They reach their peak value within a few µs and subsequenly decayvery rapidly. The magnitude of these unipolar overvoltages in amedium voltage network can reach well over 10 p.u.
Lightning overvoltages are the greatest threat to the mediumvoltage networks. Overvoltage protection must be arranged insuch a way as to confine the overvoltage to non-damaging values.
The most effective protection against overvoltages in a mediumvoltage network is therefore the use of surge arresters in thevicinity of the electrical equipment.
Temporary overvoltages
Switching overvoltages
Lightning overvoltages
√ √
√
m
m
3
2 Surge Arrester Technology
The so-called "conventional" surge arresters were exclusivelyemployed in MV networks (MV = medium voltage) until about themiddle of the eight decade of our century. They consisted of aseries connection of SiC resistors and plate spark-gaps. During theovervoltage rising there emerges a short circuit to the earth whenthe spark-gaps come into action. The series of SiCresistors limits the follow current from the power supply andallows in this way the disappeareance of the arcs between the platespark-gaps the next time the voltage reaches the zero crossing.
In the last years there were two fundamental improvements ofsurge arresters used in MV networks. On one hand the series
of SiC resistors and the plate spark-gaps were replacedwith the metalloxid resistors (MO-resistors) without plate spark-gaps, on the other hand the housings of the surge arresters madeof porcelain were replaced with new ones made of polymermaterial (synthetic material).
The arresters protect the electrical equipment no matter whethersome other types of arresters are present. Therefore it is possibleto have at work in the same installation both the conventionalspark-gap arresters and the modern MO-arresters. It is not evennecessary that an electrical equipment should be protected in allits three phases by the same type of arrester. The user cantherefore simply replace a spark-gap arrester as soon as it is out ofwork with a MO-arrester. That allows the user to introduce cost-saving the MO-arresters that have an elevated operating safety.
A fundamental advantage of the MO-arresters is the fact thatbecause of their extremely non-linear characteristic of the MO-resistors they do not need any spark-gaps. The current starts toflow already through the arrester before the overvoltage achievesthe value U . The MO-arresters reduce therefore the overvoltagesooner as the spark-gap arresters. The last ones are able toconduct after the overvoltage was increased to U . That is whytheir protection distance is shorter in many cases. This means thatthe overvoltage to the electrical equipment is higher when a spark-gap arrester instead of a MO-arrester is installed as both types ofarresters are at the same distance from equipment to be protected.
The response voltage of the spark-gaps increases when the risetime becomes steeper (< 1 . This causes a deterioration of theprotection possiblity of the spark-gap arresters in case of anovervoltage wave with steep front .
If the outside insulation of the arrester is very dirty the potentialdistribution along the humid surface is very uneven. It is possiblethat between the sheds partial arcings appear which can induceovervoltages in the active part. This situation is especially criticalwith a spark-gap arrester, because it can bring it regularly to sparkover and at the end destroy it. This is the reason why MO-arresterswithout spark-gaps have a fundamentally higher pollution resi-stence.
If more spark-gap arresters are parallel connected usually onlyone arrester switches on during an overvoltage. This reduces thenthe overvoltage to a value that lies under the sparking voltage ofthe other parallel arresters. Therefore it is not possible to distributethe energy of the overvoltage on more spark-gap arresters whichare parallel connected. In case that this energy is too high thearrester that switched on will be overloaded. This appliesespecially to the spark-gap arresters which are parallel connectedto capacitor batteries with a higher reactive power.
connection
connection
µs)
2.1 MO-Arresters and Spark-Gap Arresters
p
p
[1] IEC Publication 99-5, First edition 1996-02 : Surge arresters Part 5 : Selection andapplication recommendations.
[2] R. Rudolph und A. Mayer: Überspannungsschutz von Mittelspannungskabeln. Bull.SEV/VSE 76 (1985) 4, S. 204-208.
[3] R. Rudolph: Bemessung, Prüfung und Einsatz von Metalloxid-Ableitern. Bull.SEV/VSE 75 (1984) 23, S. 1407-1412.
[4] A. Mayer und R. Rudolph: Funkenstreckenlose Überspannungsableiter ermöglichenoptimalen Überspannungsschutz. Brown Boveri Technik 72(1985) 12, S. 576-585.
[5] W. Schmidt: Metalloxid, ein fast idealer Überspannungsableiter. Bull.SEV/VSE 7 / 1998, S. 13-20.
[6] IEC Publication 60099-4, Edition 1.1, 1998-08: Surge arresters Part 4: Metal-oxidesurge arresters without gaps for a.c. systems.
[7] ANSI/IEEE Publication C62.11 1993: IEEE Standard for Metal-Oxide Surge Arrestersfor Alternating Current Power Circuits.
[8] R. Rudolph: ZnO-Ableiter. Eine Alternative zu konventionellen Ableitern. Elektrotechnikund Maschinenbau 5 (1983), S. 195-200.
[9] IEC Publication 71-1 (1993-12): Insulation coordination - Part 1: Definitions, principlesand rules.
[10] IEC Publication 71-2 (1996-12): Insulation coordination Part 2: Application guide.
[11] G. Balzer und K.H. Weck: Isolationskoordination von gasisolierten Schaltanlagen.ETG - Fachbericht 32 (1990), S. 71-89.
[12] VDEW Störungs- und Schadensstatistik 1990. Verlags- und Wirtschaftsgesellschaftder Elektrizitätswerke m.b.H.
[13] A.J. Eriksson et al.: Guide to procedures for estimating the lightning performance oftransmission lines. Report of WG 01 of CIGRE Study Committee 33, Oct. 1991.
[14] K. Berger: Methoden und Resultate der Blitzforschung auf dem Monte San Salvatore beiLugano in den Jahren 1963 bis 1971. Bull. SEV/VSE 63 (1972) 24, S. 1403-1422.
[15] Surge arrester application guide. IEC 37 (Sec) 85, Jan 1992.
[16] R.B. Anderson and A.J. Eriksson: Lightning parameters for engineering application.Electra, 69 (1980), S. 65-102.
[17] A.J. Eriksson et al.: A study of lightning stresses on metal oxide surge arresters.Cigre paper 33-08 (1986).
[18] M. Christoffel: Der Einfluss von Kabelstrecken auf die Überspannungsvorgänge inÜbertragungssystemen mittlerer und hoher Spannungen. Brown Boveri Mitt. 51 (1964)6, S. 369-376.
[19] A. Braun: Schirmspannungen und Schirmverluste bei Mittelspannungs-VPE-Kabeln.Elektrizitätswirtschaft 88 (1989) 26, S. 1898-1906.
[20] M. Darveniza und D.R. Mercer: Lightning protection of pole mounted transformers.IEEE Transactions on Power Delivery, Vol. 4, No. 2, April 1989, S. 1087-1093.
[21] G. Balzer: Schaltvorgänge in Mittelspannungsnetzen und deren Berücksichtigung beider Planung. Brown Boveri Technik, 73 (1983) 5, S. 270-278.
[22] Non-linear metal-oxide resistor polymeric housed surge arresters without sparkgaps.IEC 37 / 154 / CD; March 1996
[23] W.Schmidt: Die neuen POLIM -Überspannungsableiter mit Silikonisolation fürMittelspannungsnetze. ABB Revue 2/96
24
Bibliography
23
During unusual operational conditions, for example when asystem flashover takes place, all the parallel arresters in thenetwork are overloaded equally by the operational frequencyovervoltage. If metal oxide arresters are in use, it is possible toinduce a particular arrester to become over-charged first, ratherthan a random one. For example, an indoor arrester in a substationbuilding is selected with U approx. 10% higher than that of theoutdoor arrester. As soon as an abnormal operational frequencyovervoltage occurs, the outdoor arrester will be over-charged first.This arrester limits the overvoltage by producing an outdoorflashover and thus preventing arcing inside the substationbuilding.
A similar situation exists when very high temporary overvoltagesare expected in a MV network, and these occur only veryinfrequently. In order to avoid an over-charge on the arrester alsoin this rare case, a 15% higher U is necessary. Such arrestersmust be employed with electrical equipment. The drawback is, thatthe protection is reduced by 15%.
Two sets of arresters provide an acceptable solution to theproblem. One set with 15% higher U is installed next to theelectrical equipment, and a second set with a lower U is placedsome distance away. In this way two metal oxide arresters areswitched parallel in each phase. Should a lightning overvoltageoccur, both sets would be conductive and together produce thesame protection level for the electrical equipment as would bepossible with a single arrester set with a lower U . During the abovementioned very high overvoltage, only the arresters which are ata distance from the electrical equipment become over-charged.Therefore, the resulting flashovers cannot damage the equipment.Furthermore, since such an overload rarely occurs, thesubsequent interruption of operation can be risked.
When installing the arrester, two points must be carefullyobserved. Both are equally important for achieving the bestpossible protection level. The lightning endangered line must beguided to the high voltage connection of the arrester first, and onlythereafter led to the equipment to be protected. A short distancebetween the high voltage connection of the arrester and theequipment is certainly important, but not of crucial significance.
Secondly the galvanic connection from the earth side of thearrester to the earth of the electrical equipment must be as short aspossible. This distance must be below 2 m for lines with earthedcross arms. The distance for wooden pole lines must be shorterthan :
1 m for U 24 kV
0.6 m for U > 24 kV
If this is not possible, the cross arms on the last 3 poles before thesubstation or the electrical equipment must be earthed or anotherset of arresters must be installed one pole in front of thesubstation. In this case the upper limit for the earth connection is 2m. The connection lines to the arrester in the case of a cable mustbe as short as possible.
c
c
c
c
c
m
m
<
20
MO
SiC
200A
13
10
5.66
0
Semi-logarithmic plot of the current-voltage characteristics ofresistors for Uc = 4 kVMO and SiC
10-4 10 410-2 10 21 [A]
√2 x Uc
U = 4 p.up
I = 10 kAn
[kV]
Figure 1
4
d.c. voltage measurement current wave 8 / 20 s
The characteristic curve in 1 corresponds to a resistor withU = 4 kV. In the case of a d.c. voltage load with 2 x U = 5.66 kV ad.c.current in the range of 0.1 mA flows. The capacitive componentat 50 Hz and the value of U lies in the vicinity of 0.5 mA. Theprotective level U when I = 10 kA amounts to 13 kV.
In comparison, the diagram also shows the curve of a SiC resistor,which has also U =13 kV. Since SiC exhibits a considerably lessnon-linearity, the continous current of a SiC arrester without spark-gaps at a nominal load would amount to about 200 A. Naturally, forthermal reasons, such an arrester would not be feasible. Besides itwould strain the electrical network much too much. Consequentlyconventional arresters need spark-gaps in series, which take overthe voltage in continuous operation.
Figure
c c
c
p n
p
3 Metal Oxide Resistors as Arrester Elements
The voltage current characteristic of metal oxide resistors isextremely non-linear. In Figure 1 the characteristic curve for suchresistors is plotted. I is the nominal discharge current (in 1e.g. I =10 kA). U is the protection level. It is defined as themaximum voltage at the resistor during the flow of I . U is definedas the r.m.s.value of the Maximum Continuous Operating Voltage(MCOV) of the resistor.
n
p
n c
Figure
n
All the parallel MO-columns of the MO-arresters without spark-gaps conduct current at the same time. The energy of theovervoltage is in this way distributed on all the parallel arresters, sothat the energy capacity as a limiting parameter disappears.
follow
follow
MO-arresters can be used both with 50 Hz and with 16 2/3 Hzbecause they do not conduct any follow current. On the otherhand in the spark-gap arresters the current flows with16 2/3 Hz three times longer than with 50 Hz. As a result the spark-gaps and the SiC resistors are loaded with a corresponding higherenergy. In order to prevent this it is necessary to reduce the peakvalue of the current. This asks for spark-gap arresterswith a higher nominal voltage, which however makes a consi-derable increasing of the protection level necessary. For thebetter understanding of the facts it is necessary to add that the MO-arrester may be used with d.c. voltage, the arrester with platespark-gaps however cannot resist this strain.
The MO-resistors make up the active part of the MO-arrester. TheMO-resistors are compressed and sintered in the form of roundblocks out of different metall oxides in powder form. The diametersof the MO-resistors of ABB High Voltage Technologies Ltd, madefor the usage in the medium voltage, lie between 38 mm and 75mm. The height of the blocks is generally between 23 mm and 46mm. The diameter of the MO-resistors decides the carryingcapacity of the current, the height of the MO-resistors (or resistorstack) decides the voltage in continuous operation and the energycapability. The diameter of MO-resistors correlate with the linedischarge classes corresponding to IEC 60099-4, as shown inTable 1.
Table 1Correlation between typical MO-resistors and the line dischargeclasses acc. to IEC. Line discharge class 5 is important only inhigh-voltage systems above 420 kV, and not mentioned here.
The contact areas of the MO-resistors are metallized with softaluminium up to the edge of the block, the surface is passivatedwith glass. In this way the MO-material of the MO-resistorsproduced by ABB High Voltage Technologies Ltd is completelycovered. The 2 shows MO-resistors which are used in themedium voltage arresters.
The energy capability of the MO-resistors depends besides thevolume also on the construction of the arrester (carrying-off ofheat). Chapter 4 gives more details concerning this.
Figure
Figure 2
MO-resistors (collection) produced by ABB
Line discharge classacc. to IEC 60099-4
Diameter of blocks in mm
Square wave, 2000 s in A
Energy capability in (kJ / kV )
µ
Uc
1
38
250
3.6
2
47
550
5.5
3
62
1000
9.0
4
75
1350
13.3
4 Medium Voltage Arresters of ABB
•
•
•
It was the wish to increase the reliability and the safety of thearresters and correspondingly to it also the increasing of theenergy supply that brought about the development of the MO-arresters with silicon housing in the 1980s. For more than 30years is silicon known as an excellent insulation material for thehigh voltage technology, as for instance the long rod insulators andbushings. The first MO-arresters with silicon housing of the typicalABB execution (direct moulding) were used in 1986. Now, in 1999there are everywhere in the world more than 600 000 arresters inuse under very different ambient conditions.
Generally an arrester is made up of two parts, the active part,consisting of one or more piled up MO-resistors, and an insulatinghousing, that guarantees both the insulation and the mechanicalstrength.The MO-resistors have been shortly described in the last chapter.
Fundamentally there are three different possibilities ofconstruction:
a glass-fibre reinforced tube is shed with an insulating material.These so called hollow insulators have the same problems as theporcelain insulators, they need a sealing and pressure reliefsystem, they can have inner partial discharges under pollution.
the active part is wrapped with glass-fibre material and is soakedwith resin, which makes up the whole into a rigid body. Theinsulating polymeric housing is then thrust over the resin block orshrunk on it. This costruction has the disadvantage that it forciblebreaks apart when the MO-blocks are overloaded. Anotherdisadvantage is the fact that there are different insulatingmaterials, which also means that there are more boundary layers.Therefore it is necessary to take special measures forsealing.
the active part is hold mechanically together with glass-fibrereinforced loops or bands. The silicon is directly put on the MO-resistors. This direct moulding has the advantage that no gasvolume stays in the arrester. Sealing problems and inner partialdischarges are in this way out of question. There are no interfacesbetween polymeric materials where humidity can go in. Anexplosion or a shattering of the housing is not possible.
All the medium voltage arresters of ABB are build upcorresponding to the third principle (direct moulding).
The requirements to the arresters depend on the operationnecessities and the type of the equipment to be protected. That iswhy ABB offers a large selection of different types of arresters forthe medium voltage network and for special applications. Theconstruction, the functioning and the characteristics of thearresters are described for instance in [5]. In the Table 2 there arethe electrical main data of the medium voltage arresters to befound.
4.1 Arrester construction
The outdoor arresters have sheded housings made of silicon. Thetype MWD for the use indoor has a smooth silicon housing. (seeFigure 3 and 3a)
Arrester type
POLIM-DN
POLIM-D
POLIM-DA
MWK / MWD
POLIM-I
POLIM-S
POLIM-H
IkA
nI Ain t in s
E / UkJ / kV
cU / Up c
5
10
10
10
10
10
20
65
100
100
100
100
100
100
2000
2000
2000
2000
2000
2000
2000
3.5
5.5
5.5
9.0
13.3
350
550
550
1000
1350
3.33
3.07
3.07
3.00
3.19
3.33
3.5 3.6
2.6
250
150
High currentkA
Square wave
Figure 3MO-surge arresters with silicone housing. (POLIM-family)
Table 2Electrical main data of the MV-surge arresters from ABB(most common types). E / U as tested in the operating duty test.c
Figure 3aMO-surge arresters with silicone housing
left:right:
5
MWK for outdoor applicationMWD for indoor application
If, for a certain arrester type, the reactive power of the parallelcapacitor bank exeeds the limiting values in Table 9, an arresterwith better energy qualities must be selected. For networks notoperating under standard voltages, the limiting values in thecolumn with the lower standard voltage apply for S . If the reactivepower is very large, arresters connected parallel must be chosen.In such a case the arrester supplier must be informed in order totake measures to guarantee a sufficiently good current distributionin the parallel arresters. The supplier should also be consultedwhen arresters with U < U are to be used.
Line traps are air-core chokes which are switched in connectionwith the high voltage lines. Their inductivity L is in the range of mH.If no measures are taken, the lightning currents in the conductormust flow through the line trap. Even relatively small current ratesof rise of several kA /µs would produce overvoltages on the linetrap amounting to several 1000 kV and resulting in a flashover. Inorder to prevent this, MV-arresters are connected parallel to theline trap. These will take over lightning currents and limit theovervoltage to its residual voltage.
When a short to earth occurs in a high voltage network, the faultcurrent I flows through the conductor. This power frequencycurrent would overload the arrester. U must therefore be selectedso that the current flows through the line trap. It induces atemporary overvoltage of U = x L x I , standard for U , at theline trap. If the fault current duration is t < 3s, then T = 1.28. Thisresults in the following for U :
U x L x I
U ---------- = ----------------T 1.28
I : maximum fault current through the line trap
L : inductivity of the line trap
K
c m
K
c
TOV K c
c
TOV K
c
K
10.10 Line Traps (Parallel Protection)
ω
ω>
On the other hand most of the d.c. current networks arerailnetworks. If the arresters are used on a rolling material(locomotive) the safety is especially important (safety of persons).
The arresters produced by ABB are suitable to be used on d.c.current networks and especially also in the railnetworks and onlocomotives and traction vehicles.The producer has to be informed by all means if the arrester is usedin d.c. current networks. For the dimensioning of the arrester it isalso necessary to take contact with the producer.
During many discussions with the users of surge arresters it wasnoticed that a profound consulting on the use of arresters iswelcome. This is necessary both by changes in technology, as forinstance from the spark-gap arresters with insulation of porcelainto the MO-arresters with silicon housing, and by the choosing ofthe arresters for additional equipment of alredy existinginstallations or the planing of new installation in the medium andlow voltage networks. Especially in the new fields of application, asfor instance in the d.c. current networks, or the designs for theprotection against overvoltages and lightning danger in wholeinstallations it is necessary a profound observation. The evaluationof the existing installations from the point of view of the powertransfer (higher system voltage) or the reliability and availabilitystipulate clear protection concepts, taking into account an optimaleconomic and technical solution.
Hence we offer gladly consultig and calculation for the protectionagainst overvoltage and lightning, which exceed the abovedescribed instructions.
12 Consulting Concernig Questions on theUse of Arresters
13. Conclusions
11 Arresters for D.C. Voltage
For the time being there are no international valid regulations ordirections for the use of the arresters in d.c.voltage networks. Onprinciple in d.c.voltage networks there also appear voltagesproduced by lightning or other activities, which may endanger theequipment and the insulation. In this case it is also necessary theuse of an arrester as protection against overvoltages. For suchsituation the MO surge arresters are especially suitable, becausethey do not conduct any follow current after the limiting of theovervoltage, and therefore it is not necessary to extinguish anyd.c.current arc. There are two fundamental items to be taken intoconsideration when using the MO-arresters in the d.c.currentnetworks. On one hand it is necessary to make sure that the MOmaterial is stable for a long period of time from the point of view ofthe d.c. voltage continuous operation. This is not the case with allthe MO materials which are comercially available.
22
Lightning overvoltages are a hazard for all the electrical equipmentin a MV network. However, surge arresters assure reliable protect-ion against excessive overvoltage stresses. The closer the arresterto the electrical equipment, the better the protection.
When determining the arrester U , two contradictory requirementsmust be considered.On the one hand, U must be as small as possible so that thearrester can limit the overvoltage to the lowest possible values. Onthe other hand, U must be selected at a value high enough to allowthe arrester to fulfill the requirements of network operation.Modern MO-arresters with no spark-gaps meet both theserequirements and provide sufficient protection from overvoltage,as well as simultaneously assuring safe network operation.
c
P
c
21
During this process the capacitor is charged with a higher voltage[21]. The overvoltage of the capacitor between phase and earth[15] reaches a max. of 3 p.u. If the capacitors are connected in astar, they are discharged between phase and earth by the arresterparallel to the bank. During the discharge up to a voltage of 2 xU , in terms of power, the arrrester is loaded with:
SE = ----------- x [3 - (U / U ) ]
S : 3-phase reactive power of the capacitor bankE : the discharge energy taken up by the arrester
Assuming that the arrester must carry out this process 3 timeswith no cool down phase, it follows with U U that
E 6 x S---- -------------U x U
The power consumption capacity E of the arrester must beadjusted to the reactive power of the bank. Table 9 shows themaximum allowable reactive power values for different types ofABB MV-arresters parallel to the bank.
If the neutral of the capacitor bank is insulated, the arrester cannotdischarge the charged capacitor between phase and earth. Thearrester does not get charged. It must be noted that after a re-strikeof the breaker, the neutral of the bank increases to 2 p.u. A voltageflashover of the neutral to earth results in the arrester having todischarge the capacitor. The arresters parallel to a bank with aninsulated neutral must, in terms of power, be adjusted to theirreactive power.
If the bank remains disconnected from the network after a shut-down, the arresters discharge the voltage to zero, not merely to
2 x U . Below 2 x U the discharge current through the arresteris very small, so that the remaining discharge takes a long time.During this time the arrester can cool down. It releases more heatthan it receives during the remaining discharge. Therefore it wasjustified in the above calculation of E to take into account only theenergy taken up by the arrester, up to the discharge at 2 x U .
√
√ √
√
c
K
c c m2
K
c
c m
c K
c m
c c
c
c
>
>
The arrester type POLIM-H 15 is sufficient. Its protection level at I =10 kA is 43.5 kV. This special arrester guarantees a 11% lowerprotection level. In addition this arrester has also advantages withregard to operational safety thanks to its substantially higherenergy absorption capability (see Table 2).
Generators have a larger capacity between phase and earth. Asseen in Figure 14, this capacity results in a considerably shorterarrester protective distance. Therefore it is especially important toplace arresters near the generator.
High voltage motors can become over-stressed by multiple re-starts being switched off during the run-up. This is applicablewhen the cut-off current is less than 600 A. In order to protectthese motors it is recommended that surge arresters be placeddirectly at the motor terminals or, alternatively, at the circuitbreaker. The layout of U according to the recommendations canbe seen in section 8.
In special cases, i.e. aged winding insulation, it becomesnecessary to additionally decrease the protection level of thearrester. One way of doing this is to decrease U . This procedure isjustifiable when temporary overvoltages taken into account for Uoccur very infrequently. The fact that in such a rare case thearrester could become overloaded has been deliberately taken intoaccount. Resulting drawbacks, such as operation interruptionsand arrester replacement are outweighted by the advantage ofbetter overvoltage protection. However U smaller than U / 3may never be selected.
However such a decrease of U is not recommended in a generator.The risk exists that this would cause a two-phase short circuit atthe generator terminals. The resulting asymmetrical short circuitcurrent in the windings produces an extremely high mechanicalload on the rotor.
In the case of locomotives, the highest standards with respect tooperational safety are placed on the arresters. Therefore, thearresters of the POLIM-H series are recommended. The strongmechanical construction meets all the requirements for railwayoperation. The completely moulding in silicon guarantees thehighest mechanical safety even during extreme shock loads. Whenan arrester is overloaded the special construction of this arresterprevents the housing from bursting. This arrester type is proved upto a fault current in the network of 65 kA and can be consideredsure from the point of view of explosion and disintegration.Additional advantages of this arrester type are its low protectionlevel and its high energy absorption capability.
Normally no overvoltage occurs when a capacitor bank isswitched. The circuit breaker interrupts the current in the naturalcurrent zero crossing and the voltage in the capacitor to earthreaches a max. of 1.5 p.u. As a result of the network voltage vary-ing at the power frequency, a voltage across the open circuitbreaker of 2.5 p.u. is caused. If the breaker re-strikes, a highfrequency transient effect takes place between the capacitorvoltage and the operating voltage.
10.7 Arrester Protection for Motors
10.8 Overvoltage Protection in Locomotives
10.9 Arresters Parallel to a Capacitor Bank
c
c
c
c m
c
√
3.6 9.05.5 13.3
U [kV]m S [MVA]K S [MVA]K S [MVA]K S [MVA]K
3.6
7.2
12
17.5
24
36
0.67
1.35
2.26
3.29
4.52
6.78
1.03
2.07
3.45
5.03
6.90
10.36
1.69
3.39
5.65
8.24
11.30
16.95
2.50
5.01
8.35
12.18
16.70
25.05
E/U [kJ/kV]c
Table 9Arrester parellel to capacitor bank. Maximum allowable reactivepower S of the bank for the indicated arrester types. Threedischarges without a cool down phase are allowed for thearresters.E/U : The arrester energy absorbtion capability in relation to U .
K
c c
Arrester typeU Uc m>
POLIM-D POLIM-SMWKMWD
POLIM-I
POLIM-H
ω
ω
In the last 15 years most of the modern MO-arresters were set innew installations in high-voltage networks [4]. Up until a few yearsago conventional arresters,consisting of SiC resistors and seriesspark-gaps were still installed in distribution systems. Now a days,MO-arresters without spark-gaps are in use, especially those withpolymer housing. The reasons for this technological change areincreasing operation efficiency, lower protection level with verysteep overvoltages and high pollution resistance [5].
Due to the simple construction of the active part without spark-gaps, which means a very high stability from the mechanical pointof view, and also due to the simple construction generallyspeaking, such arresters can also undertake the support insulatorfunction and are not in danger of exploding in case of an overload.Silicone is the best insulating material concerning dirt, and that iswhy the arresters which are insulated with silicone behavefavourable especially with high pollution.
Silicone rubber (usually referred to simply as 'silicone') is anexcellent insulating material for high-voltage components.Comparisons with traditional insulating materials, such asceramic, glass and other synthetics (eg, thylene- ropylene-
iene onomer, or short ) show clearly the superiority ofsilicone. As already short mentioned, during the manufacture ofthe surge arrester the silicone insulation is bonded to the arresterassembly through casting (or injection) of the liquid rubber inmoulds at a high temperature. Different moulds can be used toobtain an optimum match between the insulator and thecomponent assembly. The arrester insulator thus producedexhibits the following properties:
No hydrocarbon is present in the main chemical chain; thisproperty is partially the reason for the high immunity of theinsulator to serious surface pollution and is also largelyresponsible for preventing carbonized creepage paths fromforming.
The material is water-repellent, so that even after excessivecontact with water only very few drops adhere to the surface. Thisproperty is referred to in the industry as hydrophobicity. Drops ofwater that do cling to the surface are quickly removed by the effectof gravity or through normal exposure to wind.
The hydrophobicity effect is also partly transferred to any dirtdeposits on the surface, which as a result does not become coatedwith layers of semiconducting material as quickly as with othertypes of insulation. Because of this, the surface leakage currentsflowing on silicone insulators are extremely low. The technicalliterature explains the transfer of the hydrophobicity effect to dirtdeposits as being due to low-molar silicone evaporating. Thehydrophobicity effect is temporarily reduced by strong electricalpartial discharges or extreme exposure to water, but to a lesserdegree with silicone than with other materials. This is clearlyshown by the artificial aging af EPDM and silicone in accordancewith IEC 1109, see Figure 5. After 5,000 hours of alternatedprecipitation, salt-fog and UV radition, the silicone still retains 50percent of its water-repellent properties, whereas EPDM losesthese properties. Tests have further shown that the hydrophobicityeffect can be restored to its original state by drying the silicone forseveral hours.The salt-fog test to IEC 507 also demonstrated that, assuming thesame salinity in each case, the creepage paths required for siliconeinsulation are, on average, 30 percent shorter than the pathsnecessary with ceramic isolators, see Figure 5a. As a result, thecreepage path of the silicone isolators could be reduced by thisamount.
4.2 Insulation made of silicone rubber
E PD M EPDM
•
•
The diameter of the MO-resistors is proportional to the energyabsorption capability E and the nominal discharge current I . Thespecial arresters of the type POLIM-S and POLIM-H have resistorslike the ones of the high voltage arresters. These types of arrestersset new standards in the medium voltage network ; they are able toresist the strongest stress and at the same time they guarantee alow protection level. The continuous operating voltage U of theMV-arresters in the Table 3 reaches from 4 kV up to 36 kV.
In addition to the already mentioned types ABB manufactures alsothe special arrester POLIM-C. This arrester is also built upaccording to the already mentioned principle of direct moulding.The nominal discharge current is I = 10 kA and the continuousoperating voltage U reaches from1 kV up to 7.2 kV. In the mediumvoltage system this type of arrester is used among otherapplications for the protection of non-earthed cable sheath.
The functioning of an arrester, which consists only of seriesconnections of MO-resistors (without spark-gaps), is extremelysimple. During an overvoltage at the arrester terminals the currentrises continuously and without delay through the arrester corre-sponding to the shown U-I characteristic, Figure 4, which meansthat no really spark over takes place, but the arrester goescontinuously to the conducting condition. After the decreasing ofthe overvoltage the current becomes low again corresponding tothe U-I characteristic. Unlike the spark-gap arresters there flowsno follow current.
n
c
n
c
The number of the resistors in a stack depends on the U of thearrester. The stack of cylindrical MO-resistors is moulded inpolymeric material (silicone) as described.
The resistor stack behaves itself capacitive under U . The straycapacitances of each individual resistor towards the earth causethe nonlinearity of the voltage distribution along the axes ofarrester under U . This nonlinearity increases with the lenght of theresistor stack [3]. That is why high voltage arresters need gradingrings, which compensate mainly the unfavourable influence of thestray capacitances. Due to the relatively short length of the activepart of the distribution surge arresters the nonlinear voltagedistribution is so low that it can be neglected. These arresters donot need any grading rings.
c
c
c
U
[p.u.]
1.0
0
0.5
10-4 10-3 10-2 10-1 100 101 102 103 104
4/10 s
1/5 s
8/20 s
30/60 s
2000 s
I [A]
Figure 4Normalised current-voltage ( U-I)characteristic of a MO-surge arrester with I = 10 kAn
6
The short-time tests acc. to IEC 507 provide the basis for thedimensions of the insulator. In certain cases, the insulator be-haviour may deviate under actual field conditions as a result ofother parameters (eg, due to the shape of the sheds). However, it isgenerally true for silicone as well as for the ceramic insulators thatextreme pollution calls for a longer creepage path.The mentioned temporary reduction in hydrophobicity was takeninto account in the design of the POLIM arresters, and the specificcreepage path was not reduced. All of the discussed surgearresters with silicone insulation have been designed with aspecific creepage path of at least 25 mm per kV, providing a morethan adequate safety margin. Whenever possible, all the pollutionand lifetime tests were carried out on complete MO arresters. Thetests were performed to the various standards (eg, the 1,000-hourhumidity room test to IEC 1109, the 5,000-hour aging cycle testand the salt-fog test to IEC 507) and showed that the siliconeinsulation performs better after ten years in service that the othertypes of insulation.
Figure 5Change of hydrophobicity of EPDM (black) and silicone (white)in the accelerated ageing test acc. to IEC 1109.
Figure 5aComparison of the specific creapage distance of porcelain (black)and silicone insulators (white), depending on the salt content in thesalt fog test acc. to IEC 507
7
5
4
3
2
1
00
1000 2000 3000 4000 5000h
HC
Hyd
rophobic
ity
test timetv
6
70%=
100%=
^
^
00
1
2
3
4
5
8
cm/kVrms
kg/m33 4 5 7 10 15 20 30 40 80
Cre
apag
e
salt content of water
2.5
4.3 Energy absorption capability and cool-down time
4.4 Nominal Discharge Current and Energy AbsorptionCapability
The arresters in the network can work reliable if their energyabsorption capability is bigger than the energy strain expected inthe system operation. Some examples of the stress on thearresters in the network are shown in the Table 3. The arresters ofthe line discharge class 1 have the highest energy stress with thehigh current (65 kA respectively 100 kA). To prove the thermalstability under this stress, a special type test has to be performed.
The guaranteed energy absorption capability of the types of theline discharge class 2 and higher can be proved by the means ofrectangular current stresses, similar to the examination of the highvoltage arresters.The guarantee data contain a certain amount of energy reserve andtherefore do not mean the limit of the thermal stability of thearrester.
Anyway the arresters will be very strongly warmed up when theyhave to carry very high lightning currents. Therefore they needbetween two such stresses a suitable cool-down time. Thisreduction is however not important because it is most unlikely thatthe same arrester has to carry a second very high ligtning currentduring its cool-down time. That is the reason why the test sampleis allowed to be cooled-down to 60 C during the type test withhigh current [6] between the two surges.
The needed cool-down time of the arrester depends among otherthings on the ambient temperature and the height of the operatingvoltage. It increases with the ambient temperature and the opera-ting voltage. In the most unfavourable case, with 45 C airtemperature and U the following values are valid:Cool-down time between two high ligtning current stresses (65 kArespectively 100 kA):Type POLIM-S and POLIM-H no break necessaryThe other arrester types 75 minutes
Cool-down time between two energy stresses acc. the Table 2:Type POLIM-S and POLIM-H 60 minutesThe other arrester types 60 minutes
The arresters with a nominal discharge current of 5 kA proved to beenough reliable in the MV- network. That is why generally the typePOLIM-D respectively the type POLIM-DN are used.
°
°c
Table 3Energy load of arresters in MV-networks
Arrester type
POLIM-DN
POLIM-D
POLIM-DA
MWK, MWDPOLIM-I
POLIM-S
POLIM-H
200 km line
kJ/kV Uc
10 km cable
kJ/kV Uc kJ/kV Uc kJ/kV UckA kA
I8/20 s
n
High current4/10 s
0.4
0.4
0.4
0.4
0.4
0.4
0.33
0.33
0.33
0.33
0.33
0.33
5
10
10
10
10
20
0.48
0.47
1.0
3.4
3.3
3.2
100
100
100
65
100
100
0.55
0.26
0.53
3.6
3.5
2.6
3.5 p.u. Charging voltage current wave form
Conditions are different when arresters must contain switchingovervoltages rather than lightning overvoltages. The former couldoccur during switching, when an inductive current is interruptedbefore it reaches its natural zero crossing. When such switchingovervoltages occur, the current load on the arrester is very low, sothat a nominal discharge current of 5 kA is sufficient. On this casethe maximum overvoltage is considerably lower than U for MO-arresters. Therefore, shorter distances between arresters andbetween the arrester and earth are possible, facilitating theinstallation of arresters in the cells. The lower values for thesedistances are prescribed in the respective national regulations andare sufficient for metal oxide arresters.
The maximum voltage for arresters with spark-gaps reaches Ualso during switching overvoltages. The minimum distances forthese arresters must therefore be longer in order to preventflashovers. This can make arrester installation in the cellssignificantly more difficult, particularly when there are especiallytight spacing conditions.
Overvoltage protection is the result of arresters placed betweenphase and earth. If a loaded generator is suddenly disconnectedfrom the network, its terminal voltage increases until the voltageregulator has readjusted itself after a few seconds. The relationshipbetween this temporary overvoltage and the normal operatingvoltage is called the load rejection factor . This factor can reach avalue of up to 1.5. In the worst case, the arrresters could becharged with a temporary overvoltage of U = x U , wichmust be taken into account when choosing U .
x U
U ----------------T
The duration t of U determines T and lies in a range from 3 to 10seconds. In the following example, U of type MWK arresters isdetermined:
U = 14 kV = 1.4
t = 10 s T = 1.26 (from Figure 8)
1.4 x 14 kVU ------------------- = 15.56 kV
1.26
The arrester type needed is called MWK 16. Its U is 16 kV and theprotection level at I = 10 kA is 49.1 kV.
The high operational safety requirements for generators make theuse of arresters with the lowest possible protection level desirable.Therefore the special POLIM-H series arrester is recommended forgenerator protection. Not only is the protection level lower, but alsoat the same time is T larger.
For t =10 s, T = 1.31 is permitted, which results in:
1.4 x 14 kVU ------------------ = 14.96 kV
1.31
p
p
L
TOV L m
c
L m
c
TOV
c
m L
c
c
n
c
10.6 Generator Connected to a Lightning EndangeredDistribution Line
>
>
>
10.4 Transformer Connected to a Lightning EndangeredLine on One Side Only
10.5 Arresters in Metal Enclosed MV Substations
In general, all transformer exits which are directly linked tolightning endangered lines must be equipped with arrestersbetween phase and earth. If a transformer connects a high voltagenetwork with a MV network, and only the line on the high voltageside is lightning endangered, arresters are required there. Becauseovervoltages occur very quickly, up to 40% of the overvoltage onthe high voltage side is capacitively carried over to the MV sidethrough the transformer (10). It is therefore often necessary toforesee overvoltage protection for the transformer on the MV side,even though no lightning overvoltages occur on the MV side,according to the above assumptions. According to (9), thisovervoltage protection can be a long MV cable, a low-inductancecapacitor, or a combination of these two elements. They must beconnected as close as possible to the MV exit of the transformerand together possess a capacity of at least 50 nF per phase.
The overvoltage protection can also come from a MV arrester. Thissolution has two essential advantages. First, it must be noted thatinductively transferred overvoltage can be incraesed bycapacitors. Carefully chosen damping resistors in series to thecapacitors make possible a partial decrease of this additionalvoltage load on the transformers. When a MO-arrester without aspark-gap is used, this additional load does not occur.
Secondly, primary voltage encroaches on the MV-voltage during avoltage flashover in the transformer and can cause additionaldamage there. When the MV side is protected with arresters, thearrester sacrifices itself, causing a short to earth, and leaving thedamage essentially restricted to the transformer. The advantage ofan arrester versus a capacitor is particularly evident when thetransformer is connected to a generator, and the arrester protectsthe generator from subsequent damage.
Similar conditions exist with a distribution transformer, whichconnects a MV network to a low-voltage network. Again, lightningovervoltage from the MV network is transferred through thetransformer capacitively to the low voltage side. Therefore,arresters in the low voltage side are necessary, even when only theMV side is lightning endangered. If, on the other hand, only the lowvoltage side is endangered, frequently no arresters are mounted onthe MV side. In this case, it is assumed that the low voltagearrester can also adequately protect the MV side from overvoltagecoming from the low voltage side. Several cases of transformerfailure caused by lightning overvoltage on the 415 volt side arereported in [20]. The author's opinion is that these overvoltages,when they are of long durations, are transferred inductively withthe turn ratio of the transformer to the 11 kV side. In any case, the415 volt arresters were unable to prevent flashovers in the 11 kVwindings. In regions with high lightning activity, it is thereforerecommended to install arresters on the MV side of thetransformer.
It is often necessary to place arresters in a metal enclosed MVsubstation. If a cable connects the cell with a lightning endangeredline, the nominal discharge current of the arrester at the cable headin the cell should be 10 kA. The voltage on the arrester can beexpected to reach U . In order to prevent flashovers in the cell, theminimum distances between the arresters and the arresters andearth recommended by the supplier must be observed.
p
20
19
Naturally, cables in overhead lines are lightning endangered onboth sides. Therefore it must be taken into account that in cableswith one-sided protection, overvoltage can also come from theunprotected side. In this case, the protection effectiveness of thearrester at the other end would be strongly reduced. The allowablelength of cables in overhead lines with one-sided protection istherefore smaller. The length is especially short for cables inconnection with wooden pole lines, as shown in Table 7. The givenvalues for L are valid for arresters with I = 10 kA. The surgeimpedance across the entire cable section must be constant so thatthe voltage reflections do not cause a decrease in L . This is thecase, for example, with cable junctions or when a cable sectionwith a single cable is connected to a section with two parallelcables.
Due to thermal principles, the sheath for single conductor cablesare generally only earthed on one side. Under these circumstancesthe sheath on the unearthed side can take on up to 50 % of thevoltage peak value of the overvoltage entering on the innerconductor. The sheath insulation cannnot withstand thisovervoltage demand. Momentary flashovers can occur betweenthe sheath and the earth, consequently damaging the outer sheathinsulation.
Therefore, the unearthed cable sheath must be protected againstlightning overvoltage with an arrester [2]. The special arresterPOLIM-C is particularly well-suited for this purposes. The voltageinduced along the cable sheath during a short circuit is decisive forU of the arrester. According to [19] it reaches maximum 0.3 kV perkA of fault current and km of cable length. When T = 1.28 and thefault current duration is t < 3 s, the following results:
U 0.24 x I x L in kV
I : max. 50 Hz fault current in kA
L : length of the unearthed cable section in km
Figure 17
K n
K
c
c K K
K
K
10.2 Cable Sheath Protection
>
10.3 Transformers at the End of Cables
According to the direction in Figure 17, a cable of at least 100 m inlength is connected on one end to a lightning endangered line. Atthe other end, a bus bar consisting of sections and connects thecable end on the other side with a transformer. Arrester A1 takesover the overvoltage protection on the line side. The cable end andthe transformer must each be protected with an additional arresterwhen the connecting distance between the two is especially long.The following example indicates under what circumstancesarrester A2 offers sufficient overvoltage protection, in addition toarrester A1.
The overvoltage reflection U at the junction from the line to thecable causes a strong flattening of the voltage rate of rise in thecable. However, this has practically no influence on the allowablelength of the connection , because with increasing length of thevoltage U increases very quickly. Optimal overvoltage protection,therefore, requires that arrester A2 be placed as close as possibleto the cable end, in
Table 8
Maximum permissible distance a between cable end andtransformer according Figure 17 with b=O. The cable is connectedto a lightning endangered line and protected at both ends withMO-surge arresters (type MWK or MWD with U = U )The transformer has no additional arrester protection.Z : Surge impedance of the cable.
The line section is different. Here U increases more slowly withthe increasing length of . Therefore, the transformer is adequatelyprotected, even at a relatively far distance from the arrester. Themaximum allowable values for are indicated in Table 8. Thecapacity of the transformer is assumed to be 2 nF. Smaller valuesresult in greater distances of .
a b
b b
a
a
a
a
K
c m
K
T
order to shorten the distance (see section 10.1).b
MO-surge arrester withU = 3.8 p.u. beiI = 10 kA
p
n
wooden poles earthed crossarms
U [kV]m a [m] a [m]a [m] a [m]
3.6
7.2
12
17.5
24
36
300
43
20
17
19
16
30 3060 60
300
37
14
10
12
11
500
53
20
16
19
20
500
53
14
10
12
11
Z [ ]K Ω
Transformer at the end of a cable
F:U:K:A1, A2:a, b:U :U :MV::LV:
K
T
lightning endangered linelightning overvoltagelong cablearresterslength of the connecting linesmaximum voltage at the cable endmaximum voltage at the transformermedium voltage sidelow voltage side
F
A1 A2
KUK UT MV LVU
ab
The energy absorption capability of these types is much higherthan the expected stresses in the network, exepting the very highligtning currents. These currents can also be commanded by thearresters, it is however most unlikely that they appear. Such highlightning currents can flow through the arrester only when alightning hits directly the top of the arrester. The energy values aregiven in Table 2 and 3.By aerial lines with wooden poles even far away lightning strokescan cause relatively high currents that flow through the arrester. Ifthe sparkover voltage of the wire against the earth is U = 3000 kVand the characteristic wave impedance of the wire is Z = 450from the equation (3) ensue that lightning currents up to13 kA areto be expected in the arrester. This current produce in arresterswith I = 5 kA a residual stress which lies 15% over U . In this waythe protection of the electrical equipment gets worse. For instanceif it lies at the end of an aerial line of 10 km it will be once in threeyears exposed to such an increased voltage stress. That is whyABB has also in the assortment of the MV-arresters the typesMWK, MWD, POLIM-I, POLIM-S and POLIM-H. They possesnominal discharge currents of 10 kA respectively 20 kA. Theiremployment is recommended everywhere where in terms ofstress, operation safety and protection level the highestrequirements are set.
This is the case in
regions with high lightning activity
erial lines with concrete or wooden poles and non-earthed
crossarms
arresters, which are placed on locations where people are often
to be found
on lines, which set exeptional high requirements
concerning the operation safety
protection of engines, generators and cables
areas with high industrial pollution, or when the arresters arenot farther than 1000 m from the sea.
In cases where the 10 kA arresters are recommended is also ahigher energy absorption capability advantageous and an arrestertype with a line discharge class 2 or more should be chosen. That iswhy these arresters have a higher energy capability of at least 5.5kJ/kV (MWK, POLIM-I, POLIM-S)
The peculiarity of some electrical equipment, as for instance
arc furnace
big capacitor batteries
very long cable sections
expensive rotating machines
can make a higher energy absorption capability necessary. In suchcases the special type POLIM-H with I = 20 kA and with 13.3kJ/kV is preferred.
n p
Uc
n
Uc
••
•
•
••
••••
5 Special Operating Conditions
5.1 Network Short Circuit Power
Any arrester can be overloaded. The causes are extremely highstroke currents, a large number of multiple strokes [16, 17] or theso-called system flashover. This is understood to be a short circuitbetween two different voltage levels. The voltage at the arrester onthe lower voltage level rises above the allowable limit. An overloadof any kind causes a flashover or puncture in the resistor. An arcresults in the arrester and the current from this arc is defined by theshort circuit power of the network.
In porcelain housed arresters the ensuing electric arc causes thegas pressure in the housing to elevate quickly. If the network shortcircuit current is not too high, the pressure relief valve in thearrester opens before the housing bursting pressure is reached.On the other hand, if the current is extremely high, the possibility ofthe housing exploding cannot be ruled out.With ABB silicon-polymer arresters there is no danger of burstingin the case of an overload. There is no air space between the activepart of the arrester and its silicon insulation, thus there is no placefor pressure to build up. In the case of an overload, holes appear inthe casing which immediately leads to initiation of the externalflashover.The MV-arresters of the types POLIM-D, MWK and MWD areproved with short circuit currents up to 20 kA. The types POLIM-I,POLIM-S and POLIM-H are tested with short circuit currents up to65 kA. Because of their special construction the arresters are alsoup to the highest short circuit currents insured against explosionand destruction.
The guaranteed values for U are valid for an ambient temperatureof up to 45 C. In the case of outdoor arresters, extreme solarradiation (1.1 kW/m ) is taken into account. If there are other heatsources in the vicinity of the arrester, the increase in radiationexposure must also be taken into account and the value of Uincreased if necessary. If the ambient temperature exceeds 45 C,U must be increased by 2 % for every 5 C of temperature eleva-tion.
MV-arresters produced by ABB are operationally reliable even inareas of high earthquake activity. Silicon arresters from ABB canalso have a support function. In the case of cantilever strength, itmust be distinguished between temporary and operational loadsaccording to DIN 48113. The permissible loads result from theproduct of arrester altitude and maximum permissible momentumload. In Table 4 there are the mechanical data of different arrestertypes to be read.
5.2 Elevated Ambient Temperatures
5.3 Mechanical Stability
c
2
c
c
8
POLIM-DN
POLIM-D
POLIM-DA
MWK, MWD
POLIM-I
POLIM-S
POLIM-H
250
250
350
350
2500
4000
6000
50
50
50
68
100
100
100
625
625
1000
1200
2000
3000
4000
Arrester type Canti lever strengthNm
TorsionNm
VerticalLoad
N
Table 4Mechanical data of MV-surge arresters, produced by ABB
5.4 Air Pollution
Silicon is the best insulating material against pollution. This ismainly because the material is water-repellent. Silicon arrestersbehave more favourably under conditions of heavy air pollutionthan porcelain housed arresters or other polymer insulationmaterials. In addition the self-cleaning feature of silicon itself isoutstanding. Pollutants and dirt cannot adhere well to the flexiblecovering and are washed away by rain.
9
6 Protection Characteristics of the SurgeArrester, Stability
6.1 Surge Arrester Protection Level
The protection level U is the maximum voltage at the arresterterminals during the flow of the nominal discharge current which,according to definition, shows a current form of 8/20 µs. The peakvalue of the current is reached after approx. 8 µs and after approx.20 µs it has decayed to 50 % of the peak value. In the case of spark-gap arresters U is additionally given by the standard lightningimpulse sparkover voltage. This is the lowest prospective peakvalue of a standard lightning voltage impulse (1.2/50 µs) which,when applied to the arrester, causes sparkovers on everyapplication. Virtually the same protection level is possible throughMO and spark-gap arresters having identical continuous servicevoltage U . It lies at about U = 3.33 U or under this value. Moreprecise values are available in the corresponding booklets.
The protection characteristics of an arrester consists not only ofthe value U , but of two additional features. The first is thebehaviour of the arrester during steep wave fronts, which isespecially important for MV equipment. The test for MO-arresterstakes place with the nominal discharge current, the front time ofwhich is reduced from 8 µs to 1 µs. The residual voltage over thearrester reaches a maximum of 1.13 U at this steep current wave.Because of the non linearity of the current-voltage-characteristicof the MO-arrester, the front time of this residual voltage lies in theorder of magnitude of 50 ns.
In comparison with it the front-of-wave sparkover voltage is oftenreferred to for spark-gap arresters. It lies at approx. 1.15 U . In thistest the length of the rise time is adjusted to approx. 400 ns.
p
p
c p c
p
p
p
A true comparison with a MO-arrester necessitates a rise timewhich also lies in the range of 50 ns. With such a steep front thesparkover voltage reaches a value of at least 1.4 U . It follows thatby a steep rise the limiting voltage of the spark-gap arrester is atleast 24 % higher than that of the MO-arrester.
The behaviour of the arrester during switching overvoltages is afurther feature of the protection characteristics. In spark-gaparresters the sparkover voltage reaches approx. the value of Uwith these relatively slowly rising overvoltages. MO-arresters haveno sparkover voltage. With MV-arresters the switching protectionlevel is given through the residual voltage at 500 A of the currentwave 30/60 s. The residual voltage reaches 0.77.... 0.83 Udepending on the arrester type. The limiting voltage duringswitching overvoltages of spark-gap arresters is at least 20 %higher than that of MO-arresters.At the same continuous operating voltage the MO-arresterstherefore demonstrate a more favorable protection characteristicthan spark-gap arresters. The above mentioned figures are validfor arresters employed in networks with high-ohmic insulatedneutral. Regarding the operational safety, MO-arresters have anadditional advantage in the fact that they can also resist temporaryovervoltages as shown in Figure 8.MO and spark-gap arresters must be dimensioned differently innetworks with solidly earthed neutral systems [8]. The result isthat U can be chosen 28 % lower than the rated voltage of thespark-gap arrester. Thereby a protection characteristic results forthe MO technology which, depending on the wave form, lies 28 %to 42 % lower.
p
p
p
c
6.2 Questions of Stability of MO Surge Arresters
In Figure 7, P is the power loss of the MO-resistors in an arresterwhen U is applied. It is evident how P exponentially increases withthe MO temperature, which also results in an increased warming ofthe active parts. The cooling of the resistors results from the flowof heat Q from the active part to the exterior. At temperatures abovethe critical point is P > Q. Here the cooling is not sufficient todissipate the power loss. The resistors would continue to heat upand the arrester would be destroyed by overheating. Throughsuitably dimensioning of the resistors and through designmeasures that enable the cool down of the blocks, it is possible toraise the critical point to such a level, that even if during theoperation the highest energies are likely to occur, this critical pointis impossible to be reached.On the other hand, the mechanism described clearly shows thelimits of the energy absorption capacity of the MO-arrester. Theamount of energy fed to it must never exceed the critical point.Then P < Q and the MO discs cool down until the stable operatingpoint is again reached.
c
Figure 7Power loss P of the MO discs and heat flow Q from the activearrester parts to the exterior as a function of the MOtemperature T at the continuous operating voltage Uc
P,Qthermal runaway
critical
stableoperating point
point
T
Q P
5.5 Altitude Adjustement for Arrester Housing
Normal MV-arresters from ABB can be used at altitudes of up to1800 m above sea level.At higher altitudes the air density is so low that the withstandvoltage of the arrester housing may be no longer sufficient againstexternal flashovers. In this case the unaltered active parts of thearrester (same protection level) must be placed in an elongatedhousing with a larger flashover distance.
As an orientation value one may consider that for every 1000 mover 1800 m above sea level the flashover distance of the housingmust be enlarged by 12 %. For example, at an altitude of 3300 mabove sea level the flashover distance of the housing must be 18 %larger than of a normal arrester.
Figure 6Repelling water on silicone surface (hydrophobicity-effect)
The shorter the sum of the connecting lines + compared with Lin Figure 14, the lower the failure rate. In other words, + must beas small as possible , and L must be as large as possible. The latteris achieved by choosing the proper line direction. As can be seen inFigure 15, the line must be first connected with the arrester andthen connected with the transformer. In this case b = 0 and Lbecomes maximal. The connecting line L can be held short byplacing the arrester as close as possible to the transformer. Bothmeasures together make it possible to fulfill the requirements of
+ << L and therefore keep the failure rate considerably below10%.
If the transformer is connected to a wooden pole line, and if
b < 1 m when U 24 kV
b < 0.6 m when U > 24 kV
cannot be maintained, then the line is to be modified so thatregarding the overvoltage at the substation and the protectivedistance, it behaves as favourably as a line with earthed crossarms.
The necessary measures for this are relatively simple: the crossarms of the last three poles before the station are to be earthed. Theovervoltage which runs into the station from the modified linesnow have the same form as if it came from a line with continuousearthed crossarms. The disadvantage of this solution is thatadditional lightning overvoltages cause flashovers between theconductor and the earth owing to the reduced insulation level of theline. A more efficient method than the earthing the cross armswould be to install another set of arresters one pole in front of thesubstation. The effect is a very strong reduction in the amplitude ofthe incoming overvoltage. This in turn leads to a protectivebehaviour of the arrester at the equipment which is better than thatof earthed cross arms.
On one hand the protective distance of an arrester is, in somecases, not especially long. This applies mostly to electrical equip-ment which is subject to capacitance in substations with a highnetwork voltage and which are connected to wooden poles (seeFigure 14), on the other hand pieces of electrical equipment in asubstation are seldom placed close together. Usually they are sofar apart from each other that one arrester could not protect severalpieces of equipment at the same time. Under this conditions, eachpiece of electrical equipment requires a separate arrester set (onearrester each per phase to earth).
The essential difference between the electrical data of overheadlines and cables is the surge impedance of their conductors toearth. Values for overhead distribution lines are approximately300 to 450 and for cables in the 20 to 60 range. First of all,this difference causes a marked decrease of the lightning over-voltage as soon as the travelling wave reaches the cable entrance.The reduced voltage wave flows through the cable and it isreflected at the end so that the voltage is nearly doubled.Subsequently the wave returns to the cable entrance and is oncemore reflected, etc. In this way, the overvoltage in the cable is builtup gradually although the overvoltage slope in the cable is actuallylower, the peak value is near that of lightning overvoltage on theline [18].
a b
a b
a b
m
m
<
10 Some Special Cases
10.1 Overvoltage Protection in Cable Sections
Ω Ω Ω Ω
The flashover of a bus bar or a conductor of a line toward the earthcauses a short operation shutdown at the most. Subsequentdamage is, however extremly rare. In cables, flashover behaviouris completely different. Flashovers in cable insulation can causedisturbances and require extensive repairs. Flashovers along thecable heads can damage these and exibit the same damage as withinsulation flashovers. Cables must therefore be treated as stationequipment and protected against lightning overvoltage witharresters.The arresters are to be placed directly next to the cable heads. Thejunction lines should be as short as possible. It must be noted thatthe earth connection of the arrester is attached to the cable sheath.
Longer cables require arrester protection at both ends. For shortcables sections onesided protection is, in some cases, sufficient.This is because an arrester at only one end can still offer sufficientlightning overvoltage protection to the other end.A cable which connects the overhead line with the substation isoften only endagered by lightning on the line. The arrester musttherefore be mounted to the line at the cable junction. No secondarrester is necessary at the other end of the cable, as long as thecable length L does not exceed the values which are given in theTable 7. At first glance, it should be noted that L is unlimited in 3.6kV networks. This is because of the relatively high BIL of 13.6 p.u.at this network level. The arrester at the line side of the cable limitsthe overvoltage to approximately 4 p.u. As a result of voltagereflections in the cable, the overvoltage at the other end of the cableis higher, but under 10 p.u. At this level, the overvoltage isharmless to the cable. This, however, does not apply toequipments in the substation. With these equipments additionalvoltage reflections can increase the overvoltage, so that for theirprotection, in case of necessity, arresters must be provided. Themaximum allowable length for a cable section with onesidedprotection is higher for MO-arresters than for those with spark-gaps. This is based on the favourable protection properties of MO-arresters, which begin conducting before U is reached.
K
K
p
18
Arrester withU = 3.8 p.u. for MOU = 4 p.u. for SiCand I = 10 kA
p
p
n
Type of Line Wooden pole Wooden poleearthedcrossarms
earthedcrossarms
Arrester Type MO MO MO MOSiC SiC SiC SiC
U[kV]
m Z[ ]
KΩ
L[m]
K L[m]
K L[m]
K L[m]
KL[m]
K L[m]
K L[m]
K L[m]
K
3.6
7.2
12
17.5
24
36
30
30
30
30
30
30
60
60
60
60
60
60
8
64
40
25
28
22
8
45
30
21
23
20
8
64
40
26
28
22
8
50
32
22
24
20
7
9
9
6
10
8
3
4
4
3
5
4
17
22
19
15
17
15
10
13
14
13
15
14
8
30
15
6
6
1
8
20
11
4
5
1
8
28
14
5
5
1
8
19
10
4
4
1
6
9
7
4
5
1
3
4
3
2
3
1
17
1411
7
3
31
10
9
4
4
1
ULK
ULK
Table 7Maximum allowable length L of cable sections with one-sidedarrester protection. The cable is connected to a lightningendangered line.Lightning overvoltage and momentary value of system voltagehaving different polarities.
Junction length arrester to cable 1 mZ :MO:SiC:
K
K Surge impedance of the cableMetal oxide arresterArrester with spark-gap
9.4 Fault Hazards in Electrical Equipment and TheirDistance from the Surge Arrester
An arrester placed at a distance L from the electrical equipmentlimits the overvoltage to a value of BIL / 1.2 as long as theovervoltage steepness S at the station is not larger than
However, on the average this steepness will be exceeded onceevery 400 years. In this case an overvoltage in the electricalequipment can reach a value above its BIL causing permanentdamage. If the life expectancy of the equipment, e.g. a transformer,is put at about 40 years, then in the time interval t = 400 yearsthere exists a 90 % probability that no damage will occur. Howeverthis includes a failure rate caused by overvoltages during this 40years which amounts to 10 %. Even though an arrester is mountedat the distance L from the transformer.
s
17
S=800 kV / s
U va
b
c
T
UT
U=660 kV
30
20 C=0
C=0,5nF
C=2nF
b (m)
L (m)
10
5
3
2
10 1 2 3 4 5
Arrester protective distance L in the networklevel U = 17.5 kV and 24 kV with respect tothe conductor length .
If + L, then U BIL / 1,2
mb
a b T
C: transformer T capacitance betweenphase and earthMO-arresterspark-gap arresterU = 4 p.u. when I = 5 kAline with wooden polesline with earthed cross arms
p n
14a):14b):
Figure 14b
Arrangements for arresters and electrical equipment
1: poor2: good3: excellent
evaluation of the connections: F:U:A:T: electrical equipment (transformer)C: capacitance of T to earth
lightning endangered linelightning overvoltagearrester
AC C C
U F
1 2 3
A AT T T
Figure 15
MO-surge arrester type POLIM-D 12 N with disconnector,installed on a distribution transformer
Figure 16This is of special significance in regard to arrester protection fortransformers, because they have a capacitance to earth whichshould not be underestimated. Additionally noteworthy is themarked decrease of L with the conductor length . The connectionfrom the lightning endangered line to the high voltage connectionof the arrester should therefore be as direct as possible. In Figure15, three connection possibilities are schematically representedand evaluated.The larger protective distance of the arrester in lines with earthedcross arms (Figure 14 b) stems from the less magnitude of theovervoltage running into the substation (lower flashover voltageline to earth). From this a lower current through the arrester and alower limiting voltage result which enable a larger value for L.In networks where U = 12 kV, the protective distance of thearrester are about 10% longer than represented in Figure 14. Onthe other hand, when U =36 kV, the distance is about 30%shorter. At this network operating level, it should also be noted thatwhen S = 1550 kV / µs (wooden pole lines), the value of L sharplydecreases as soon as > 0.6The protective properties of the arresters are somewhat reducedwith different polarity of the lightning overvoltage and themomentary value of the phase voltage, this is taken into account inthe calculation of L. Additionally it is assumed a very short galva-nic connection between the earth side of the arrrester to thetransformer tank. This is to be taken into consideration whenconnecting the arrester.
b
b
m
m
Otherwise it becomes necessary to increase the length of theconductor length in Figure 13 due to the additional earthconnection. Branching between the arrester and electrical equip-ment to other electrical equipment creates additional voltageoscillations which in most cases results in a reduction of L.
b
1550 kV/µs for wooden pole lines800 kV/µs for earthed cross arm lines
1.0
1.1
1.2
UTOV
Uc
1.3
1.35
1.25
1.15
1.05
T
1.4
101 100 1000 10'000 t (s)
a
b
10
6.3 Temporary Overvoltages
The meaning of Temporary Over-Voltages U is the operatingfrequency overvoltages of a limited duration. The spark-gaparresters require special measures regarding these voltages. Inthese arresters the follow current increases very strongly with theoperating voltage. If this voltage lies above the rated voltage of thearrester, the follow current through the arrester will be too high.Under these conditions, the spark-gaps can no longer extinguishthe arc, that is they ignite it again in each of the following half cyclesuntil the arrester is destroyed by overheating. In networks withtemporary overvoltages the rated voltage of the spark-gap arrestermust be raised to U , which also requires the raising of theprotection level of the arrester.
TOV
TOV
Figure 8Strength T=U / U with respect to temporary overvoltages Uas a function of their duration t at an ambient temperature (airtemperature outside the arrester) of 45 C. The curve a applies toan arrester without preload, the curve b to an arrester, preloadedwith the guaranteed energy E. t is the time duration of theovervoltage with power frequency.The curves apply for the MO-surge arrester type MWK.
In MO-arresters there is no follow current because this isprevented by the extremely non-linear voltage current charac-teristic (Figure 4). It is for this reason that MO arresters are capableof bearing increased operational voltages over a longer period oftime. The strength T of the arrester in the presence of such tempor-ary overvoltages is described in Figure 8.U = T x U
T is then a measure for the permissible height of U .The curve in Figure 8 is valid for arresters without a significantenergy preloading. The higher T and respectively U , the greaterthe power generated in the arrester. Because the MO temperaturecannot exceed a certain value for reasons of stability, is the energysupplied to the arrester also limited. For that reason thepermissible load duration t decreases with the magnitude of Tresp. U . The curve is valid for arresters which at the time t = 0are already preloaded with the specified energy E. Naturally, curveb lies beneath curve a. Arresters which are already preloaded withthe E / U values specified in Table 2 can nevertheless withstandtemporary overvoltages correstonding to curve . This impliesthat the entire energy absorption capability of the arrester exceedsthese guaranteed data. In the time interval t the energy can besupplied to the arrester at any given moment in the form of energyimpulses. The sum of the impulses however must not exceed theamount E.
TOV c TOV
TOV c
TOV
TOV
TOV
c
°
a
b
b
The following example should illustrate the use of Figure 8:An arrester MWK 24 with U = 24 kV could be operated for as longa time as one wishes with U . The environmental temperaturesurrounding the arrester amounts to a maximum of 45 C. At thetime t = 0 the arrester is charged with the specified energy E = 5.5kJ/kV U .Immediately following the temporary overvoltage U = 28 kVoccurs. Thus:T= U / U = 28 kV / 24kV = 1.17
For T= 1.17 it follows that from curve the time t = 400 s.Therefore the duration of U is limited to 400 seconds. Followingthis the arrester is again capable of bearing U and cools down. If itis expected that U has to occur for longer than 400 seconds onthe line, then an arrester with the corresponding elevated U mustbe selected.
c
c
c
TOV
TOV c
TOV
c
TOV
c
°
b
7 Tests
The tests for ABB arresters follow internationally agreed uponrecommendations. IEC 60099-4 has been valid for the MO-arresters since August 1998 [6]. In the USA - Norm ANSI C62.11-1993 is applied [7], which coincides with the IEC. The MV-arresters from ABB fulfill both norms.The tests are made inaccordance to type, routine, and acceptance tests.Furthermore the arresters are submitted to special tests, which arenot mentioned in the international regulations.
At the completion of the development of an arrester design, typetests are carried out. They furnish proof that they comply with therelevant standard. The following tests are designated for MV-arresters:
these testsdemonstrate that the external housing insulation meets theexpected voltage demands.
The function of these tests is to certify thatthe protection level of the arrester does not exceed the guaranteeddata.
this test isperformed to prove that the MO resistors withstand possibledielectric and energy demands without puncture, flashover andcracking.
in this test resistors are subjectedto a temperature of 115 C for 1000 hours with a voltage above U .In doing so it is watched if and how intensive the power losses ofthe resistors increase over their life span. The life span isunderstood to be 110 years according to [7]. In this time ABBresistors demonstrate no increase of power losses: therefore theyare not subject to any ageing process.
the following values are of significance inthis test:
This is the peak value of the ohmic current component by which thereference voltage is measured. I should be large enough so thatthis measurement cannot be influenced by the stray capacitance ofthe arrester components. The reference current must be specifiedby the manufacturer. For ABB MV-arresters the following valuesare valid for I :
1.4 mA for POLIM-DN1.4 mA for POLIM-D, MVK1.6 mA for POLIM-DA2.2 mA for MWK, MWD, POLIM-I, POLIM-C3.6 mA for POLIM-S5.0 mA for POLIM-H
7.1 Type Tests
Isolation withstand tests on the arrester housing:
Residual voltage tests:
Long duration current impulse withstand test:
Time accelerated ageing test:
Operating duty tests:
• Reference current I
° c
ref
ref
ref
11
7.2. Routine Tests
Routine tests are carried out on every arrester or parts of it (e.g. onthe resisors) in order to ascertain that the product meets therequirements of the design specification.
the measured value of thereference voltage U must lie within the stated tolerance rangeallowed by the manufacturer. The lower limit of the U guaranteesthe termal stability of the arrester. The higher the value of U in theroutine test of an arrester, the smaller the power losses at U andtherefore better stabililty during network operation.
this proves that the guaranteed protectionlevel of the arrester is not exceeded. Residual voltage can bemeasured on the individual resistors at nominal current.
this test serves to prove that the arrester isfree of partial discharge. The measurement takes place at a voltageof 1.05 x U on the entire arrester. According to IEC [6] a partialdischage level of < 50 pC is permissible. ABB arresters are testedmore strictly though and must be kept within the 5 pC limit.
this test proves that the porcelain housinghermetically seals the active parts of the arrester. This test is notdone on silicone polymer arresters because the active parts aredirectly sealed in silicone polymer.
In addition to the IEC recommended tests, ABB MV-arresters aresubject to the two following tests:
measurement of the continous current at U for every arrestertime accelerated ageing test over 300 hours on at least tworesistors in every production lot. This test insures that in everyassembly only resistors without any ageing process areused.
Reference voltage measurement:
Residual voltage test:
Partial discharge test:
Leakage test:
•
•
ref
ref
ref
c
c
c
Figure 9MO-surge arrester type MWK after overload test with20kA (0,2 sec) short circuit current.
• Reference voltage U
• Rated voltage U
•
•
•
•
•
Power frequency voltage versus time characteristic:
Pressure-relief test:
Artificial pollution test:
ref
r
This is defined as the operation frequency voltage at the arrester atwhich I flows. U is determined by the peak value of the voltagedivided by 2.
This is the highest permissible r.m.s. value of the power frequencyvoltage for which the arrester is dimensioned in order to operatecorrectly under temporary overvoltage conditions as establishedin the operating duty tests
U is determined by the arrester supplier and lies with ABBarresters at 1.25 x U . The voltages U and U applied during thetest are correspondingly to be raised if:
The resistors show an increase in the power losses in theaccelerated ageing test
The reference voltage of the test sample is higher than theguaranteed minimum value for the arrester.
The operating duty tests serve as proof of the thermal stability ofthe arrester. It does this in two steps. First the conditioning of theresistors is carried out. This is done by applying a voltage of 1.2 xU to the resistors. To this voltage 20 impulse with nominaldischarge current are superimposed. The conditioning can becarried out at the complete arrester, too.
Afterwards the resistors are installed in the arrester housing andloaded with a first high current impulse. After the test sample hascooled down, it is heated to 60 C and loaded with a second highcurrent impulse. At the latest 100ms after the second impulse thetest sample is subjected to the power frequency voltage U for 10 sand then to U for 30 minutes. In the last phase the test mustdemonstrate if the test sample remains thermally stable orbecomes unstable.The test described here is valid for MV-arresters with a nominaldischarge current of 5 kA and 10 kA of the line discharge class 1.
The arrester has passed the test if
Thermal stability has been achievedChanges in the residual voltage, measured before and after thetest, do not exceed 5 %Examination reveals no evidence of puncture, flashover orcracking of the resistors.
this testserves to confirm through experimental means the curves inFigure 8 which are generally proved mathematically. Therefore itserves as a proof of the sufficient stability of the arrester againsttemporary overvoltages.
for arresters with a pressure-relief device.These tests prove that the arrester housing can endure faultcurrent without bursting under predetermined test conditions. Thearresters with housing made of synthetic material which do nothave a pressure-relief device are tested in a special way: they areelectrical overstressed purposefully in order to watch theirbehaviour in case of overloading.
this test demonstrates that the internalparts of the arrester get no damage through external pollution. Theelevated temperature strain of the active parts produced by theuneven voltage distribution along the soiled external insulation areto be observed in particular. With non-ceramic insulations likesilicone the short time tests are not significant.
ref ref
r
c r c
c
r
c
°
Here it is necessary to carry out long term tests in order to provethe ageing stability of the insulation material and theimpermeability of the construction. All the arresters produced byABB are succesfully tested with cyclical long term tests.
S=1550 kV / s
U va
b
c
T
UT
U=3000 kV
b (m)
30
20
10
C=0
C=0,5nF
C=2nF
L (m)
5
3
2
10 0,5 b (m) 1 1,5
SU v
b
Up
cA
a
T
UT
16
It is assumed here that 8 lightning strokes per year per 100 kmoverhead distribution line occur, on the premisies that multi-phaselightning strokes appear more often than single-phase ones. Onthe average, this steepness S is extended once every 400 years.The time-function of the overvoltage increase is parabolic and hasthe steepness S when the value U is reached:
t x Su( t ) = ------------ (2)
4 x U
Equation (2) is defined for the time interval 0 t 2 x U/S. U = 660kV is assumed for lines with earthed cross arms. This isapproximately the flashover voltage of a 20 kV line insulator whenthere are chopped voltage impulses with a steepness of 800 kV / µsand negative polarity.If one puts the values U and S into equation (2), then it becomesclear that the temporal rise in overvoltage u(t) runs about the samefor both types of line. Because the arrester limits the voltage to wellbelow U, the higher value S in wooden pole lines has no effectregarding the protective distance of the arrester. Nevertheless, theprotective distances for both of these line types are different. Thereason lies in the difference in height U of the incomingovervoltage wave. The lightning current i that passes through thearrester reaches the approximately peak value.
2 x U - Ui = ----------------- (3)
Z
Therefore, in the case of wooden pole lines (U = 3000 kV), when Z =450 , a current of 13 kA can be expected through the arrester.
In regard to lines with earthed cross arms (U = 660 kV ), the currentlies below 3 kA. This difference influences the limiting voltage ofthe arrester. This lies therefore higher in the case of wooden polelines which leads, with this sort of line, to a shorter protectivedistance of the arrester.
Using BIL and U from Table 5 and the above values of S inequation (1), the following protective distances result:
L = 2.3. m in the case of wooden pole linesL = 4.5 m in the case of earthed crossed arm lines
These values are valid for the simplified assumption according toFigure 11. Therefore they need to be corrected as depicted inFigure 13. Generally speaking, the electrical equipment, in thiscase a transformer, has a capacity C to earth. This causes voltageoscillations in the connections and , with the result being thatthe voltage U increases with C. This leads to a reduction of theprotective distance. However the parabolic rise of the lightningovervoltage has an opposite influence. The arrester limits theovervoltage to well below its peak value. The maximum steepness,which occurs only in the region of the voltage maximum, thereforehas no effect.In deriving L according to equation (1) it is assumed that thearrester will become conductive only when the voltage at itsterminals has reached the value U . This is the case with spark-gaparresters. MO-arresters without spark-gaps are conductive beforethe terminal voltage has reached U . Therefore the protectiveproperties begin working at an earlier point. Under certaincircumstances therefore, MO arresters protect remote electricalequipment better, which is equivalent to a longer protectivedistance.
2 2
p
p
T
p
p
< <
Ω
9.3. Influences on the Protective Distance through
Electical Equipment, the Types of Arresters and
the Arrangement of the Arresters.
a b
For the configuration according to Figure 13, protective distancesof the arresters were calculated. The increase of the overvoltagewave is assumed to be parabolic and it is assumed that the arresterhas a value of U = 4 p.u. when I = 5 kA. With 1m, the result forthe network voltages up to 7.2 kV is.
L = 20 m in the case of wooden pole lines, C = 0L = 6 m in the case of wooden pole lines, C = 2 nFL = 25 m in the case of earthed cross arms, C = 0L = 15 m in the case of earthed cross arms, C = 2 nF
Figure 13
These values apply to both MO and spark-gap arresters. Theinfluence of capacitance C of the electrical equipment on the lengthof L is clearly seen. The protective distance of the arrester for thenetwork operating levels of U = 17.5 kV and 24 kV are describedin Figure 14. Here it is also clear how L decreases with theincreasing capacitance of the electrical equipment.
Figure 14a
p n
m
b <
Overvoltage at transformer T
U: incoming overvoltage wavev:S: maximum steepness of UA: arresterU : protection level of Ap
propagation velocity of Ua, b: length of the connecting linesT: transformerC: capacitance of T between
phase and earthU : overvoltage at TT
15
S =
u(t) =
Z x di / dt
Z x i(t)
t
2
2
Z
i(t)
u(t)i/2i/2
Figure 12
In 10% of all lightnings, the maximum stroke current change di/dtis higher than 32 kA/µs. When Z = 450 , every 10 lightningstroke will cause a maximum voltage steepness S 7200 kV/µs. Asteepness of this order is to be expected in the substation only ifthe lightning strikes neareby. The probability of this happening isrelatively small. As an example, a lightning stroke with a currentrate of rise of over 32 kA/µs striking within 25 m of a station wouldoccur on the average once every 5000 years.Substantially smaller voltage rates of rise are to be expected at thestation when the lightning stroke occurs far from the substation.Due to corona damping, the front of the overvoltage wave flattensout as it proceeds from the point of the stroke towards the station.If S is the steepness at the location of the stroke, the steepnessalong the length d of the line decreases to the value
The constant K is dependent upon the geometry of the overheadline. In [11] it is approximated to be K = 5 x 10 µs/kVm for MVlines. Supposing that the location of the stroke is d =135 m remotefrom the station a lightning stroke causes an infinitely large voltagerate of rise S . According to the above formula , the steepness S atthe substation will be less than 1500 kV/µs due to the coronadamping. That means that only lightnings which strike the con-ductor in front of the station in a route track of d =135 m can havethe effect of S > 1500 kV/µs at the station.It can be derived from [12] that approximately 8 lightning strokeshit a 100 km long overhead distribution line per year. This numberis valid for German MV networks of 10 , 20 and 30 kV. In Germanythe average ground flash density is 3 strokes per year per km .According to [13] this amounts to approximately 25 lightningstrokes per year per 100 km of distribution line. This factor 3 largervalue which was mathematically determined supposes a MV lineon a level terrain . The difference must be attributed to the fact thatMV lines are often not out in the open . Frequently they are shieldedfrom lightning by neighboring lines, buildings and forests . Thefollowing demonstration uses the empirical value of 8 lightningstrokes per year per 100 km overhead line. However it is to be keptin mind that more lightning strokes are to be expected inunfavorable topographical conditions. In areas with extremelyhigh lightning activity the possibility of 100 strokes per year per100 km overhead line cannot be ruled out.The probability of a lightning stroke in a route section of d = 135 mis therefore 0.01 per year. In an MV substation, lightningovervoltages which have the steepness over 1500 kV/µs can beexpected at the most once every 100 years .
Ω th
o
-6
o
2
>
These two arbitrarily chosen examples should show that largevoltage rates of rise occur less often than small ones. The expectedvalue of a steepness is always linked with the probability of theoccurrence. It is customary, instead of the probability, to indicatethe time interval t , which on the average passes between twoevents. Certainly, in the above example not all lightnings whichstrike the conductor in a route section of d = 135 m cause in thestation a steepness higher than 1500 kV/µs. With some of thelightnings the steepness of the current increase is too low. Manylightnings strike more than just one of the three conductors, whichleads to a reduction in the current rate of rise in the individual linesand therefore lowers the voltage rate of rise.Of further signifance is the fact that the stroke current rise isconcave [13]. That is why the highest steepness of the overvoltageoccurs in the range of the voltage maximum, as shown in the Fig-ure 11. In voltage waves with a high stroke current peak value aflashover from the line to earth takes place before the voltagemaximum has been reached. The upper part of the wave is therebycut off so that the highest steepness does not become effective.Therefore only a fraction of the lightnings which hit the routesection d =135 m of the line generates S > 1500 kV/µs at the station.The probability of S > 1500 kV/µs is therefore significantly less than0.01 per year. This can be evaluated with the help of the lightningcurrent statistics from Berger [14]. Assuming a parabolicprogression of the current increase, then the values indicated inTable 6 result for the expected steepness in a MV substation. Thelower values for S in the lines with earthed cross arms are a result ofthe smaller flashover voltages of the insulators versus the flashovervoltages along the wooden poles.The values of t in Table 6 were determined under the assumptionthat 8 lightning strokes per year and per 100 km of distribution linewould occur. For the value t only lightnings which strike the linewithin 300 m of the station are of significance. If this stretch of theline is free standing, that is not shielded from lightning by theneighbouring lines, buildings and woods, then the value of t is 3times smaller. If in addition there is an extremely high degree oflightning activity in the vicinity, then the value is even 12 timessmaller.
Table 6Expected steepness S from lightning overvoltages in MV sub-stations: The shown values of S will, on the average, be exceededonce in the time interval t .1 in the case of single phase lightning strikes2 in the case of three phase lightning strikes
Overhead line Wooden poles Earthed crossarms
U [kV] 3000 660
S [kV/µs] 1550 800
s
s
s
s
s
Time intervalt [years]s
1940
1630
1450
1200
820
1850
1530
1350
1100
660
1060
920
820
700
520
820
730
660
580
440
1S[kV/ s]
1S[kV/ s]
2S[kV/ s]
2S[kV/ s]
Wooden poles with 3000kVflashover voltage
20 kV network withearthed cross arms
600
400
300
200
100
Type of overheaddistribution line
Lightning overvoltage caused by a lightning strike on an overhead Line.
F:Z:t:i(t):di / dt:u(t):S:
overhead Linesurge impedance of Ftimetotal stroke current as a function of timemaximum steepness of i(t)lightning overvoltage as a function of timemaximum steepness of u(T)
F
1S = -----------------
1/S + K x do
7.3. Acceptance Tests
7.4 Special Tests
If acceptance tests are stipulated at the time of order, the followingtests are carried out on a number of the to be delivered arresters(the number of arresters to be tested is determined by taking thecube root of the delivery amount and rounding it down to a wholenumber):
reference voltage measurement
measurement of the residual voltage of the arrester at nominaldischarge current
partial discharge level measurement at 1.05 x U with the morestringend value < 5 pC, as compared with the IEC.
In the newest editon of the relevant IEC instructions [6] theperformed tests refer to the arresters with porcelain insulation. Inthe IEC working paper for MO-arresters with polymeric housing[22] there are disscused tests special for arresters with polymerhousing. In conformity with this working paper and also exceedingit ABB performed the following tests for the MV-arresters withsilicone insulation.
this test shows the behaviour of the arrester underoverload. During the test the arrester is loaded deliberately withincreasing voltage up to destruction and up to the appearance ofthe system short current. Because of the special construction(completely moulded) and the chosen insulation material(silicone) the ABB MV-arresters are safe from explosion anddestruction up to the highest tested currents. Silicone is a selfextinguishing material. Fire is not caused by downfalling burninginsulation-material.
the test shows the long time behaviour of theinsulation material and the construction in case of cyclicalenvironmental situations like warmth, humidity, rain, saltfog andUV rays during the continuous voltage applied. The test extends ona totally duration of 5000 hours.
the insulation material is exposed 1000 hours to theUV radiation and it is additionally damp. The insulation chara-cteristics of silicon are not negatively influenced because of it. Onthe contrary, the UV radiation promotes the process of thepermanent renewal of the hydrophobicity of the siliconsurface.
the construction as well as the materials usedof the ABB MV-arresters with silicone housing enduretemperatures up to - 60 C without changes of the electrical andmechanical characteristics. Furthermore cyclical freezing up to-40 C in water showed that the construction and especially thesurface of the silicon are not injured through the formation ofice.
long term tests of more than 2 years, in which thearresters were exposed to a relatively air humidity of more than90%, showed that the electrical behaviour of the arrester was notinfluenced because of the penetration of humidity, or the arresterdid not get out of order.
•
•
•
Overload test:
Weather-aging test:
UV radiation:
Deep temperature:
Humidity:
c
°
°
8 Selection of Surge Arresters andDetermination of Uc
For the arrester to meet the needs of the network system, twoconditons are necessary to be fulfilled in the selection of themaximum continuous operating voltage U :
U must be higher than the constant power frequency voltage atthe arrester terminal.
T x U must be higher than the expected temporary overvoltage atthe arrester terminal. According to Figure 8, T is determined by theduration t of the temporary overvoltage. Thus in determining T, t isalso to be taken into account. For reasons of safety, the lower curvein Figure 8 will generally be used.
In selecting the arresters in a three-phase network, the location ofthe arrester plays the deciding role: between phase and earth,between transformer neutral and earth or between phases. Themaximum operating voltage at the arrester terminal connectioncan be calculated with the help of the maximum voltage Ubetween phases. If this is not known, then U should be replacedwith the highest voltage of the system or the highest voltage for theelectrical equipment.
In three-phase networks special attention must be paid to thetemporary overvoltage U . It occurs most frequently duringearth faults. Its value is given by the method of neutral systemearthing. Additionally the system management is of significancebecause it determines the duration t of the temporary overvoltageand with that it decides the value of T (t) for U .
UUc ----------
T(t)
Under the conditions for earth-fault, the voltage increases at"healthy” phases to a maximum of U :
U U for an arrester between phase and earth
UU ------ for the arrester between transformer neutral and earth
3
In every network there exists inductance and capacitance whichproduce oscillating circuits. If their resonant frequency is similarto that of the operating frequency, then the voltage between phaseand earth can basically become higher than that of U in single-pole earth faults. The system management must avoid theoccurrence of such resonances. If this is not possible, then U of acorresponding magnitude should be chosen.
c
c
c
m
m
TOV
c
TOV
m
c m
m
c
m
c
•
•
>
>
>
8.1 Networks with Earth Fault Compensation or withHigh- Ohmic Insulated Neutral
The voltage at transformer neutral can reach a maximum of Um / 3:√
√
12
Figure 11
2 x S x (a + b)U = U + -------------------- v = 300 m/µs
vExperience has shown that a safety factor of 1.2 is sufficientbetween the BIL of the electrical equipment and the lightningovervoltage U at the electrical equipment.
BIL 2 x S x (a + b)------ U = U + --------------------1.2 v
If the limiting value is set at L = a + b, then the required equation (1)follows for
v BILL = --------- x [ ----------- - U ] (1)
2 x S 1.2
If the sum of the connecting lines + is smaller than theprotective distance L of the arrester, then the electrical equipmentis adequately protected at point E. In order to determine theprotective distance L from the equation (1), the steepness S mustbe known. The expected value of S is estimated in the followingsection.
Figure 12 shows a lightnig stroke on a conductor of a distributionline. The time function of the stroke current is designated by i(t).From the point where the lightning hits the conductor, the lightningcurrent i/2 flows out in both directions. If Z is the surge impedanceof the conductor to earth, then this current generates a lightningovervoltage u(t) with the steepness of the voltage increase S(t)between the conductor and the earth. As indicated in Figure 12,S(t) is not constant with time. From now on the maxiumumsteepness of the rise of an overvoltage wave will be indicated by S.
E P
E
E p
p
>
a b
9.2 Expected Steepness S from Lightning Overevoltagesin MV Substations
kV 3.6 7.2 12 17.5 24 36
kV 40 60 75 95 125 170
kV 12 24 40 58.3 79.9 119.9
3.33 2.5 1.88 1.63 1.56 1.42
U
BIL
U
BIL / U
m
p
p
Overvoltage at the line end E
U: incoming overvoltage wavev: propagation velocity of US: steepness (front of wave) of UA: arrester
U : protection level of Aa, b: length of the connecting linesE: line endU : overvoltage at E
p
E
SU v
b
UpA
E
a UE
13 14
8.2 Networks with High-Ohmic Insulated NeutralSystems and Automatic Earth Fault Clearing
8.3.
8.4 Networks with Low-Ohmic Neutral Earthing which donot have Uniformly C 1 .4
Temporary overvoltages are of the same magnitude as those inSection 8.1. Though early cut-off of earth faults enables areduction of U by the factor T. If, for example, the earth fault cut-off results after a maximum of t = 10 s, then, with the help ofFigure 8 it follows that T = 1.26.
UU ---------- for an arrester between phase and earth
TU
U -----------
T x 3
In this type of network, there are at least enough transformers inlow-ohmic neutral earthing, that during an earth fault the phasevoltage never exceeds 1.4 p.u. in the entire system (earth faultfactor C 1.4 ). Therefore is U 1.4 x U / 3. It can beassumed that the clearing time of the earth fault amounts to at themost t = 3 s. It follows for instance for the arrester MWK that T =1.28, and therefore:
The voltage of the neutral of non-earth transformers reaches amaximum of U = 0.4 x U :
For arresters in the vicinity of neutral earth transformers, U can bechosen according to Section 8.3, because C 1.4 is applicablehere.
Care is required if the arrester is located just a few kilometers fromthe transformer, e.g. if there is a remote connection between anoverhead line and a cable. If the ground is dried out or consists ofrock, then it has a relatively high resistance. This can lead to aphase voltage at the location of the arrester which approaches U .In this case it is recommended
UU ----------- for arresters between phase and earth.
TGenerally speaking, the earth fault monitoring would switch off theearth fault quickly (t < 3 s ): therefore T = 1.28.Under extremely poor earthing conditions, e.g. in desert regions,only a slight fault current flows in the case of a remote earth fault. Ifthis is not caught by monitoring, switching off will not take place.The arresters in the vicinity of the earth fault are then loaded for along period of time with Um. In such cases it is advisable to chooseU U .
For keeping in mind: If, as in the above described network, thearrester is located at a transformer with a low-ohmic neutralearthing, then U 1.4 x U / 1.28 x 3 is permissible. It isrecommended that the earth connections of the arresters have agalvanic connection to the transformer tank and these connectionsshould be kept as short as possible.
c
m
c
m
c
e TOV m
TOV m
c
e
m
m
c
c m
c m
>
>
< <
<
>
>
>
for an arrester between transformer neutral and earth
√
√
√
Networks with Solidly Earthed Neutral Systems (C 1.4)e
e
<
<
8.5 Low-Ohmic Neutral Earthing Networks and C > 1.4
8.6 Arresters between Phases ( Neptune Design )
e
This concerns networks which are earthed with an impedance sothat the fault current is limited, for example, to 2 kA. In the case ofan earth fault the voltage increases for a "healthy” phase to U .With a resistive earthed neutral the voltage can be 5 % higher thanU . If the clearing time of the earth fault does not exceed t = 10 s,then results T = 1.26 (for the MWK):
1.05 x UU ------------- = 0.83 U
T
In special cases like, e.g. transformers in arc furnace installations,switching overvoltages can occur which are insufficiently limitedby arresters between phase and earth. In this case arrestersbetween phases are to be used:
U U for arresters between phases.
The arrester arrangement is then composed of 6 arresters, 3between phase and earth and 3 between the phases.
Figure 10 shows the modification of an arrester arrangement,which is known as the Neptune design because of itsconfiguration. It is composed of 4 identical arresters. Twoarresters in series each are fitted between phase and earth andbetween the phases. This configuration delivers overvoltageprotection between the phases. However it does have anappreciable disadvantage compared to the above describedconfiguration with 6 arresters. In the case of an earth fault, e.g. inthe phase of arrester A1, arresters A1 and A4 are parallelconnected. Since the arresters behave in a capacitive mannerduring continuous operating voltage, all 4 arresters together buildup an asymmetrical capacitive system. This results in arresters A2and A3 reaching a value of 0.667 x U . All 4 arresters musttherefore be dimensioned for
U 0.667 x U
The protection level of this arrangement in which there are alwaystwo arresters in series is the same as that of an arrester with U1.334 x U , whereas by a configuration with 6 arresters U Uis sufficent. The protection level of the Neptune design isconsequently 33 % higher as that of the configuration with 6arresters.
Figure 10
m
m
m
c m
c m
m
c m
c
m c m
>
>
>
>
8.7 Operating Voltage with Harmonic Oscillation
9.1 Theoretical Projection for the Protective Distance L
Harmonic currents generate harmonic oscillations underoperating voltage frequencies. For this reason it is possible that thepeak value of phase-to-phase voltage can be greater than 2 xU . If this difference is less than 5 % a correspondingly higher Umust be used as long as U is lower then 1.05 x U / 3 forarresters between phase and earth and lower than 1.05 x U forarresters between phases. On the other hand, if due to theharmonic wave the voltage increase is higher than 5 % the choiceof U must be discussed with the arrester supplier. The same isvalid for forms of voltage which can often be seen in the vicinity ofthyristor converters: voltage steps, ignition peaks, asymmetry inthe two half cycles.
The more the Basic Insulation Level (BIL) exceeds the protectionlevel U of the arrester, the better the electrical equipment isprotected against lightning overvoltages. Modern arresters withU = 3.33 x U and under this value, maintain U 4 p.u. evenwhen placed in a system with high-ohmic insulated neutral. Forelectrical equipment which is subject to lightning overvoltages ,the [9] recommends the indicated BIL values given in Table 5. Inaddition, the IEC [10] recommends for MV networks BIL > 1.4 xU . As can be seen in Table 5, modern arresters fulfill thisrequirement.
Table 5Withstand voltage (BIL) acc. IEC [9] and protection level of modernsurge arresters with U = 4p.u.
The factor 1.4 is generously calculated because it is to be taken intoaccount that the overvoltage can exceed U in the electricalequipment. Reflection effects cause increasing overvoltage at theelectrical equipment with increasing distance from the arrester.After a certain distance the arrester protection is insufficient. Theprotective distance L is understood to be the maximum distancebetween the arrester and the electrical equipment allowingsufficient protection.
In order to effectively lay out overvoltage protection, it is necessaryto know these protective distances. They will be estimated forarresters in MV-systems below.
On the overhead distribution line in Figure 11 an overvoltage Uruns as a travelling wave with the speed v toward the line terminalE. At point E is the electrical equipment to be protected. For thefollowing example it is considered that the electrical equipment tobe protected is high-ohmic (transformator, open connection).When the travelling wave reaches E, it is reflected and the voltageincreases to 2 x U. It is the function of the arrester A to protectelectrical equipment from reaching unacceptable high voltagevalues. Under the simplified assumption that the front of wavesteepness S of the incoming overvoltage wave is time constant, thefollwing relationship is valid for the maximum value U :
√
√m c
c m
m
c
P
P C P
P
p
P
E
9. Protective Distance of the Surge Arrester
<
U Uc m>
U Uc m>
U 0,667 x Uc m>
T T
A1 A2
A4
A3
a) b)
Overvoltage protection between phase and earthand between phases.
T:a):b):A1, A2, A3, A4
transformerprotection with 6 arrestersneptun designfour identical arresters with U 0.667 x Uc m>
1.4 x U 1.1 x UU
1.28 x 3 3
m m
c > ------------- = ------------- for an arrester between phase and earth√ √
0.4 x UU ------------- = 0.32 x U for an arrester between
1.28 transformer neutral and earth.
m
c m>
Figure 11
2 x S x (a + b)U = U + -------------------- v = 300 m/µs
vExperience has shown that a safety factor of 1.2 is sufficientbetween the BIL of the electrical equipment and the lightningovervoltage U at the electrical equipment.
BIL 2 x S x (a + b)------ U = U + --------------------1.2 v
If the limiting value is set at L = a + b, then the required equation (1)follows for
v BILL = --------- x [ ----------- - U ] (1)
2 x S 1.2
If the sum of the connecting lines + is smaller than theprotective distance L of the arrester, then the electrical equipmentis adequately protected at point E. In order to determine theprotective distance L from the equation (1), the steepness S mustbe known. The expected value of S is estimated in the followingsection.
Figure 12 shows a lightnig stroke on a conductor of a distributionline. The time function of the stroke current is designated by i(t).From the point where the lightning hits the conductor, the lightningcurrent i/2 flows out in both directions. If Z is the surge impedanceof the conductor to earth, then this current generates a lightningovervoltage u(t) with the steepness of the voltage increase S(t)between the conductor and the earth. As indicated in Figure 12,S(t) is not constant with time. From now on the maxiumumsteepness of the rise of an overvoltage wave will be indicated by S.
E P
E
E p
p
>
a b
9.2 Expected Steepness S from Lightning Overevoltagesin MV Substations
kV 3.6 7.2 12 17.5 24 36
kV 40 60 75 95 125 170
kV 12 24 40 58.3 79.9 119.9
3.33 2.5 1.88 1.63 1.56 1.42
U
BIL
U
BIL / U
m
p
p
Overvoltage at the line end E
U: incoming overvoltage wavev: propagation velocity of US: steepness (front of wave) of UA: arrester
U : protection level of Aa, b: length of the connecting linesE: line endU : overvoltage at E
p
E
SU v
b
UpA
E
a UE
13 14
8.2 Networks with High-Ohmic Insulated NeutralSystems and Automatic Earth Fault Clearing
8.3.
8.4 Networks with Low-Ohmic Neutral Earthing which donot have Uniformly C 1 .4
Temporary overvoltages are of the same magnitude as those inSection 8.1. Though early cut-off of earth faults enables areduction of U by the factor T. If, for example, the earth fault cut-off results after a maximum of t = 10 s, then, with the help ofFigure 8 it follows that T = 1.26.
UU ---------- for an arrester between phase and earth
TU
U -----------
T x 3
In this type of network, there are at least enough transformers inlow-ohmic neutral earthing, that during an earth fault the phasevoltage never exceeds 1.4 p.u. in the entire system (earth faultfactor C 1.4 ). Therefore is U 1.4 x U / 3. It can beassumed that the clearing time of the earth fault amounts to at themost t = 3 s. It follows for instance for the arrester MWK that T =1.28, and therefore:
The voltage of the neutral of non-earth transformers reaches amaximum of U = 0.4 x U :
For arresters in the vicinity of neutral earth transformers, U can bechosen according to Section 8.3, because C 1.4 is applicablehere.
Care is required if the arrester is located just a few kilometers fromthe transformer, e.g. if there is a remote connection between anoverhead line and a cable. If the ground is dried out or consists ofrock, then it has a relatively high resistance. This can lead to aphase voltage at the location of the arrester which approaches U .In this case it is recommended
UU ----------- for arresters between phase and earth.
TGenerally speaking, the earth fault monitoring would switch off theearth fault quickly (t < 3 s ): therefore T = 1.28.Under extremely poor earthing conditions, e.g. in desert regions,only a slight fault current flows in the case of a remote earth fault. Ifthis is not caught by monitoring, switching off will not take place.The arresters in the vicinity of the earth fault are then loaded for along period of time with Um. In such cases it is advisable to chooseU U .
For keeping in mind: If, as in the above described network, thearrester is located at a transformer with a low-ohmic neutralearthing, then U 1.4 x U / 1.28 x 3 is permissible. It isrecommended that the earth connections of the arresters have agalvanic connection to the transformer tank and these connectionsshould be kept as short as possible.
c
m
c
m
c
e TOV m
TOV m
c
e
m
m
c
c m
c m
>
>
< <
<
>
>
>
for an arrester between transformer neutral and earth
√
√
√
Networks with Solidly Earthed Neutral Systems (C 1.4)e
e
<
<
8.5 Low-Ohmic Neutral Earthing Networks and C > 1.4
8.6 Arresters between Phases ( Neptune Design )
e
This concerns networks which are earthed with an impedance sothat the fault current is limited, for example, to 2 kA. In the case ofan earth fault the voltage increases for a "healthy” phase to U .With a resistive earthed neutral the voltage can be 5 % higher thanU . If the clearing time of the earth fault does not exceed t = 10 s,then results T = 1.26 (for the MWK):
1.05 x UU ------------- = 0.83 U
T
In special cases like, e.g. transformers in arc furnace installations,switching overvoltages can occur which are insufficiently limitedby arresters between phase and earth. In this case arrestersbetween phases are to be used:
U U for arresters between phases.
The arrester arrangement is then composed of 6 arresters, 3between phase and earth and 3 between the phases.
Figure 10 shows the modification of an arrester arrangement,which is known as the Neptune design because of itsconfiguration. It is composed of 4 identical arresters. Twoarresters in series each are fitted between phase and earth andbetween the phases. This configuration delivers overvoltageprotection between the phases. However it does have anappreciable disadvantage compared to the above describedconfiguration with 6 arresters. In the case of an earth fault, e.g. inthe phase of arrester A1, arresters A1 and A4 are parallelconnected. Since the arresters behave in a capacitive mannerduring continuous operating voltage, all 4 arresters together buildup an asymmetrical capacitive system. This results in arresters A2and A3 reaching a value of 0.667 x U . All 4 arresters musttherefore be dimensioned for
U 0.667 x U
The protection level of this arrangement in which there are alwaystwo arresters in series is the same as that of an arrester with U1.334 x U , whereas by a configuration with 6 arresters U Uis sufficent. The protection level of the Neptune design isconsequently 33 % higher as that of the configuration with 6arresters.
Figure 10
m
m
m
c m
c m
m
c m
c
m c m
>
>
>
>
8.7 Operating Voltage with Harmonic Oscillation
9.1 Theoretical Projection for the Protective Distance L
Harmonic currents generate harmonic oscillations underoperating voltage frequencies. For this reason it is possible that thepeak value of phase-to-phase voltage can be greater than 2 xU . If this difference is less than 5 % a correspondingly higher Umust be used as long as U is lower then 1.05 x U / 3 forarresters between phase and earth and lower than 1.05 x U forarresters between phases. On the other hand, if due to theharmonic wave the voltage increase is higher than 5 % the choiceof U must be discussed with the arrester supplier. The same isvalid for forms of voltage which can often be seen in the vicinity ofthyristor converters: voltage steps, ignition peaks, asymmetry inthe two half cycles.
The more the Basic Insulation Level (BIL) exceeds the protectionlevel U of the arrester, the better the electrical equipment isprotected against lightning overvoltages. Modern arresters withU = 3.33 x U and under this value, maintain U 4 p.u. evenwhen placed in a system with high-ohmic insulated neutral. Forelectrical equipment which is subject to lightning overvoltages ,the [9] recommends the indicated BIL values given in Table 5. Inaddition, the IEC [10] recommends for MV networks BIL > 1.4 xU . As can be seen in Table 5, modern arresters fulfill thisrequirement.
Table 5Withstand voltage (BIL) acc. IEC [9] and protection level of modernsurge arresters with U = 4p.u.
The factor 1.4 is generously calculated because it is to be taken intoaccount that the overvoltage can exceed U in the electricalequipment. Reflection effects cause increasing overvoltage at theelectrical equipment with increasing distance from the arrester.After a certain distance the arrester protection is insufficient. Theprotective distance L is understood to be the maximum distancebetween the arrester and the electrical equipment allowingsufficient protection.
In order to effectively lay out overvoltage protection, it is necessaryto know these protective distances. They will be estimated forarresters in MV-systems below.
On the overhead distribution line in Figure 11 an overvoltage Uruns as a travelling wave with the speed v toward the line terminalE. At point E is the electrical equipment to be protected. For thefollowing example it is considered that the electrical equipment tobe protected is high-ohmic (transformator, open connection).When the travelling wave reaches E, it is reflected and the voltageincreases to 2 x U. It is the function of the arrester A to protectelectrical equipment from reaching unacceptable high voltagevalues. Under the simplified assumption that the front of wavesteepness S of the incoming overvoltage wave is time constant, thefollwing relationship is valid for the maximum value U :
√
√m c
c m
m
c
P
P C P
P
p
P
E
9. Protective Distance of the Surge Arrester
<
U Uc m>
U Uc m>
U 0,667 x Uc m>
T T
A1 A2
A4
A3
a) b)
Overvoltage protection between phase and earthand between phases.
T:a):b):A1, A2, A3, A4
transformerprotection with 6 arrestersneptun designfour identical arresters with U 0.667 x Uc m>
1.4 x U 1.1 x UU
1.28 x 3 3
m m
c > ------------- = ------------- for an arrester between phase and earth√ √
0.4 x UU ------------- = 0.32 x U for an arrester between
1.28 transformer neutral and earth.
m
c m>
15
S =
u(t) =
Z x di / dt
Z x i(t)
t
2
2
Z
i(t)
u(t)i/2i/2
Figure 12
In 10% of all lightnings, the maximum stroke current change di/dtis higher than 32 kA/µs. When Z = 450 , every 10 lightningstroke will cause a maximum voltage steepness S 7200 kV/µs. Asteepness of this order is to be expected in the substation only ifthe lightning strikes neareby. The probability of this happening isrelatively small. As an example, a lightning stroke with a currentrate of rise of over 32 kA/µs striking within 25 m of a station wouldoccur on the average once every 5000 years.Substantially smaller voltage rates of rise are to be expected at thestation when the lightning stroke occurs far from the substation.Due to corona damping, the front of the overvoltage wave flattensout as it proceeds from the point of the stroke towards the station.If S is the steepness at the location of the stroke, the steepnessalong the length d of the line decreases to the value
The constant K is dependent upon the geometry of the overheadline. In [11] it is approximated to be K = 5 x 10 µs/kVm for MVlines. Supposing that the location of the stroke is d =135 m remotefrom the station a lightning stroke causes an infinitely large voltagerate of rise S . According to the above formula , the steepness S atthe substation will be less than 1500 kV/µs due to the coronadamping. That means that only lightnings which strike the con-ductor in front of the station in a route track of d =135 m can havethe effect of S > 1500 kV/µs at the station.It can be derived from [12] that approximately 8 lightning strokeshit a 100 km long overhead distribution line per year. This numberis valid for German MV networks of 10 , 20 and 30 kV. In Germanythe average ground flash density is 3 strokes per year per km .According to [13] this amounts to approximately 25 lightningstrokes per year per 100 km of distribution line. This factor 3 largervalue which was mathematically determined supposes a MV lineon a level terrain . The difference must be attributed to the fact thatMV lines are often not out in the open . Frequently they are shieldedfrom lightning by neighboring lines, buildings and forests . Thefollowing demonstration uses the empirical value of 8 lightningstrokes per year per 100 km overhead line. However it is to be keptin mind that more lightning strokes are to be expected inunfavorable topographical conditions. In areas with extremelyhigh lightning activity the possibility of 100 strokes per year per100 km overhead line cannot be ruled out.The probability of a lightning stroke in a route section of d = 135 mis therefore 0.01 per year. In an MV substation, lightningovervoltages which have the steepness over 1500 kV/µs can beexpected at the most once every 100 years .
Ω th
o
-6
o
2
>
These two arbitrarily chosen examples should show that largevoltage rates of rise occur less often than small ones. The expectedvalue of a steepness is always linked with the probability of theoccurrence. It is customary, instead of the probability, to indicatethe time interval t , which on the average passes between twoevents. Certainly, in the above example not all lightnings whichstrike the conductor in a route section of d = 135 m cause in thestation a steepness higher than 1500 kV/µs. With some of thelightnings the steepness of the current increase is too low. Manylightnings strike more than just one of the three conductors, whichleads to a reduction in the current rate of rise in the individual linesand therefore lowers the voltage rate of rise.Of further signifance is the fact that the stroke current rise isconcave [13]. That is why the highest steepness of the overvoltageoccurs in the range of the voltage maximum, as shown in the Fig-ure 11. In voltage waves with a high stroke current peak value aflashover from the line to earth takes place before the voltagemaximum has been reached. The upper part of the wave is therebycut off so that the highest steepness does not become effective.Therefore only a fraction of the lightnings which hit the routesection d =135 m of the line generates S > 1500 kV/µs at the station.The probability of S > 1500 kV/µs is therefore significantly less than0.01 per year. This can be evaluated with the help of the lightningcurrent statistics from Berger [14]. Assuming a parabolicprogression of the current increase, then the values indicated inTable 6 result for the expected steepness in a MV substation. Thelower values for S in the lines with earthed cross arms are a result ofthe smaller flashover voltages of the insulators versus the flashovervoltages along the wooden poles.The values of t in Table 6 were determined under the assumptionthat 8 lightning strokes per year and per 100 km of distribution linewould occur. For the value t only lightnings which strike the linewithin 300 m of the station are of significance. If this stretch of theline is free standing, that is not shielded from lightning by theneighbouring lines, buildings and woods, then the value of t is 3times smaller. If in addition there is an extremely high degree oflightning activity in the vicinity, then the value is even 12 timessmaller.
Table 6Expected steepness S from lightning overvoltages in MV sub-stations: The shown values of S will, on the average, be exceededonce in the time interval t .1 in the case of single phase lightning strikes2 in the case of three phase lightning strikes
Overhead line Wooden poles Earthed crossarms
U [kV] 3000 660
S [kV/µs] 1550 800
s
s
s
s
s
Time intervalt [years]s
1940
1630
1450
1200
820
1850
1530
1350
1100
660
1060
920
820
700
520
820
730
660
580
440
1S[kV/ s]
1S[kV/ s]
2S[kV/ s]
2S[kV/ s]
Wooden poles with 3000kVflashover voltage
20 kV network withearthed cross arms
600
400
300
200
100
Type of overheaddistribution line
Lightning overvoltage caused by a lightning strike on an overhead Line.
F:Z:t:i(t):di / dt:u(t):S:
overhead Linesurge impedance of Ftimetotal stroke current as a function of timemaximum steepness of i(t)lightning overvoltage as a function of timemaximum steepness of u(T)
F
1S = -----------------
1/S + K x do
7.3. Acceptance Tests
7.4 Special Tests
If acceptance tests are stipulated at the time of order, the followingtests are carried out on a number of the to be delivered arresters(the number of arresters to be tested is determined by taking thecube root of the delivery amount and rounding it down to a wholenumber):
reference voltage measurement
measurement of the residual voltage of the arrester at nominaldischarge current
partial discharge level measurement at 1.05 x U with the morestringend value < 5 pC, as compared with the IEC.
In the newest editon of the relevant IEC instructions [6] theperformed tests refer to the arresters with porcelain insulation. Inthe IEC working paper for MO-arresters with polymeric housing[22] there are disscused tests special for arresters with polymerhousing. In conformity with this working paper and also exceedingit ABB performed the following tests for the MV-arresters withsilicone insulation.
this test shows the behaviour of the arrester underoverload. During the test the arrester is loaded deliberately withincreasing voltage up to destruction and up to the appearance ofthe system short current. Because of the special construction(completely moulded) and the chosen insulation material(silicone) the ABB MV-arresters are safe from explosion anddestruction up to the highest tested currents. Silicone is a selfextinguishing material. Fire is not caused by downfalling burninginsulation-material.
the test shows the long time behaviour of theinsulation material and the construction in case of cyclicalenvironmental situations like warmth, humidity, rain, saltfog andUV rays during the continuous voltage applied. The test extends ona totally duration of 5000 hours.
the insulation material is exposed 1000 hours to theUV radiation and it is additionally damp. The insulation chara-cteristics of silicon are not negatively influenced because of it. Onthe contrary, the UV radiation promotes the process of thepermanent renewal of the hydrophobicity of the siliconsurface.
the construction as well as the materials usedof the ABB MV-arresters with silicone housing enduretemperatures up to - 60 C without changes of the electrical andmechanical characteristics. Furthermore cyclical freezing up to-40 C in water showed that the construction and especially thesurface of the silicon are not injured through the formation ofice.
long term tests of more than 2 years, in which thearresters were exposed to a relatively air humidity of more than90%, showed that the electrical behaviour of the arrester was notinfluenced because of the penetration of humidity, or the arresterdid not get out of order.
•
•
•
Overload test:
Weather-aging test:
UV radiation:
Deep temperature:
Humidity:
c
°
°
8 Selection of Surge Arresters andDetermination of Uc
For the arrester to meet the needs of the network system, twoconditons are necessary to be fulfilled in the selection of themaximum continuous operating voltage U :
U must be higher than the constant power frequency voltage atthe arrester terminal.
T x U must be higher than the expected temporary overvoltage atthe arrester terminal. According to Figure 8, T is determined by theduration t of the temporary overvoltage. Thus in determining T, t isalso to be taken into account. For reasons of safety, the lower curvein Figure 8 will generally be used.
In selecting the arresters in a three-phase network, the location ofthe arrester plays the deciding role: between phase and earth,between transformer neutral and earth or between phases. Themaximum operating voltage at the arrester terminal connectioncan be calculated with the help of the maximum voltage Ubetween phases. If this is not known, then U should be replacedwith the highest voltage of the system or the highest voltage for theelectrical equipment.
In three-phase networks special attention must be paid to thetemporary overvoltage U . It occurs most frequently duringearth faults. Its value is given by the method of neutral systemearthing. Additionally the system management is of significancebecause it determines the duration t of the temporary overvoltageand with that it decides the value of T (t) for U .
UUc ----------
T(t)
Under the conditions for earth-fault, the voltage increases at"healthy” phases to a maximum of U :
U U for an arrester between phase and earth
UU ------ for the arrester between transformer neutral and earth
3
In every network there exists inductance and capacitance whichproduce oscillating circuits. If their resonant frequency is similarto that of the operating frequency, then the voltage between phaseand earth can basically become higher than that of U in single-pole earth faults. The system management must avoid theoccurrence of such resonances. If this is not possible, then U of acorresponding magnitude should be chosen.
c
c
c
m
m
TOV
c
TOV
m
c m
m
c
m
c
•
•
>
>
>
8.1 Networks with Earth Fault Compensation or withHigh- Ohmic Insulated Neutral
The voltage at transformer neutral can reach a maximum of Um / 3:√
√
12
11
7.2. Routine Tests
Routine tests are carried out on every arrester or parts of it (e.g. onthe resisors) in order to ascertain that the product meets therequirements of the design specification.
the measured value of thereference voltage U must lie within the stated tolerance rangeallowed by the manufacturer. The lower limit of the U guaranteesthe termal stability of the arrester. The higher the value of U in theroutine test of an arrester, the smaller the power losses at U andtherefore better stabililty during network operation.
this proves that the guaranteed protectionlevel of the arrester is not exceeded. Residual voltage can bemeasured on the individual resistors at nominal current.
this test serves to prove that the arrester isfree of partial discharge. The measurement takes place at a voltageof 1.05 x U on the entire arrester. According to IEC [6] a partialdischage level of < 50 pC is permissible. ABB arresters are testedmore strictly though and must be kept within the 5 pC limit.
this test proves that the porcelain housinghermetically seals the active parts of the arrester. This test is notdone on silicone polymer arresters because the active parts aredirectly sealed in silicone polymer.
In addition to the IEC recommended tests, ABB MV-arresters aresubject to the two following tests:
measurement of the continous current at U for every arrestertime accelerated ageing test over 300 hours on at least tworesistors in every production lot. This test insures that in everyassembly only resistors without any ageing process areused.
Reference voltage measurement:
Residual voltage test:
Partial discharge test:
Leakage test:
•
•
ref
ref
ref
c
c
c
Figure 9MO-surge arrester type MWK after overload test with20kA (0,2 sec) short circuit current.
• Reference voltage U
• Rated voltage U
•
•
•
•
•
Power frequency voltage versus time characteristic:
Pressure-relief test:
Artificial pollution test:
ref
r
This is defined as the operation frequency voltage at the arrester atwhich I flows. U is determined by the peak value of the voltagedivided by 2.
This is the highest permissible r.m.s. value of the power frequencyvoltage for which the arrester is dimensioned in order to operatecorrectly under temporary overvoltage conditions as establishedin the operating duty tests
U is determined by the arrester supplier and lies with ABBarresters at 1.25 x U . The voltages U and U applied during thetest are correspondingly to be raised if:
The resistors show an increase in the power losses in theaccelerated ageing test
The reference voltage of the test sample is higher than theguaranteed minimum value for the arrester.
The operating duty tests serve as proof of the thermal stability ofthe arrester. It does this in two steps. First the conditioning of theresistors is carried out. This is done by applying a voltage of 1.2 xU to the resistors. To this voltage 20 impulse with nominaldischarge current are superimposed. The conditioning can becarried out at the complete arrester, too.
Afterwards the resistors are installed in the arrester housing andloaded with a first high current impulse. After the test sample hascooled down, it is heated to 60 C and loaded with a second highcurrent impulse. At the latest 100ms after the second impulse thetest sample is subjected to the power frequency voltage U for 10 sand then to U for 30 minutes. In the last phase the test mustdemonstrate if the test sample remains thermally stable orbecomes unstable.The test described here is valid for MV-arresters with a nominaldischarge current of 5 kA and 10 kA of the line discharge class 1.
The arrester has passed the test if
Thermal stability has been achievedChanges in the residual voltage, measured before and after thetest, do not exceed 5 %Examination reveals no evidence of puncture, flashover orcracking of the resistors.
this testserves to confirm through experimental means the curves inFigure 8 which are generally proved mathematically. Therefore itserves as a proof of the sufficient stability of the arrester againsttemporary overvoltages.
for arresters with a pressure-relief device.These tests prove that the arrester housing can endure faultcurrent without bursting under predetermined test conditions. Thearresters with housing made of synthetic material which do nothave a pressure-relief device are tested in a special way: they areelectrical overstressed purposefully in order to watch theirbehaviour in case of overloading.
this test demonstrates that the internalparts of the arrester get no damage through external pollution. Theelevated temperature strain of the active parts produced by theuneven voltage distribution along the soiled external insulation areto be observed in particular. With non-ceramic insulations likesilicone the short time tests are not significant.
ref ref
r
c r c
c
r
c
°
Here it is necessary to carry out long term tests in order to provethe ageing stability of the insulation material and theimpermeability of the construction. All the arresters produced byABB are succesfully tested with cyclical long term tests.
S=1550 kV / s
U va
b
c
T
UT
U=3000 kV
b (m)
30
20
10
C=0
C=0,5nF
C=2nF
L (m)
5
3
2
10 0,5 b (m) 1 1,5
SU v
b
Up
cA
a
T
UT
16
It is assumed here that 8 lightning strokes per year per 100 kmoverhead distribution line occur, on the premisies that multi-phaselightning strokes appear more often than single-phase ones. Onthe average, this steepness S is extended once every 400 years.The time-function of the overvoltage increase is parabolic and hasthe steepness S when the value U is reached:
t x Su( t ) = ------------ (2)
4 x U
Equation (2) is defined for the time interval 0 t 2 x U/S. U = 660kV is assumed for lines with earthed cross arms. This isapproximately the flashover voltage of a 20 kV line insulator whenthere are chopped voltage impulses with a steepness of 800 kV / µsand negative polarity.If one puts the values U and S into equation (2), then it becomesclear that the temporal rise in overvoltage u(t) runs about the samefor both types of line. Because the arrester limits the voltage to wellbelow U, the higher value S in wooden pole lines has no effectregarding the protective distance of the arrester. Nevertheless, theprotective distances for both of these line types are different. Thereason lies in the difference in height U of the incomingovervoltage wave. The lightning current i that passes through thearrester reaches the approximately peak value.
2 x U - Ui = ----------------- (3)
Z
Therefore, in the case of wooden pole lines (U = 3000 kV), when Z =450 , a current of 13 kA can be expected through the arrester.
In regard to lines with earthed cross arms (U = 660 kV ), the currentlies below 3 kA. This difference influences the limiting voltage ofthe arrester. This lies therefore higher in the case of wooden polelines which leads, with this sort of line, to a shorter protectivedistance of the arrester.
Using BIL and U from Table 5 and the above values of S inequation (1), the following protective distances result:
L = 2.3. m in the case of wooden pole linesL = 4.5 m in the case of earthed crossed arm lines
These values are valid for the simplified assumption according toFigure 11. Therefore they need to be corrected as depicted inFigure 13. Generally speaking, the electrical equipment, in thiscase a transformer, has a capacity C to earth. This causes voltageoscillations in the connections and , with the result being thatthe voltage U increases with C. This leads to a reduction of theprotective distance. However the parabolic rise of the lightningovervoltage has an opposite influence. The arrester limits theovervoltage to well below its peak value. The maximum steepness,which occurs only in the region of the voltage maximum, thereforehas no effect.In deriving L according to equation (1) it is assumed that thearrester will become conductive only when the voltage at itsterminals has reached the value U . This is the case with spark-gaparresters. MO-arresters without spark-gaps are conductive beforethe terminal voltage has reached U . Therefore the protectiveproperties begin working at an earlier point. Under certaincircumstances therefore, MO arresters protect remote electricalequipment better, which is equivalent to a longer protectivedistance.
2 2
p
p
T
p
p
< <
Ω
9.3. Influences on the Protective Distance through
Electical Equipment, the Types of Arresters and
the Arrangement of the Arresters.
a b
For the configuration according to Figure 13, protective distancesof the arresters were calculated. The increase of the overvoltagewave is assumed to be parabolic and it is assumed that the arresterhas a value of U = 4 p.u. when I = 5 kA. With 1m, the result forthe network voltages up to 7.2 kV is.
L = 20 m in the case of wooden pole lines, C = 0L = 6 m in the case of wooden pole lines, C = 2 nFL = 25 m in the case of earthed cross arms, C = 0L = 15 m in the case of earthed cross arms, C = 2 nF
Figure 13
These values apply to both MO and spark-gap arresters. Theinfluence of capacitance C of the electrical equipment on the lengthof L is clearly seen. The protective distance of the arrester for thenetwork operating levels of U = 17.5 kV and 24 kV are describedin Figure 14. Here it is also clear how L decreases with theincreasing capacitance of the electrical equipment.
Figure 14a
p n
m
b <
Overvoltage at transformer T
U: incoming overvoltage wavev:S: maximum steepness of UA: arresterU : protection level of Ap
propagation velocity of Ua, b: length of the connecting linesT: transformerC: capacitance of T between
phase and earthU : overvoltage at TT
9.4 Fault Hazards in Electrical Equipment and TheirDistance from the Surge Arrester
An arrester placed at a distance L from the electrical equipmentlimits the overvoltage to a value of BIL / 1.2 as long as theovervoltage steepness S at the station is not larger than
However, on the average this steepness will be exceeded onceevery 400 years. In this case an overvoltage in the electricalequipment can reach a value above its BIL causing permanentdamage. If the life expectancy of the equipment, e.g. a transformer,is put at about 40 years, then in the time interval t = 400 yearsthere exists a 90 % probability that no damage will occur. Howeverthis includes a failure rate caused by overvoltages during this 40years which amounts to 10 %. Even though an arrester is mountedat the distance L from the transformer.
s
17
S=800 kV / s
U va
b
c
T
UT
U=660 kV
30
20 C=0
C=0,5nF
C=2nF
b (m)
L (m)
10
5
3
2
10 1 2 3 4 5
Arrester protective distance L in the networklevel U = 17.5 kV and 24 kV with respect tothe conductor length .
If + L, then U BIL / 1,2
mb
a b T
C: transformer T capacitance betweenphase and earthMO-arresterspark-gap arresterU = 4 p.u. when I = 5 kAline with wooden polesline with earthed cross arms
p n
14a):14b):
Figure 14b
Arrangements for arresters and electrical equipment
1: poor2: good3: excellent
evaluation of the connections: F:U:A:T: electrical equipment (transformer)C: capacitance of T to earth
lightning endangered linelightning overvoltagearrester
AC C C
U F
1 2 3
A AT T T
Figure 15
MO-surge arrester type POLIM-D 12 N with disconnector,installed on a distribution transformer
Figure 16This is of special significance in regard to arrester protection fortransformers, because they have a capacitance to earth whichshould not be underestimated. Additionally noteworthy is themarked decrease of L with the conductor length . The connectionfrom the lightning endangered line to the high voltage connectionof the arrester should therefore be as direct as possible. In Figure15, three connection possibilities are schematically representedand evaluated.The larger protective distance of the arrester in lines with earthedcross arms (Figure 14 b) stems from the less magnitude of theovervoltage running into the substation (lower flashover voltageline to earth). From this a lower current through the arrester and alower limiting voltage result which enable a larger value for L.In networks where U = 12 kV, the protective distance of thearrester are about 10% longer than represented in Figure 14. Onthe other hand, when U =36 kV, the distance is about 30%shorter. At this network operating level, it should also be noted thatwhen S = 1550 kV / µs (wooden pole lines), the value of L sharplydecreases as soon as > 0.6The protective properties of the arresters are somewhat reducedwith different polarity of the lightning overvoltage and themomentary value of the phase voltage, this is taken into account inthe calculation of L. Additionally it is assumed a very short galva-nic connection between the earth side of the arrrester to thetransformer tank. This is to be taken into consideration whenconnecting the arrester.
b
b
m
m
Otherwise it becomes necessary to increase the length of theconductor length in Figure 13 due to the additional earthconnection. Branching between the arrester and electrical equip-ment to other electrical equipment creates additional voltageoscillations which in most cases results in a reduction of L.
b
1550 kV/µs for wooden pole lines800 kV/µs for earthed cross arm lines
1.0
1.1
1.2
UTOV
Uc
1.3
1.35
1.25
1.15
1.05
T
1.4
101 100 1000 10'000 t (s)
a
b
10
6.3 Temporary Overvoltages
The meaning of Temporary Over-Voltages U is the operatingfrequency overvoltages of a limited duration. The spark-gaparresters require special measures regarding these voltages. Inthese arresters the follow current increases very strongly with theoperating voltage. If this voltage lies above the rated voltage of thearrester, the follow current through the arrester will be too high.Under these conditions, the spark-gaps can no longer extinguishthe arc, that is they ignite it again in each of the following half cyclesuntil the arrester is destroyed by overheating. In networks withtemporary overvoltages the rated voltage of the spark-gap arrestermust be raised to U , which also requires the raising of theprotection level of the arrester.
TOV
TOV
Figure 8Strength T=U / U with respect to temporary overvoltages Uas a function of their duration t at an ambient temperature (airtemperature outside the arrester) of 45 C. The curve a applies toan arrester without preload, the curve b to an arrester, preloadedwith the guaranteed energy E. t is the time duration of theovervoltage with power frequency.The curves apply for the MO-surge arrester type MWK.
In MO-arresters there is no follow current because this isprevented by the extremely non-linear voltage current charac-teristic (Figure 4). It is for this reason that MO arresters are capableof bearing increased operational voltages over a longer period oftime. The strength T of the arrester in the presence of such tempor-ary overvoltages is described in Figure 8.U = T x U
T is then a measure for the permissible height of U .The curve in Figure 8 is valid for arresters without a significantenergy preloading. The higher T and respectively U , the greaterthe power generated in the arrester. Because the MO temperaturecannot exceed a certain value for reasons of stability, is the energysupplied to the arrester also limited. For that reason thepermissible load duration t decreases with the magnitude of Tresp. U . The curve is valid for arresters which at the time t = 0are already preloaded with the specified energy E. Naturally, curveb lies beneath curve a. Arresters which are already preloaded withthe E / U values specified in Table 2 can nevertheless withstandtemporary overvoltages correstonding to curve . This impliesthat the entire energy absorption capability of the arrester exceedsthese guaranteed data. In the time interval t the energy can besupplied to the arrester at any given moment in the form of energyimpulses. The sum of the impulses however must not exceed theamount E.
TOV c TOV
TOV c
TOV
TOV
TOV
c
°
a
b
b
The following example should illustrate the use of Figure 8:An arrester MWK 24 with U = 24 kV could be operated for as longa time as one wishes with U . The environmental temperaturesurrounding the arrester amounts to a maximum of 45 C. At thetime t = 0 the arrester is charged with the specified energy E = 5.5kJ/kV U .Immediately following the temporary overvoltage U = 28 kVoccurs. Thus:T= U / U = 28 kV / 24kV = 1.17
For T= 1.17 it follows that from curve the time t = 400 s.Therefore the duration of U is limited to 400 seconds. Followingthis the arrester is again capable of bearing U and cools down. If itis expected that U has to occur for longer than 400 seconds onthe line, then an arrester with the corresponding elevated U mustbe selected.
c
c
c
TOV
TOV c
TOV
c
TOV
c
°
b
7 Tests
The tests for ABB arresters follow internationally agreed uponrecommendations. IEC 60099-4 has been valid for the MO-arresters since August 1998 [6]. In the USA - Norm ANSI C62.11-1993 is applied [7], which coincides with the IEC. The MV-arresters from ABB fulfill both norms.The tests are made inaccordance to type, routine, and acceptance tests.Furthermore the arresters are submitted to special tests, which arenot mentioned in the international regulations.
At the completion of the development of an arrester design, typetests are carried out. They furnish proof that they comply with therelevant standard. The following tests are designated for MV-arresters:
these testsdemonstrate that the external housing insulation meets theexpected voltage demands.
The function of these tests is to certify thatthe protection level of the arrester does not exceed the guaranteeddata.
this test isperformed to prove that the MO resistors withstand possibledielectric and energy demands without puncture, flashover andcracking.
in this test resistors are subjectedto a temperature of 115 C for 1000 hours with a voltage above U .In doing so it is watched if and how intensive the power losses ofthe resistors increase over their life span. The life span isunderstood to be 110 years according to [7]. In this time ABBresistors demonstrate no increase of power losses: therefore theyare not subject to any ageing process.
the following values are of significance inthis test:
This is the peak value of the ohmic current component by which thereference voltage is measured. I should be large enough so thatthis measurement cannot be influenced by the stray capacitance ofthe arrester components. The reference current must be specifiedby the manufacturer. For ABB MV-arresters the following valuesare valid for I :
1.4 mA for POLIM-DN1.4 mA for POLIM-D, MVK1.6 mA for POLIM-DA2.2 mA for MWK, MWD, POLIM-I, POLIM-C3.6 mA for POLIM-S5.0 mA for POLIM-H
7.1 Type Tests
Isolation withstand tests on the arrester housing:
Residual voltage tests:
Long duration current impulse withstand test:
Time accelerated ageing test:
Operating duty tests:
• Reference current I
° c
ref
ref
ref
9
6 Protection Characteristics of the SurgeArrester, Stability
6.1 Surge Arrester Protection Level
The protection level U is the maximum voltage at the arresterterminals during the flow of the nominal discharge current which,according to definition, shows a current form of 8/20 µs. The peakvalue of the current is reached after approx. 8 µs and after approx.20 µs it has decayed to 50 % of the peak value. In the case of spark-gap arresters U is additionally given by the standard lightningimpulse sparkover voltage. This is the lowest prospective peakvalue of a standard lightning voltage impulse (1.2/50 µs) which,when applied to the arrester, causes sparkovers on everyapplication. Virtually the same protection level is possible throughMO and spark-gap arresters having identical continuous servicevoltage U . It lies at about U = 3.33 U or under this value. Moreprecise values are available in the corresponding booklets.
The protection characteristics of an arrester consists not only ofthe value U , but of two additional features. The first is thebehaviour of the arrester during steep wave fronts, which isespecially important for MV equipment. The test for MO-arresterstakes place with the nominal discharge current, the front time ofwhich is reduced from 8 µs to 1 µs. The residual voltage over thearrester reaches a maximum of 1.13 U at this steep current wave.Because of the non linearity of the current-voltage-characteristicof the MO-arrester, the front time of this residual voltage lies in theorder of magnitude of 50 ns.
In comparison with it the front-of-wave sparkover voltage is oftenreferred to for spark-gap arresters. It lies at approx. 1.15 U . In thistest the length of the rise time is adjusted to approx. 400 ns.
p
p
c p c
p
p
p
A true comparison with a MO-arrester necessitates a rise timewhich also lies in the range of 50 ns. With such a steep front thesparkover voltage reaches a value of at least 1.4 U . It follows thatby a steep rise the limiting voltage of the spark-gap arrester is atleast 24 % higher than that of the MO-arrester.
The behaviour of the arrester during switching overvoltages is afurther feature of the protection characteristics. In spark-gaparresters the sparkover voltage reaches approx. the value of Uwith these relatively slowly rising overvoltages. MO-arresters haveno sparkover voltage. With MV-arresters the switching protectionlevel is given through the residual voltage at 500 A of the currentwave 30/60 s. The residual voltage reaches 0.77.... 0.83 Udepending on the arrester type. The limiting voltage duringswitching overvoltages of spark-gap arresters is at least 20 %higher than that of MO-arresters.At the same continuous operating voltage the MO-arresterstherefore demonstrate a more favorable protection characteristicthan spark-gap arresters. The above mentioned figures are validfor arresters employed in networks with high-ohmic insulatedneutral. Regarding the operational safety, MO-arresters have anadditional advantage in the fact that they can also resist temporaryovervoltages as shown in Figure 8.MO and spark-gap arresters must be dimensioned differently innetworks with solidly earthed neutral systems [8]. The result isthat U can be chosen 28 % lower than the rated voltage of thespark-gap arrester. Thereby a protection characteristic results forthe MO technology which, depending on the wave form, lies 28 %to 42 % lower.
p
p
p
c
6.2 Questions of Stability of MO Surge Arresters
In Figure 7, P is the power loss of the MO-resistors in an arresterwhen U is applied. It is evident how P exponentially increases withthe MO temperature, which also results in an increased warming ofthe active parts. The cooling of the resistors results from the flowof heat Q from the active part to the exterior. At temperatures abovethe critical point is P > Q. Here the cooling is not sufficient todissipate the power loss. The resistors would continue to heat upand the arrester would be destroyed by overheating. Throughsuitably dimensioning of the resistors and through designmeasures that enable the cool down of the blocks, it is possible toraise the critical point to such a level, that even if during theoperation the highest energies are likely to occur, this critical pointis impossible to be reached.On the other hand, the mechanism described clearly shows thelimits of the energy absorption capacity of the MO-arrester. Theamount of energy fed to it must never exceed the critical point.Then P < Q and the MO discs cool down until the stable operatingpoint is again reached.
c
Figure 7Power loss P of the MO discs and heat flow Q from the activearrester parts to the exterior as a function of the MOtemperature T at the continuous operating voltage Uc
P,Qthermal runaway
critical
stableoperating point
point
T
Q P
5.5 Altitude Adjustement for Arrester Housing
Normal MV-arresters from ABB can be used at altitudes of up to1800 m above sea level.At higher altitudes the air density is so low that the withstandvoltage of the arrester housing may be no longer sufficient againstexternal flashovers. In this case the unaltered active parts of thearrester (same protection level) must be placed in an elongatedhousing with a larger flashover distance.
As an orientation value one may consider that for every 1000 mover 1800 m above sea level the flashover distance of the housingmust be enlarged by 12 %. For example, at an altitude of 3300 mabove sea level the flashover distance of the housing must be 18 %larger than of a normal arrester.
Figure 6Repelling water on silicone surface (hydrophobicity-effect)
The shorter the sum of the connecting lines + compared with Lin Figure 14, the lower the failure rate. In other words, + must beas small as possible , and L must be as large as possible. The latteris achieved by choosing the proper line direction. As can be seen inFigure 15, the line must be first connected with the arrester andthen connected with the transformer. In this case b = 0 and Lbecomes maximal. The connecting line L can be held short byplacing the arrester as close as possible to the transformer. Bothmeasures together make it possible to fulfill the requirements of
+ << L and therefore keep the failure rate considerably below10%.
If the transformer is connected to a wooden pole line, and if
b < 1 m when U 24 kV
b < 0.6 m when U > 24 kV
cannot be maintained, then the line is to be modified so thatregarding the overvoltage at the substation and the protectivedistance, it behaves as favourably as a line with earthed crossarms.
The necessary measures for this are relatively simple: the crossarms of the last three poles before the station are to be earthed. Theovervoltage which runs into the station from the modified linesnow have the same form as if it came from a line with continuousearthed crossarms. The disadvantage of this solution is thatadditional lightning overvoltages cause flashovers between theconductor and the earth owing to the reduced insulation level of theline. A more efficient method than the earthing the cross armswould be to install another set of arresters one pole in front of thesubstation. The effect is a very strong reduction in the amplitude ofthe incoming overvoltage. This in turn leads to a protectivebehaviour of the arrester at the equipment which is better than thatof earthed cross arms.
On one hand the protective distance of an arrester is, in somecases, not especially long. This applies mostly to electrical equip-ment which is subject to capacitance in substations with a highnetwork voltage and which are connected to wooden poles (seeFigure 14), on the other hand pieces of electrical equipment in asubstation are seldom placed close together. Usually they are sofar apart from each other that one arrester could not protect severalpieces of equipment at the same time. Under this conditions, eachpiece of electrical equipment requires a separate arrester set (onearrester each per phase to earth).
The essential difference between the electrical data of overheadlines and cables is the surge impedance of their conductors toearth. Values for overhead distribution lines are approximately300 to 450 and for cables in the 20 to 60 range. First of all,this difference causes a marked decrease of the lightning over-voltage as soon as the travelling wave reaches the cable entrance.The reduced voltage wave flows through the cable and it isreflected at the end so that the voltage is nearly doubled.Subsequently the wave returns to the cable entrance and is oncemore reflected, etc. In this way, the overvoltage in the cable is builtup gradually although the overvoltage slope in the cable is actuallylower, the peak value is near that of lightning overvoltage on theline [18].
a b
a b
a b
m
m
<
10 Some Special Cases
10.1 Overvoltage Protection in Cable Sections
Ω Ω Ω Ω
The flashover of a bus bar or a conductor of a line toward the earthcauses a short operation shutdown at the most. Subsequentdamage is, however extremly rare. In cables, flashover behaviouris completely different. Flashovers in cable insulation can causedisturbances and require extensive repairs. Flashovers along thecable heads can damage these and exibit the same damage as withinsulation flashovers. Cables must therefore be treated as stationequipment and protected against lightning overvoltage witharresters.The arresters are to be placed directly next to the cable heads. Thejunction lines should be as short as possible. It must be noted thatthe earth connection of the arrester is attached to the cable sheath.
Longer cables require arrester protection at both ends. For shortcables sections onesided protection is, in some cases, sufficient.This is because an arrester at only one end can still offer sufficientlightning overvoltage protection to the other end.A cable which connects the overhead line with the substation isoften only endagered by lightning on the line. The arrester musttherefore be mounted to the line at the cable junction. No secondarrester is necessary at the other end of the cable, as long as thecable length L does not exceed the values which are given in theTable 7. At first glance, it should be noted that L is unlimited in 3.6kV networks. This is because of the relatively high BIL of 13.6 p.u.at this network level. The arrester at the line side of the cable limitsthe overvoltage to approximately 4 p.u. As a result of voltagereflections in the cable, the overvoltage at the other end of the cableis higher, but under 10 p.u. At this level, the overvoltage isharmless to the cable. This, however, does not apply toequipments in the substation. With these equipments additionalvoltage reflections can increase the overvoltage, so that for theirprotection, in case of necessity, arresters must be provided. Themaximum allowable length for a cable section with onesidedprotection is higher for MO-arresters than for those with spark-gaps. This is based on the favourable protection properties of MO-arresters, which begin conducting before U is reached.
K
K
p
18
Arrester withU = 3.8 p.u. for MOU = 4 p.u. for SiCand I = 10 kA
p
p
n
Type of Line Wooden pole Wooden poleearthedcrossarms
earthedcrossarms
Arrester Type MO MO MO MOSiC SiC SiC SiC
U[kV]
m Z[ ]
KΩ
L[m]
K L[m]
K L[m]
K L[m]
KL[m]
K L[m]
K L[m]
K L[m]
K
3.6
7.2
12
17.5
24
36
30
30
30
30
30
30
60
60
60
60
60
60
8
64
40
25
28
22
8
45
30
21
23
20
8
64
40
26
28
22
8
50
32
22
24
20
7
9
9
6
10
8
3
4
4
3
5
4
17
22
19
15
17
15
10
13
14
13
15
14
8
30
15
6
6
1
8
20
11
4
5
1
8
28
14
5
5
1
8
19
10
4
4
1
6
9
7
4
5
1
3
4
3
2
3
1
17
1411
7
3
31
10
9
4
4
1
ULK
ULK
Table 7Maximum allowable length L of cable sections with one-sidedarrester protection. The cable is connected to a lightningendangered line.Lightning overvoltage and momentary value of system voltagehaving different polarities.
Junction length arrester to cable 1 mZ :MO:SiC:
K
K Surge impedance of the cableMetal oxide arresterArrester with spark-gap
19
Naturally, cables in overhead lines are lightning endangered onboth sides. Therefore it must be taken into account that in cableswith one-sided protection, overvoltage can also come from theunprotected side. In this case, the protection effectiveness of thearrester at the other end would be strongly reduced. The allowablelength of cables in overhead lines with one-sided protection istherefore smaller. The length is especially short for cables inconnection with wooden pole lines, as shown in Table 7. The givenvalues for L are valid for arresters with I = 10 kA. The surgeimpedance across the entire cable section must be constant so thatthe voltage reflections do not cause a decrease in L . This is thecase, for example, with cable junctions or when a cable sectionwith a single cable is connected to a section with two parallelcables.
Due to thermal principles, the sheath for single conductor cablesare generally only earthed on one side. Under these circumstancesthe sheath on the unearthed side can take on up to 50 % of thevoltage peak value of the overvoltage entering on the innerconductor. The sheath insulation cannnot withstand thisovervoltage demand. Momentary flashovers can occur betweenthe sheath and the earth, consequently damaging the outer sheathinsulation.
Therefore, the unearthed cable sheath must be protected againstlightning overvoltage with an arrester [2]. The special arresterPOLIM-C is particularly well-suited for this purposes. The voltageinduced along the cable sheath during a short circuit is decisive forU of the arrester. According to [19] it reaches maximum 0.3 kV perkA of fault current and km of cable length. When T = 1.28 and thefault current duration is t < 3 s, the following results:
U 0.24 x I x L in kV
I : max. 50 Hz fault current in kA
L : length of the unearthed cable section in km
Figure 17
K n
K
c
c K K
K
K
10.2 Cable Sheath Protection
>
10.3 Transformers at the End of Cables
According to the direction in Figure 17, a cable of at least 100 m inlength is connected on one end to a lightning endangered line. Atthe other end, a bus bar consisting of sections and connects thecable end on the other side with a transformer. Arrester A1 takesover the overvoltage protection on the line side. The cable end andthe transformer must each be protected with an additional arresterwhen the connecting distance between the two is especially long.The following example indicates under what circumstancesarrester A2 offers sufficient overvoltage protection, in addition toarrester A1.
The overvoltage reflection U at the junction from the line to thecable causes a strong flattening of the voltage rate of rise in thecable. However, this has practically no influence on the allowablelength of the connection , because with increasing length of thevoltage U increases very quickly. Optimal overvoltage protection,therefore, requires that arrester A2 be placed as close as possibleto the cable end, in
Table 8
Maximum permissible distance a between cable end andtransformer according Figure 17 with b=O. The cable is connectedto a lightning endangered line and protected at both ends withMO-surge arresters (type MWK or MWD with U = U )The transformer has no additional arrester protection.Z : Surge impedance of the cable.
The line section is different. Here U increases more slowly withthe increasing length of . Therefore, the transformer is adequatelyprotected, even at a relatively far distance from the arrester. Themaximum allowable values for are indicated in Table 8. Thecapacity of the transformer is assumed to be 2 nF. Smaller valuesresult in greater distances of .
a b
b b
a
a
a
a
K
c m
K
T
order to shorten the distance (see section 10.1).b
MO-surge arrester withU = 3.8 p.u. beiI = 10 kA
p
n
wooden poles earthed crossarms
U [kV]m a [m] a [m]a [m] a [m]
3.6
7.2
12
17.5
24
36
300
43
20
17
19
16
30 3060 60
300
37
14
10
12
11
500
53
20
16
19
20
500
53
14
10
12
11
Z [ ]K Ω
Transformer at the end of a cable
F:U:K:A1, A2:a, b:U :U :MV::LV:
K
T
lightning endangered linelightning overvoltagelong cablearresterslength of the connecting linesmaximum voltage at the cable endmaximum voltage at the transformermedium voltage sidelow voltage side
F
A1 A2
KUK UT MV LVU
ab
The energy absorption capability of these types is much higherthan the expected stresses in the network, exepting the very highligtning currents. These currents can also be commanded by thearresters, it is however most unlikely that they appear. Such highlightning currents can flow through the arrester only when alightning hits directly the top of the arrester. The energy values aregiven in Table 2 and 3.By aerial lines with wooden poles even far away lightning strokescan cause relatively high currents that flow through the arrester. Ifthe sparkover voltage of the wire against the earth is U = 3000 kVand the characteristic wave impedance of the wire is Z = 450from the equation (3) ensue that lightning currents up to13 kA areto be expected in the arrester. This current produce in arresterswith I = 5 kA a residual stress which lies 15% over U . In this waythe protection of the electrical equipment gets worse. For instanceif it lies at the end of an aerial line of 10 km it will be once in threeyears exposed to such an increased voltage stress. That is whyABB has also in the assortment of the MV-arresters the typesMWK, MWD, POLIM-I, POLIM-S and POLIM-H. They possesnominal discharge currents of 10 kA respectively 20 kA. Theiremployment is recommended everywhere where in terms ofstress, operation safety and protection level the highestrequirements are set.
This is the case in
regions with high lightning activity
erial lines with concrete or wooden poles and non-earthed
crossarms
arresters, which are placed on locations where people are often
to be found
on lines, which set exeptional high requirements
concerning the operation safety
protection of engines, generators and cables
areas with high industrial pollution, or when the arresters arenot farther than 1000 m from the sea.
In cases where the 10 kA arresters are recommended is also ahigher energy absorption capability advantageous and an arrestertype with a line discharge class 2 or more should be chosen. That iswhy these arresters have a higher energy capability of at least 5.5kJ/kV (MWK, POLIM-I, POLIM-S)
The peculiarity of some electrical equipment, as for instance
arc furnace
big capacitor batteries
very long cable sections
expensive rotating machines
can make a higher energy absorption capability necessary. In suchcases the special type POLIM-H with I = 20 kA and with 13.3kJ/kV is preferred.
n p
Uc
n
Uc
••
•
•
••
••••
5 Special Operating Conditions
5.1 Network Short Circuit Power
Any arrester can be overloaded. The causes are extremely highstroke currents, a large number of multiple strokes [16, 17] or theso-called system flashover. This is understood to be a short circuitbetween two different voltage levels. The voltage at the arrester onthe lower voltage level rises above the allowable limit. An overloadof any kind causes a flashover or puncture in the resistor. An arcresults in the arrester and the current from this arc is defined by theshort circuit power of the network.
In porcelain housed arresters the ensuing electric arc causes thegas pressure in the housing to elevate quickly. If the network shortcircuit current is not too high, the pressure relief valve in thearrester opens before the housing bursting pressure is reached.On the other hand, if the current is extremely high, the possibility ofthe housing exploding cannot be ruled out.With ABB silicon-polymer arresters there is no danger of burstingin the case of an overload. There is no air space between the activepart of the arrester and its silicon insulation, thus there is no placefor pressure to build up. In the case of an overload, holes appear inthe casing which immediately leads to initiation of the externalflashover.The MV-arresters of the types POLIM-D, MWK and MWD areproved with short circuit currents up to 20 kA. The types POLIM-I,POLIM-S and POLIM-H are tested with short circuit currents up to65 kA. Because of their special construction the arresters are alsoup to the highest short circuit currents insured against explosionand destruction.
The guaranteed values for U are valid for an ambient temperatureof up to 45 C. In the case of outdoor arresters, extreme solarradiation (1.1 kW/m ) is taken into account. If there are other heatsources in the vicinity of the arrester, the increase in radiationexposure must also be taken into account and the value of Uincreased if necessary. If the ambient temperature exceeds 45 C,U must be increased by 2 % for every 5 C of temperature eleva-tion.
MV-arresters produced by ABB are operationally reliable even inareas of high earthquake activity. Silicon arresters from ABB canalso have a support function. In the case of cantilever strength, itmust be distinguished between temporary and operational loadsaccording to DIN 48113. The permissible loads result from theproduct of arrester altitude and maximum permissible momentumload. In Table 4 there are the mechanical data of different arrestertypes to be read.
5.2 Elevated Ambient Temperatures
5.3 Mechanical Stability
c
2
c
c
8
POLIM-DN
POLIM-D
POLIM-DA
MWK, MWD
POLIM-I
POLIM-S
POLIM-H
250
250
350
350
2500
4000
6000
50
50
50
68
100
100
100
625
625
1000
1200
2000
3000
4000
Arrester type Canti lever strengthNm
TorsionNm
VerticalLoad
N
Table 4Mechanical data of MV-surge arresters, produced by ABB
5.4 Air Pollution
Silicon is the best insulating material against pollution. This ismainly because the material is water-repellent. Silicon arrestersbehave more favourably under conditions of heavy air pollutionthan porcelain housed arresters or other polymer insulationmaterials. In addition the self-cleaning feature of silicon itself isoutstanding. Pollutants and dirt cannot adhere well to the flexiblecovering and are washed away by rain.
The short-time tests acc. to IEC 507 provide the basis for thedimensions of the insulator. In certain cases, the insulator be-haviour may deviate under actual field conditions as a result ofother parameters (eg, due to the shape of the sheds). However, it isgenerally true for silicone as well as for the ceramic insulators thatextreme pollution calls for a longer creepage path.The mentioned temporary reduction in hydrophobicity was takeninto account in the design of the POLIM arresters, and the specificcreepage path was not reduced. All of the discussed surgearresters with silicone insulation have been designed with aspecific creepage path of at least 25 mm per kV, providing a morethan adequate safety margin. Whenever possible, all the pollutionand lifetime tests were carried out on complete MO arresters. Thetests were performed to the various standards (eg, the 1,000-hourhumidity room test to IEC 1109, the 5,000-hour aging cycle testand the salt-fog test to IEC 507) and showed that the siliconeinsulation performs better after ten years in service that the othertypes of insulation.
Figure 5Change of hydrophobicity of EPDM (black) and silicone (white)in the accelerated ageing test acc. to IEC 1109.
Figure 5aComparison of the specific creapage distance of porcelain (black)and silicone insulators (white), depending on the salt content in thesalt fog test acc. to IEC 507
7
5
4
3
2
1
00
1000 2000 3000 4000 5000h
HC
Hyd
rophobic
ity
test timetv
6
70%=
100%=
^
^
00
1
2
3
4
5
8
cm/kVrms
kg/m33 4 5 7 10 15 20 30 40 80
Cre
apag
e
salt content of water
2.5
4.3 Energy absorption capability and cool-down time
4.4 Nominal Discharge Current and Energy AbsorptionCapability
The arresters in the network can work reliable if their energyabsorption capability is bigger than the energy strain expected inthe system operation. Some examples of the stress on thearresters in the network are shown in the Table 3. The arresters ofthe line discharge class 1 have the highest energy stress with thehigh current (65 kA respectively 100 kA). To prove the thermalstability under this stress, a special type test has to be performed.
The guaranteed energy absorption capability of the types of theline discharge class 2 and higher can be proved by the means ofrectangular current stresses, similar to the examination of the highvoltage arresters.The guarantee data contain a certain amount of energy reserve andtherefore do not mean the limit of the thermal stability of thearrester.
Anyway the arresters will be very strongly warmed up when theyhave to carry very high lightning currents. Therefore they needbetween two such stresses a suitable cool-down time. Thisreduction is however not important because it is most unlikely thatthe same arrester has to carry a second very high ligtning currentduring its cool-down time. That is the reason why the test sampleis allowed to be cooled-down to 60 C during the type test withhigh current [6] between the two surges.
The needed cool-down time of the arrester depends among otherthings on the ambient temperature and the height of the operatingvoltage. It increases with the ambient temperature and the opera-ting voltage. In the most unfavourable case, with 45 C airtemperature and U the following values are valid:Cool-down time between two high ligtning current stresses (65 kArespectively 100 kA):Type POLIM-S and POLIM-H no break necessaryThe other arrester types 75 minutes
Cool-down time between two energy stresses acc. the Table 2:Type POLIM-S and POLIM-H 60 minutesThe other arrester types 60 minutes
The arresters with a nominal discharge current of 5 kA proved to beenough reliable in the MV- network. That is why generally the typePOLIM-D respectively the type POLIM-DN are used.
°
°c
Table 3Energy load of arresters in MV-networks
Arrester type
POLIM-DN
POLIM-D
POLIM-DA
MWK, MWDPOLIM-I
POLIM-S
POLIM-H
200 km line
kJ/kV Uc
10 km cable
kJ/kV Uc kJ/kV Uc kJ/kV UckA kA
I8/20 s
n
High current4/10 s
0.4
0.4
0.4
0.4
0.4
0.4
0.33
0.33
0.33
0.33
0.33
0.33
5
10
10
10
10
20
0.48
0.47
1.0
3.4
3.3
3.2
100
100
100
65
100
100
0.55
0.26
0.53
3.6
3.5
2.6
3.5 p.u. Charging voltage current wave form
Conditions are different when arresters must contain switchingovervoltages rather than lightning overvoltages. The former couldoccur during switching, when an inductive current is interruptedbefore it reaches its natural zero crossing. When such switchingovervoltages occur, the current load on the arrester is very low, sothat a nominal discharge current of 5 kA is sufficient. On this casethe maximum overvoltage is considerably lower than U for MO-arresters. Therefore, shorter distances between arresters andbetween the arrester and earth are possible, facilitating theinstallation of arresters in the cells. The lower values for thesedistances are prescribed in the respective national regulations andare sufficient for metal oxide arresters.
The maximum voltage for arresters with spark-gaps reaches Ualso during switching overvoltages. The minimum distances forthese arresters must therefore be longer in order to preventflashovers. This can make arrester installation in the cellssignificantly more difficult, particularly when there are especiallytight spacing conditions.
Overvoltage protection is the result of arresters placed betweenphase and earth. If a loaded generator is suddenly disconnectedfrom the network, its terminal voltage increases until the voltageregulator has readjusted itself after a few seconds. The relationshipbetween this temporary overvoltage and the normal operatingvoltage is called the load rejection factor . This factor can reach avalue of up to 1.5. In the worst case, the arrresters could becharged with a temporary overvoltage of U = x U , wichmust be taken into account when choosing U .
x U
U ----------------T
The duration t of U determines T and lies in a range from 3 to 10seconds. In the following example, U of type MWK arresters isdetermined:
U = 14 kV = 1.4
t = 10 s T = 1.26 (from Figure 8)
1.4 x 14 kVU ------------------- = 15.56 kV
1.26
The arrester type needed is called MWK 16. Its U is 16 kV and theprotection level at I = 10 kA is 49.1 kV.
The high operational safety requirements for generators make theuse of arresters with the lowest possible protection level desirable.Therefore the special POLIM-H series arrester is recommended forgenerator protection. Not only is the protection level lower, but alsoat the same time is T larger.
For t =10 s, T = 1.31 is permitted, which results in:
1.4 x 14 kVU ------------------ = 14.96 kV
1.31
p
p
L
TOV L m
c
L m
c
TOV
c
m L
c
c
n
c
10.6 Generator Connected to a Lightning EndangeredDistribution Line
>
>
>
10.4 Transformer Connected to a Lightning EndangeredLine on One Side Only
10.5 Arresters in Metal Enclosed MV Substations
In general, all transformer exits which are directly linked tolightning endangered lines must be equipped with arrestersbetween phase and earth. If a transformer connects a high voltagenetwork with a MV network, and only the line on the high voltageside is lightning endangered, arresters are required there. Becauseovervoltages occur very quickly, up to 40% of the overvoltage onthe high voltage side is capacitively carried over to the MV sidethrough the transformer (10). It is therefore often necessary toforesee overvoltage protection for the transformer on the MV side,even though no lightning overvoltages occur on the MV side,according to the above assumptions. According to (9), thisovervoltage protection can be a long MV cable, a low-inductancecapacitor, or a combination of these two elements. They must beconnected as close as possible to the MV exit of the transformerand together possess a capacity of at least 50 nF per phase.
The overvoltage protection can also come from a MV arrester. Thissolution has two essential advantages. First, it must be noted thatinductively transferred overvoltage can be incraesed bycapacitors. Carefully chosen damping resistors in series to thecapacitors make possible a partial decrease of this additionalvoltage load on the transformers. When a MO-arrester without aspark-gap is used, this additional load does not occur.
Secondly, primary voltage encroaches on the MV-voltage during avoltage flashover in the transformer and can cause additionaldamage there. When the MV side is protected with arresters, thearrester sacrifices itself, causing a short to earth, and leaving thedamage essentially restricted to the transformer. The advantage ofan arrester versus a capacitor is particularly evident when thetransformer is connected to a generator, and the arrester protectsthe generator from subsequent damage.
Similar conditions exist with a distribution transformer, whichconnects a MV network to a low-voltage network. Again, lightningovervoltage from the MV network is transferred through thetransformer capacitively to the low voltage side. Therefore,arresters in the low voltage side are necessary, even when only theMV side is lightning endangered. If, on the other hand, only the lowvoltage side is endangered, frequently no arresters are mounted onthe MV side. In this case, it is assumed that the low voltagearrester can also adequately protect the MV side from overvoltagecoming from the low voltage side. Several cases of transformerfailure caused by lightning overvoltage on the 415 volt side arereported in [20]. The author's opinion is that these overvoltages,when they are of long durations, are transferred inductively withthe turn ratio of the transformer to the 11 kV side. In any case, the415 volt arresters were unable to prevent flashovers in the 11 kVwindings. In regions with high lightning activity, it is thereforerecommended to install arresters on the MV side of thetransformer.
It is often necessary to place arresters in a metal enclosed MVsubstation. If a cable connects the cell with a lightning endangeredline, the nominal discharge current of the arrester at the cable headin the cell should be 10 kA. The voltage on the arrester can beexpected to reach U . In order to prevent flashovers in the cell, theminimum distances between the arresters and the arresters andearth recommended by the supplier must be observed.
p
20
21
During this process the capacitor is charged with a higher voltage[21]. The overvoltage of the capacitor between phase and earth[15] reaches a max. of 3 p.u. If the capacitors are connected in astar, they are discharged between phase and earth by the arresterparallel to the bank. During the discharge up to a voltage of 2 xU , in terms of power, the arrrester is loaded with:
SE = ----------- x [3 - (U / U ) ]
S : 3-phase reactive power of the capacitor bankE : the discharge energy taken up by the arrester
Assuming that the arrester must carry out this process 3 timeswith no cool down phase, it follows with U U that
E 6 x S---- -------------U x U
The power consumption capacity E of the arrester must beadjusted to the reactive power of the bank. Table 9 shows themaximum allowable reactive power values for different types ofABB MV-arresters parallel to the bank.
If the neutral of the capacitor bank is insulated, the arrester cannotdischarge the charged capacitor between phase and earth. Thearrester does not get charged. It must be noted that after a re-strikeof the breaker, the neutral of the bank increases to 2 p.u. A voltageflashover of the neutral to earth results in the arrester having todischarge the capacitor. The arresters parallel to a bank with aninsulated neutral must, in terms of power, be adjusted to theirreactive power.
If the bank remains disconnected from the network after a shut-down, the arresters discharge the voltage to zero, not merely to
2 x U . Below 2 x U the discharge current through the arresteris very small, so that the remaining discharge takes a long time.During this time the arrester can cool down. It releases more heatthan it receives during the remaining discharge. Therefore it wasjustified in the above calculation of E to take into account only theenergy taken up by the arrester, up to the discharge at 2 x U .
√
√ √
√
c
K
c c m2
K
c
c m
c K
c m
c c
c
c
>
>
The arrester type POLIM-H 15 is sufficient. Its protection level at I =10 kA is 43.5 kV. This special arrester guarantees a 11% lowerprotection level. In addition this arrester has also advantages withregard to operational safety thanks to its substantially higherenergy absorption capability (see Table 2).
Generators have a larger capacity between phase and earth. Asseen in Figure 14, this capacity results in a considerably shorterarrester protective distance. Therefore it is especially important toplace arresters near the generator.
High voltage motors can become over-stressed by multiple re-starts being switched off during the run-up. This is applicablewhen the cut-off current is less than 600 A. In order to protectthese motors it is recommended that surge arresters be placeddirectly at the motor terminals or, alternatively, at the circuitbreaker. The layout of U according to the recommendations canbe seen in section 8.
In special cases, i.e. aged winding insulation, it becomesnecessary to additionally decrease the protection level of thearrester. One way of doing this is to decrease U . This procedure isjustifiable when temporary overvoltages taken into account for Uoccur very infrequently. The fact that in such a rare case thearrester could become overloaded has been deliberately taken intoaccount. Resulting drawbacks, such as operation interruptionsand arrester replacement are outweighted by the advantage ofbetter overvoltage protection. However U smaller than U / 3may never be selected.
However such a decrease of U is not recommended in a generator.The risk exists that this would cause a two-phase short circuit atthe generator terminals. The resulting asymmetrical short circuitcurrent in the windings produces an extremely high mechanicalload on the rotor.
In the case of locomotives, the highest standards with respect tooperational safety are placed on the arresters. Therefore, thearresters of the POLIM-H series are recommended. The strongmechanical construction meets all the requirements for railwayoperation. The completely moulding in silicon guarantees thehighest mechanical safety even during extreme shock loads. Whenan arrester is overloaded the special construction of this arresterprevents the housing from bursting. This arrester type is proved upto a fault current in the network of 65 kA and can be consideredsure from the point of view of explosion and disintegration.Additional advantages of this arrester type are its low protectionlevel and its high energy absorption capability.
Normally no overvoltage occurs when a capacitor bank isswitched. The circuit breaker interrupts the current in the naturalcurrent zero crossing and the voltage in the capacitor to earthreaches a max. of 1.5 p.u. As a result of the network voltage vary-ing at the power frequency, a voltage across the open circuitbreaker of 2.5 p.u. is caused. If the breaker re-strikes, a highfrequency transient effect takes place between the capacitorvoltage and the operating voltage.
10.7 Arrester Protection for Motors
10.8 Overvoltage Protection in Locomotives
10.9 Arresters Parallel to a Capacitor Bank
c
c
c
c m
c
√
3.6 9.05.5 13.3
U [kV]m S [MVA]K S [MVA]K S [MVA]K S [MVA]K
3.6
7.2
12
17.5
24
36
0.67
1.35
2.26
3.29
4.52
6.78
1.03
2.07
3.45
5.03
6.90
10.36
1.69
3.39
5.65
8.24
11.30
16.95
2.50
5.01
8.35
12.18
16.70
25.05
E/U [kJ/kV]c
Table 9Arrester parellel to capacitor bank. Maximum allowable reactivepower S of the bank for the indicated arrester types. Threedischarges without a cool down phase are allowed for thearresters.E/U : The arrester energy absorbtion capability in relation to U .
K
c c
Arrester typeU Uc m>
POLIM-D POLIM-SMWKMWD
POLIM-I
POLIM-H
ω
ω
In the last 15 years most of the modern MO-arresters were set innew installations in high-voltage networks [4]. Up until a few yearsago conventional arresters,consisting of SiC resistors and seriesspark-gaps were still installed in distribution systems. Now a days,MO-arresters without spark-gaps are in use, especially those withpolymer housing. The reasons for this technological change areincreasing operation efficiency, lower protection level with verysteep overvoltages and high pollution resistance [5].
Due to the simple construction of the active part without spark-gaps, which means a very high stability from the mechanical pointof view, and also due to the simple construction generallyspeaking, such arresters can also undertake the support insulatorfunction and are not in danger of exploding in case of an overload.Silicone is the best insulating material concerning dirt, and that iswhy the arresters which are insulated with silicone behavefavourable especially with high pollution.
Silicone rubber (usually referred to simply as 'silicone') is anexcellent insulating material for high-voltage components.Comparisons with traditional insulating materials, such asceramic, glass and other synthetics (eg, thylene- ropylene-
iene onomer, or short ) show clearly the superiority ofsilicone. As already short mentioned, during the manufacture ofthe surge arrester the silicone insulation is bonded to the arresterassembly through casting (or injection) of the liquid rubber inmoulds at a high temperature. Different moulds can be used toobtain an optimum match between the insulator and thecomponent assembly. The arrester insulator thus producedexhibits the following properties:
No hydrocarbon is present in the main chemical chain; thisproperty is partially the reason for the high immunity of theinsulator to serious surface pollution and is also largelyresponsible for preventing carbonized creepage paths fromforming.
The material is water-repellent, so that even after excessivecontact with water only very few drops adhere to the surface. Thisproperty is referred to in the industry as hydrophobicity. Drops ofwater that do cling to the surface are quickly removed by the effectof gravity or through normal exposure to wind.
The hydrophobicity effect is also partly transferred to any dirtdeposits on the surface, which as a result does not become coatedwith layers of semiconducting material as quickly as with othertypes of insulation. Because of this, the surface leakage currentsflowing on silicone insulators are extremely low. The technicalliterature explains the transfer of the hydrophobicity effect to dirtdeposits as being due to low-molar silicone evaporating. Thehydrophobicity effect is temporarily reduced by strong electricalpartial discharges or extreme exposure to water, but to a lesserdegree with silicone than with other materials. This is clearlyshown by the artificial aging af EPDM and silicone in accordancewith IEC 1109, see Figure 5. After 5,000 hours of alternatedprecipitation, salt-fog and UV radition, the silicone still retains 50percent of its water-repellent properties, whereas EPDM losesthese properties. Tests have further shown that the hydrophobicityeffect can be restored to its original state by drying the silicone forseveral hours.The salt-fog test to IEC 507 also demonstrated that, assuming thesame salinity in each case, the creepage paths required for siliconeinsulation are, on average, 30 percent shorter than the pathsnecessary with ceramic isolators, see Figure 5a. As a result, thecreepage path of the silicone isolators could be reduced by thisamount.
4.2 Insulation made of silicone rubber
E PD M EPDM
•
•
The diameter of the MO-resistors is proportional to the energyabsorption capability E and the nominal discharge current I . Thespecial arresters of the type POLIM-S and POLIM-H have resistorslike the ones of the high voltage arresters. These types of arrestersset new standards in the medium voltage network ; they are able toresist the strongest stress and at the same time they guarantee alow protection level. The continuous operating voltage U of theMV-arresters in the Table 3 reaches from 4 kV up to 36 kV.
In addition to the already mentioned types ABB manufactures alsothe special arrester POLIM-C. This arrester is also built upaccording to the already mentioned principle of direct moulding.The nominal discharge current is I = 10 kA and the continuousoperating voltage U reaches from1 kV up to 7.2 kV. In the mediumvoltage system this type of arrester is used among otherapplications for the protection of non-earthed cable sheath.
The functioning of an arrester, which consists only of seriesconnections of MO-resistors (without spark-gaps), is extremelysimple. During an overvoltage at the arrester terminals the currentrises continuously and without delay through the arrester corre-sponding to the shown U-I characteristic, Figure 4, which meansthat no really spark over takes place, but the arrester goescontinuously to the conducting condition. After the decreasing ofthe overvoltage the current becomes low again corresponding tothe U-I characteristic. Unlike the spark-gap arresters there flowsno follow current.
n
c
n
c
The number of the resistors in a stack depends on the U of thearrester. The stack of cylindrical MO-resistors is moulded inpolymeric material (silicone) as described.
The resistor stack behaves itself capacitive under U . The straycapacitances of each individual resistor towards the earth causethe nonlinearity of the voltage distribution along the axes ofarrester under U . This nonlinearity increases with the lenght of theresistor stack [3]. That is why high voltage arresters need gradingrings, which compensate mainly the unfavourable influence of thestray capacitances. Due to the relatively short length of the activepart of the distribution surge arresters the nonlinear voltagedistribution is so low that it can be neglected. These arresters donot need any grading rings.
c
c
c
U
[p.u.]
1.0
0
0.5
10-4 10-3 10-2 10-1 100 101 102 103 104
4/10 s
1/5 s
8/20 s
30/60 s
2000 s
I [A]
Figure 4Normalised current-voltage ( U-I)characteristic of a MO-surge arrester with I = 10 kAn
6
4 Medium Voltage Arresters of ABB
•
•
•
It was the wish to increase the reliability and the safety of thearresters and correspondingly to it also the increasing of theenergy supply that brought about the development of the MO-arresters with silicon housing in the 1980s. For more than 30years is silicon known as an excellent insulation material for thehigh voltage technology, as for instance the long rod insulators andbushings. The first MO-arresters with silicon housing of the typicalABB execution (direct moulding) were used in 1986. Now, in 1999there are everywhere in the world more than 600 000 arresters inuse under very different ambient conditions.
Generally an arrester is made up of two parts, the active part,consisting of one or more piled up MO-resistors, and an insulatinghousing, that guarantees both the insulation and the mechanicalstrength.The MO-resistors have been shortly described in the last chapter.
Fundamentally there are three different possibilities ofconstruction:
a glass-fibre reinforced tube is shed with an insulating material.These so called hollow insulators have the same problems as theporcelain insulators, they need a sealing and pressure reliefsystem, they can have inner partial discharges under pollution.
the active part is wrapped with glass-fibre material and is soakedwith resin, which makes up the whole into a rigid body. Theinsulating polymeric housing is then thrust over the resin block orshrunk on it. This costruction has the disadvantage that it forciblebreaks apart when the MO-blocks are overloaded. Anotherdisadvantage is the fact that there are different insulatingmaterials, which also means that there are more boundary layers.Therefore it is necessary to take special measures forsealing.
the active part is hold mechanically together with glass-fibrereinforced loops or bands. The silicon is directly put on the MO-resistors. This direct moulding has the advantage that no gasvolume stays in the arrester. Sealing problems and inner partialdischarges are in this way out of question. There are no interfacesbetween polymeric materials where humidity can go in. Anexplosion or a shattering of the housing is not possible.
All the medium voltage arresters of ABB are build upcorresponding to the third principle (direct moulding).
The requirements to the arresters depend on the operationnecessities and the type of the equipment to be protected. That iswhy ABB offers a large selection of different types of arresters forthe medium voltage network and for special applications. Theconstruction, the functioning and the characteristics of thearresters are described for instance in [5]. In the Table 2 there arethe electrical main data of the medium voltage arresters to befound.
4.1 Arrester construction
The outdoor arresters have sheded housings made of silicon. Thetype MWD for the use indoor has a smooth silicon housing. (seeFigure 3 and 3a)
Arrester type
POLIM-DN
POLIM-D
POLIM-DA
MWK / MWD
POLIM-I
POLIM-S
POLIM-H
IkA
nI Ain t in s
E / UkJ / kV
cU / Up c
5
10
10
10
10
10
20
65
100
100
100
100
100
100
2000
2000
2000
2000
2000
2000
2000
3.5
5.5
5.5
9.0
13.3
350
550
550
1000
1350
3.33
3.07
3.07
3.00
3.19
3.33
3.5 3.6
2.6
250
150
High currentkA
Square wave
Figure 3MO-surge arresters with silicone housing. (POLIM-family)
Table 2Electrical main data of the MV-surge arresters from ABB(most common types). E / U as tested in the operating duty test.c
Figure 3aMO-surge arresters with silicone housing
left:right:
5
MWK for outdoor applicationMWD for indoor application
If, for a certain arrester type, the reactive power of the parallelcapacitor bank exeeds the limiting values in Table 9, an arresterwith better energy qualities must be selected. For networks notoperating under standard voltages, the limiting values in thecolumn with the lower standard voltage apply for S . If the reactivepower is very large, arresters connected parallel must be chosen.In such a case the arrester supplier must be informed in order totake measures to guarantee a sufficiently good current distributionin the parallel arresters. The supplier should also be consultedwhen arresters with U < U are to be used.
Line traps are air-core chokes which are switched in connectionwith the high voltage lines. Their inductivity L is in the range of mH.If no measures are taken, the lightning currents in the conductormust flow through the line trap. Even relatively small current ratesof rise of several kA /µs would produce overvoltages on the linetrap amounting to several 1000 kV and resulting in a flashover. Inorder to prevent this, MV-arresters are connected parallel to theline trap. These will take over lightning currents and limit theovervoltage to its residual voltage.
When a short to earth occurs in a high voltage network, the faultcurrent I flows through the conductor. This power frequencycurrent would overload the arrester. U must therefore be selectedso that the current flows through the line trap. It induces atemporary overvoltage of U = x L x I , standard for U , at theline trap. If the fault current duration is t < 3s, then T = 1.28. Thisresults in the following for U :
U x L x I
U ---------- = ----------------T 1.28
I : maximum fault current through the line trap
L : inductivity of the line trap
K
c m
K
c
TOV K c
c
TOV K
c
K
10.10 Line Traps (Parallel Protection)
ω
ω>
On the other hand most of the d.c. current networks arerailnetworks. If the arresters are used on a rolling material(locomotive) the safety is especially important (safety of persons).
The arresters produced by ABB are suitable to be used on d.c.current networks and especially also in the railnetworks and onlocomotives and traction vehicles.The producer has to be informed by all means if the arrester is usedin d.c. current networks. For the dimensioning of the arrester it isalso necessary to take contact with the producer.
During many discussions with the users of surge arresters it wasnoticed that a profound consulting on the use of arresters iswelcome. This is necessary both by changes in technology, as forinstance from the spark-gap arresters with insulation of porcelainto the MO-arresters with silicon housing, and by the choosing ofthe arresters for additional equipment of alredy existinginstallations or the planing of new installation in the medium andlow voltage networks. Especially in the new fields of application, asfor instance in the d.c. current networks, or the designs for theprotection against overvoltages and lightning danger in wholeinstallations it is necessary a profound observation. The evaluationof the existing installations from the point of view of the powertransfer (higher system voltage) or the reliability and availabilitystipulate clear protection concepts, taking into account an optimaleconomic and technical solution.
Hence we offer gladly consultig and calculation for the protectionagainst overvoltage and lightning, which exceed the abovedescribed instructions.
12 Consulting Concernig Questions on theUse of Arresters
13. Conclusions
11 Arresters for D.C. Voltage
For the time being there are no international valid regulations ordirections for the use of the arresters in d.c.voltage networks. Onprinciple in d.c.voltage networks there also appear voltagesproduced by lightning or other activities, which may endanger theequipment and the insulation. In this case it is also necessary theuse of an arrester as protection against overvoltages. For suchsituation the MO surge arresters are especially suitable, becausethey do not conduct any follow current after the limiting of theovervoltage, and therefore it is not necessary to extinguish anyd.c.current arc. There are two fundamental items to be taken intoconsideration when using the MO-arresters in the d.c.currentnetworks. On one hand it is necessary to make sure that the MOmaterial is stable for a long period of time from the point of view ofthe d.c. voltage continuous operation. This is not the case with allthe MO materials which are comercially available.
22
Lightning overvoltages are a hazard for all the electrical equipmentin a MV network. However, surge arresters assure reliable protect-ion against excessive overvoltage stresses. The closer the arresterto the electrical equipment, the better the protection.
When determining the arrester U , two contradictory requirementsmust be considered.On the one hand, U must be as small as possible so that thearrester can limit the overvoltage to the lowest possible values. Onthe other hand, U must be selected at a value high enough to allowthe arrester to fulfill the requirements of network operation.Modern MO-arresters with no spark-gaps meet both theserequirements and provide sufficient protection from overvoltage,as well as simultaneously assuring safe network operation.
c
P
c
23
During unusual operational conditions, for example when asystem flashover takes place, all the parallel arresters in thenetwork are overloaded equally by the operational frequencyovervoltage. If metal oxide arresters are in use, it is possible toinduce a particular arrester to become over-charged first, ratherthan a random one. For example, an indoor arrester in a substationbuilding is selected with U approx. 10% higher than that of theoutdoor arrester. As soon as an abnormal operational frequencyovervoltage occurs, the outdoor arrester will be over-charged first.This arrester limits the overvoltage by producing an outdoorflashover and thus preventing arcing inside the substationbuilding.
A similar situation exists when very high temporary overvoltagesare expected in a MV network, and these occur only veryinfrequently. In order to avoid an over-charge on the arrester alsoin this rare case, a 15% higher U is necessary. Such arrestersmust be employed with electrical equipment. The drawback is, thatthe protection is reduced by 15%.
Two sets of arresters provide an acceptable solution to theproblem. One set with 15% higher U is installed next to theelectrical equipment, and a second set with a lower U is placedsome distance away. In this way two metal oxide arresters areswitched parallel in each phase. Should a lightning overvoltageoccur, both sets would be conductive and together produce thesame protection level for the electrical equipment as would bepossible with a single arrester set with a lower U . During the abovementioned very high overvoltage, only the arresters which are ata distance from the electrical equipment become over-charged.Therefore, the resulting flashovers cannot damage the equipment.Furthermore, since such an overload rarely occurs, thesubsequent interruption of operation can be risked.
When installing the arrester, two points must be carefullyobserved. Both are equally important for achieving the bestpossible protection level. The lightning endangered line must beguided to the high voltage connection of the arrester first, and onlythereafter led to the equipment to be protected. A short distancebetween the high voltage connection of the arrester and theequipment is certainly important, but not of crucial significance.
Secondly the galvanic connection from the earth side of thearrester to the earth of the electrical equipment must be as short aspossible. This distance must be below 2 m for lines with earthedcross arms. The distance for wooden pole lines must be shorterthan :
1 m for U 24 kV
0.6 m for U > 24 kV
If this is not possible, the cross arms on the last 3 poles before thesubstation or the electrical equipment must be earthed or anotherset of arresters must be installed one pole in front of thesubstation. In this case the upper limit for the earth connection is 2m. The connection lines to the arrester in the case of a cable mustbe as short as possible.
c
c
c
c
c
m
m
<
20
MO
SiC
200A
13
10
5.66
0
Semi-logarithmic plot of the current-voltage characteristics ofresistors for Uc = 4 kVMO and SiC
10-4 10 410-2 10 21 [A]
√2 x Uc
U = 4 p.up
I = 10 kAn
[kV]
Figure 1
4
d.c. voltage measurement current wave 8 / 20 s
The characteristic curve in 1 corresponds to a resistor withU = 4 kV. In the case of a d.c. voltage load with 2 x U = 5.66 kV ad.c.current in the range of 0.1 mA flows. The capacitive componentat 50 Hz and the value of U lies in the vicinity of 0.5 mA. Theprotective level U when I = 10 kA amounts to 13 kV.
In comparison, the diagram also shows the curve of a SiC resistor,which has also U =13 kV. Since SiC exhibits a considerably lessnon-linearity, the continous current of a SiC arrester without spark-gaps at a nominal load would amount to about 200 A. Naturally, forthermal reasons, such an arrester would not be feasible. Besides itwould strain the electrical network much too much. Consequentlyconventional arresters need spark-gaps in series, which take overthe voltage in continuous operation.
Figure
c c
c
p n
p
3 Metal Oxide Resistors as Arrester Elements
The voltage current characteristic of metal oxide resistors isextremely non-linear. In Figure 1 the characteristic curve for suchresistors is plotted. I is the nominal discharge current (in 1e.g. I =10 kA). U is the protection level. It is defined as themaximum voltage at the resistor during the flow of I . U is definedas the r.m.s.value of the Maximum Continuous Operating Voltage(MCOV) of the resistor.
n
p
n c
Figure
n
All the parallel MO-columns of the MO-arresters without spark-gaps conduct current at the same time. The energy of theovervoltage is in this way distributed on all the parallel arresters, sothat the energy capacity as a limiting parameter disappears.
follow
follow
MO-arresters can be used both with 50 Hz and with 16 2/3 Hzbecause they do not conduct any follow current. On the otherhand in the spark-gap arresters the current flows with16 2/3 Hz three times longer than with 50 Hz. As a result the spark-gaps and the SiC resistors are loaded with a corresponding higherenergy. In order to prevent this it is necessary to reduce the peakvalue of the current. This asks for spark-gap arresterswith a higher nominal voltage, which however makes a consi-derable increasing of the protection level necessary. For thebetter understanding of the facts it is necessary to add that the MO-arrester may be used with d.c. voltage, the arrester with platespark-gaps however cannot resist this strain.
The MO-resistors make up the active part of the MO-arrester. TheMO-resistors are compressed and sintered in the form of roundblocks out of different metall oxides in powder form. The diametersof the MO-resistors of ABB High Voltage Technologies Ltd, madefor the usage in the medium voltage, lie between 38 mm and 75mm. The height of the blocks is generally between 23 mm and 46mm. The diameter of the MO-resistors decides the carryingcapacity of the current, the height of the MO-resistors (or resistorstack) decides the voltage in continuous operation and the energycapability. The diameter of MO-resistors correlate with the linedischarge classes corresponding to IEC 60099-4, as shown inTable 1.
Table 1Correlation between typical MO-resistors and the line dischargeclasses acc. to IEC. Line discharge class 5 is important only inhigh-voltage systems above 420 kV, and not mentioned here.
The contact areas of the MO-resistors are metallized with softaluminium up to the edge of the block, the surface is passivatedwith glass. In this way the MO-material of the MO-resistorsproduced by ABB High Voltage Technologies Ltd is completelycovered. The 2 shows MO-resistors which are used in themedium voltage arresters.
The energy capability of the MO-resistors depends besides thevolume also on the construction of the arrester (carrying-off ofheat). Chapter 4 gives more details concerning this.
Figure
Figure 2
MO-resistors (collection) produced by ABB
Line discharge classacc. to IEC 60099-4
Diameter of blocks in mm
Square wave, 2000 s in A
Energy capability in (kJ / kV )
µ
Uc
1
38
250
3.6
2
47
550
5.5
3
62
1000
9.0
4
75
1350
13.3
1 Introduction
Overvoltages in electrical supply networks result from the effectsof lightning strokes and switching actions and cannot be avoided.They endanger the electrical equipment because, due toeconomical reasons, the insulation cannot be designed for allpossible cases. A more economical and safe on-line network callsfor extensive protection of the electrical equipment againstunacceptable overvoltage loads. This applies to high voltage aswell as medium and low voltage networks.Overvoltage protection can be basically achieved in two ways:
intercept lightning.
•
•
•
•
•
Avoiding lightning overvoltage at the point of origin, for instancethrough shilding earth wires in front of the substation that
Limit overvoltage near the electrical equipment, for instancethrough surge arresters in the vicinity of the electrical equipment.
In high voltage networks both methods of protection are usual.The earth wire protection in medium voltage networks is generallynot very effective. Due to the small distance between the earth wireand the line wires, a direct lightning stroke hits usally the line wiresas well. In addition, induced overvoltages in the line wires (indirecteffects of the lightning strokes) cannot be avoided by the earthwires.
The magnitude of the overvoltage is usually given in p.u.(per unit). It is defined as
1 p.u. = 2 x U / 3 ,
the U means the maximum r.m.s voltage value between thephases in an undisturbed network operation [1].
Three types of overvoltages are essentially distinguished [2]:
occur for instance during load rejectionor because of earth connection faults. The duration of suchoperating frequency oscillating overvoltage lies between 0.1seconds and several hours. In general the surge does not exeed 3p.u. and therefore it is of no danger to the network operation,however it is decisive for the dimensioning of the arrester.
occur during switching actions andconsist mostly of heavily damped oscillations with frequencies upto several kHz and a magnitude up to 3 p.u.A special case is switching in inductive electrical circuits. Herethe front time of the overvoltage lies between 0.1 and 10 µs andthe peak value can reach 4 p.u.. Fast-front overvoltages are alsopossible when lines or cables are switched on.The peak value liesthen below 2.2 p.u. and does not endanger the network system.
originate in atmospheric discharges.They reach their peak value within a few µs and subsequenly decayvery rapidly. The magnitude of these unipolar overvoltages in amedium voltage network can reach well over 10 p.u.
Lightning overvoltages are the greatest threat to the mediumvoltage networks. Overvoltage protection must be arranged insuch a way as to confine the overvoltage to non-damaging values.
The most effective protection against overvoltages in a mediumvoltage network is therefore the use of surge arresters in thevicinity of the electrical equipment.
Temporary overvoltages
Switching overvoltages
Lightning overvoltages
√ √
√
m
m
3
2 Surge Arrester Technology
The so-called "conventional" surge arresters were exclusivelyemployed in MV networks (MV = medium voltage) until about themiddle of the eight decade of our century. They consisted of aseries connection of SiC resistors and plate spark-gaps. During theovervoltage rising there emerges a short circuit to the earth whenthe spark-gaps come into action. The series of SiCresistors limits the follow current from the power supply andallows in this way the disappeareance of the arcs between the platespark-gaps the next time the voltage reaches the zero crossing.
In the last years there were two fundamental improvements ofsurge arresters used in MV networks. On one hand the series
of SiC resistors and the plate spark-gaps were replacedwith the metalloxid resistors (MO-resistors) without plate spark-gaps, on the other hand the housings of the surge arresters madeof porcelain were replaced with new ones made of polymermaterial (synthetic material).
The arresters protect the electrical equipment no matter whethersome other types of arresters are present. Therefore it is possibleto have at work in the same installation both the conventionalspark-gap arresters and the modern MO-arresters. It is not evennecessary that an electrical equipment should be protected in allits three phases by the same type of arrester. The user cantherefore simply replace a spark-gap arrester as soon as it is out ofwork with a MO-arrester. That allows the user to introduce cost-saving the MO-arresters that have an elevated operating safety.
A fundamental advantage of the MO-arresters is the fact thatbecause of their extremely non-linear characteristic of the MO-resistors they do not need any spark-gaps. The current starts toflow already through the arrester before the overvoltage achievesthe value U . The MO-arresters reduce therefore the overvoltagesooner as the spark-gap arresters. The last ones are able toconduct after the overvoltage was increased to U . That is whytheir protection distance is shorter in many cases. This means thatthe overvoltage to the electrical equipment is higher when a spark-gap arrester instead of a MO-arrester is installed as both types ofarresters are at the same distance from equipment to be protected.
The response voltage of the spark-gaps increases when the risetime becomes steeper (< 1 . This causes a deterioration of theprotection possiblity of the spark-gap arresters in case of anovervoltage wave with steep front .
If the outside insulation of the arrester is very dirty the potentialdistribution along the humid surface is very uneven. It is possiblethat between the sheds partial arcings appear which can induceovervoltages in the active part. This situation is especially criticalwith a spark-gap arrester, because it can bring it regularly to sparkover and at the end destroy it. This is the reason why MO-arresterswithout spark-gaps have a fundamentally higher pollution resi-stence.
If more spark-gap arresters are parallel connected usually onlyone arrester switches on during an overvoltage. This reduces thenthe overvoltage to a value that lies under the sparking voltage ofthe other parallel arresters. Therefore it is not possible to distributethe energy of the overvoltage on more spark-gap arresters whichare parallel connected. In case that this energy is too high thearrester that switched on will be overloaded. This appliesespecially to the spark-gap arresters which are parallel connectedto capacitor batteries with a higher reactive power.
connection
connection
µs)
2.1 MO-Arresters and Spark-Gap Arresters
p
p
[1] IEC Publication 99-5, First edition 1996-02 : Surge arresters Part 5 : Selection andapplication recommendations.
[2] R. Rudolph und A. Mayer: Überspannungsschutz von Mittelspannungskabeln. Bull.SEV/VSE 76 (1985) 4, S. 204-208.
[3] R. Rudolph: Bemessung, Prüfung und Einsatz von Metalloxid-Ableitern. Bull.SEV/VSE 75 (1984) 23, S. 1407-1412.
[4] A. Mayer und R. Rudolph: Funkenstreckenlose Überspannungsableiter ermöglichenoptimalen Überspannungsschutz. Brown Boveri Technik 72(1985) 12, S. 576-585.
[5] W. Schmidt: Metalloxid, ein fast idealer Überspannungsableiter. Bull.SEV/VSE 7 / 1998, S. 13-20.
[6] IEC Publication 60099-4, Edition 1.1, 1998-08: Surge arresters Part 4: Metal-oxidesurge arresters without gaps for a.c. systems.
[7] ANSI/IEEE Publication C62.11 1993: IEEE Standard for Metal-Oxide Surge Arrestersfor Alternating Current Power Circuits.
[8] R. Rudolph: ZnO-Ableiter. Eine Alternative zu konventionellen Ableitern. Elektrotechnikund Maschinenbau 5 (1983), S. 195-200.
[9] IEC Publication 71-1 (1993-12): Insulation coordination - Part 1: Definitions, principlesand rules.
[10] IEC Publication 71-2 (1996-12): Insulation coordination Part 2: Application guide.
[11] G. Balzer und K.H. Weck: Isolationskoordination von gasisolierten Schaltanlagen.ETG - Fachbericht 32 (1990), S. 71-89.
[12] VDEW Störungs- und Schadensstatistik 1990. Verlags- und Wirtschaftsgesellschaftder Elektrizitätswerke m.b.H.
[13] A.J. Eriksson et al.: Guide to procedures for estimating the lightning performance oftransmission lines. Report of WG 01 of CIGRE Study Committee 33, Oct. 1991.
[14] K. Berger: Methoden und Resultate der Blitzforschung auf dem Monte San Salvatore beiLugano in den Jahren 1963 bis 1971. Bull. SEV/VSE 63 (1972) 24, S. 1403-1422.
[15] Surge arrester application guide. IEC 37 (Sec) 85, Jan 1992.
[16] R.B. Anderson and A.J. Eriksson: Lightning parameters for engineering application.Electra, 69 (1980), S. 65-102.
[17] A.J. Eriksson et al.: A study of lightning stresses on metal oxide surge arresters.Cigre paper 33-08 (1986).
[18] M. Christoffel: Der Einfluss von Kabelstrecken auf die Überspannungsvorgänge inÜbertragungssystemen mittlerer und hoher Spannungen. Brown Boveri Mitt. 51 (1964)6, S. 369-376.
[19] A. Braun: Schirmspannungen und Schirmverluste bei Mittelspannungs-VPE-Kabeln.Elektrizitätswirtschaft 88 (1989) 26, S. 1898-1906.
[20] M. Darveniza und D.R. Mercer: Lightning protection of pole mounted transformers.IEEE Transactions on Power Delivery, Vol. 4, No. 2, April 1989, S. 1087-1093.
[21] G. Balzer: Schaltvorgänge in Mittelspannungsnetzen und deren Berücksichtigung beider Planung. Brown Boveri Technik, 73 (1983) 5, S. 270-278.
[22] Non-linear metal-oxide resistor polymeric housed surge arresters without sparkgaps.IEC 37 / 154 / CD; March 1996
[23] W.Schmidt: Die neuen POLIM -Überspannungsableiter mit Silikonisolation fürMittelspannungsnetze. ABB Revue 2/96
24
Bibliography
conductor length
Basic Impulse Insulation Level (peak value)
conductor length
capacitance (given in nF or uF)
earth fault factor, C x U / 3 is the maximum voltage
between phase and earth in the case of an earth fault
section length of an overhead line before the substation
energy absorbed by the arrester (mainly given in kJ or kJ / kV )
discharge energy absorbed by the arrester (mainly given in kJ)
long duration current impulse
nominal discharge current (mainly given in kA, peak value)
50 Hz fault current (mainly given in kA, rms- value)
reference current (mainly given in mA, peak value)
peak current of the stroke current (mainly given in kA, peak value)
time function of the stroke current
corona damping constant
inductance of a line trap
protective distance
cable length
Maximum Continuous Operating Voltage = U (mainly given in kV, rms- value)
power losses of the arrester in the case of U
per unit, 1 p.u. = 2 x Um / 3
heat flow from the active arrester parts to the external environment (cooling)
maximum steepness of a voltage increase (mainly given in kV / µs)
time function of the steepness of a voltage increase (mainly given in kV / µs)
steepness of the lightning overvoltage at the point of the stroke (mainly given in kV / µs)
three-phase reactive power of a capacitor bank
withstand strength against temporary overvoltages U = T x U
temperature
time
e m
Uc
c
c
TOV c
√
√ √
in m
in kV
in m
in F
in m
in J
in A
in A
in A
in H
in J
in A
in A
in A
in m
in m
in V
in W
in W
in V /s
in V / s
in V / s
in Var
in C
in s
°
a
BIL
b
C
C
d
E
I
i
i(t)
K
L
e
Ec
In
IK
Iref
L
LK
MCOV
P
p.u.
Q
S
S(t)
So
SK
T
T
t
25
Index of symbols used
10.2
10.3
10.4
10.5
10.6
10.7
10.8
10.9
10.10
11
12
13
Overvoltage protection in cable sections
Cable sheath protection
Transformers at the end of cables
Transformer connected to a lightning endangered line onone side only
Arresters in metal enclosed MV-substation
Generator connected to a lightning endangered MV-line
Arrester protection for motors
Overvoltage protection in locomotives
Arresters parallel to a capacitor battery
Line traps (parallel protection)
Arresters for d.c.voltage
Consulting concerning questions on the use of arresters
Conclusions
Index of symbols used
Bibliography
8.4
8.5
8.6
8.7
9.1
9.2
9.3
9.4
10.1
9
Introduction
Surge arrrester technology
Metal oxide resistors as arrester elements
Medium voltage arresters from ABB
Special operating conditions
MO-arresters and spark-gap arresters
Construction of the arrester
Energy absorption capability and cool-down time
Nominal discharge current and energyabsorption capability
Network short circuit power
Elevated ambient temperature
Mechanical stability
Air pollution
Altitude adjustment for arrester housing
Insulation made of silicone rubber
1
2.1
4.1
4.2
4.3
4.4
5.1
5.2
5.3
5.4
5.5
2
3
4
5
Surge arrester protection level
Questions of stability of MO-surge arresters
Temporary overvoltages
Type tests
Routine tests
Acceptance tests
Special tests
Networks with earth fault compensation orwith a high-ohmic insulated neutral system
Networks with high-ohmic insulated neutralsystem and automatic earth fault clearing
Protection characteristics of the surge arrester,stability
Tests
Selection of surge arresters and determinationof Uc
6.1
6.2
6.3
7.1
7.2
7.3
7.4
6
7
8
Networks with solidly earthed neutral systems (Ce 1.4)
Networks with low-ohmic neutral transformer earthingwhich do not uniformly have Ce 1.4
Low-ohmic neutral earthing networks and Ce > 1.4
Arresters between phases (Neptune design)
Operating voltage with harmonic oscillation
Theoretical projection for the protective distance L
Expected steepness S from lightning overvoltages inMV-substations
Influences on the protective distance through electricalequipment, the types of the arresters and the arrangementof the arresters
Fault hazards in electrical equipment and their distancefrom the surge arrester
<
<
Protective distance of the surge arrester
8.2
8.1
8.3
2
Contents
Some special cases10
1
The first edition of our directions for dimensioning,testing and application of metal oxide surge arrestersin medium voltage networks, which appeared in 1994,was received very positively. We were pleased to getsuch a reception, which attested our belief that com-petent consulting with regard to the application of ourproducts is as important as the quality of the productsitself.
The technological progress made it now necessary to revise and renew the present booklet for the thirdedition.The dimensioning and the theoretical basis for the optimal application of the surge arresters are not changedand therefore they were taken as such in the new edition. Mr. René Rudolph, who was at the time of the firstedition responsible for the consulting concerning application in the surge arrester division, took on the taskof revising the tables. That was necessary because of the improvement of the technical data of the surgearresters and the enlargement of our product range. We would like to thank Mr. René Rudolph for his efforts.Mr. Bernhard Richter, who is now responsible for engineering and application of the overvoltage protectivedevices in the surge arrester division of ABB High Voltage Technologies Ltd, took on gladly the task of thegeneral revision of this booklet.Mr. Richter is a member in different working groups of IEC SC 37 A and IEC TC 81, and his activity fieldincludes, besides the development and testing, mainly the application of the surge arresters.
The silicon technology, which is used in medium voltage by ABB High Voltage Technologies Ltd, and thefurther developing of the metal oxide material opens new application spheres. All these are taken intoaccount in the new edition.
We hope, that you as a reader will be satisfied with our new revised edition and you will find it useful for yourpurpose. We welcome amendments and suggestions in order to meet all possible customer needs.
ABB High Voltage Technologies LtdWettingen, July 1999
First published: November 19942.revised edition: September 19953.revised edition: July 1999
All rights reserved.Neither the booklet or parts of it may be either copied or reproduced,transmitted in any way or translated info other languages withoutthe prior permission of ABB High Voltage Technologies Ltd.
Division Surge Arresters, Wettingen, Switzerland
ABB High Voltage Technologogies Ltd
Forewordin s
in V
in V
in V
in V
in V
in V
in V
in V
in V
in V
in
in
in V
in m/s
Ω
Ω
ts
U
Uc
UE
UK
Um
Up
Ur
Uref
UT
UTOV
Z
ZK
L
u(t)
v
δ
ω
time interval
peak value of the overvoltage of a travelling wave (mainly given in kV)
maximum continuous operating voltage of the arrester (mainly given in kV, rms)
maximum overvoltage at the end of an open line (mainly given in kV, peak value)
maximum overvoltage at cable end (mainly given in kV, peak value)
maximum voltage between phases (mainly given in kV, rms)
protection level of the arrester at I (mainly given in kV, peak value)
rated voltage (mainly given in kV, rms)
reference voltage (mainly given in kV, rms)
overvoltage at the transformer (mainly given in kV, peak value)
power frequency overvoltage of a limited duration (mainly given in kV, rms)
time function of a lightning overvoltage
speed of the travelling wave, v = 300 m / µs in the air
surge impedance of a distribution line conductor Z = 300........450
surge impendance of a cable conductor Z = 20 ...... 60
load rejection factor of a generator
angular frequency of the power frequency, at 50 Hz is = 314
n
K
Ω
Ω
ω
26
1 1
S S
CHHOS / AR 3257.99E (D)
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N Dimensioning, testing and application
of metal oxide surge arresters
in medium voltage networks