surge arrestor in mv networks

7/28/2019 Surge Arrestor in MV Networks

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Surge arresters in MVc what surge arrester for what type of needs?c choosing a surge arresterc where and how to insta ll surge a rresters?

WHAT YOU NEED TO REMEMBERc Surge arresters protect electrical installations that are exposedto storms, against atmospheric voltage surges.c Lightning impulses on any section of an electrical networkcan generate voltage surges on the MV network.c Certain network points and components are particularlysensitive to voltage surges. High impedance loads reflect thevoltage wave; the voltage surge may thus be doubled at theirterminals.c An MV surge arrester must be installed as soon as there isa possible risk of atmospheric voltage surges.c To be completely efficient, the installation and the surgearrester’s technical data must be optimal.



DIRECT LIGHTNING STROKES

A direct lightning stroke consists of the injection of a current waveinto the line. D epending on the typical impedance of the line,the current wave generates a voltage surge wave. A flashover device,at the isolator or anchoring level on the overhead line, shunts partof the current, but does not always limit the peak value.

Surge arresters in MV

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WHAT SURGE ARRESTER

FOR WHAT TYPE OF NEEDS?(cont’d)

, , , ,Q Q Q QR R R RS S S ST T T T




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WHAT SURGE ARRESTER


INDUCED VOLTAGE SURGES

A voltage surge can also be produced by a lighting stroke that hitsclose to the line, an “induced lighting stroke”.

The voltage surge is induced in the loop formed by the line and theearth through the magnetic field created by lightning that strikesnear the line.

Example:

An induced voltage surge or “indirect lightning stroke”

The insulation level of equipment installed on the network mustguarantee a lightning impulse withstand, see M T Partenaire B-1-1.

The lightning impulse withstand of equipment is characterisedusing normalised testing:

c voltage tests (for equipment which is normally insulating,for example line insulators);

c current tests (for equipment through which a lightning currentcan flow, for example surge arresters).




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WHAT SURGE ARRESTER

FOR WHAT TYPE OF NEEDS?(cont’d) 100 %

50 %

80

0

- 20

- 45

0 5025 75 100 125 150

20

Modelling of lightning impulses, according to the IEC

R eal lightning impulses may be different from the models used ina laboratory, in particular concerning their duration.

Each part of the network is characterised by its typical impedance:Zc, given in ohms.It characterises the conditions of electromagnetical wavepropagation along the conductors.

Zc = √L/C with:

L: inductance per unit lengthC : capacitance per unit length (not taking resistancies and

conductancies into account).The table below gives typical impedance values for electricalnetworks.

components typical impedance (ohms)insulated MV cable 25 - 50metal-clad substation 70line 500/800 kV 270

400 kV 300132/220 kV 360 - 38063 kV 40020/30 kV 450

transformer 500 - 50,000open circuit infinite

Critical points, which correspond to propagationcondition modifications, are located at each typicalimpedance change of the network.

Curve in current (used in laboratory)

I

Time in µs

Time in µs

Iin kA

Example of a real recording




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WHAT SURGE ARRESTER


Special case: opening points on the line force total reflectionof the incident wave.

High impedance loads, compared to the network’s typicalimpedance, must be considered as open network points(for example transformers).A t these points, the superposition of the incident wave andthe reflected wave creates local dielectric stress (critical point)

which can reach up to twice the wave value. In certain cases,waves that have already been contained by a surge arrester,are still capable of creating stress in an installation abovethe level for which it was foreseen.

Upon each wave propagation phenomenon,

transmission and reflection are produced whentypical impedancies vary.

, ,

, ,

, ,

Utime t

time t + ξ

Xtransformeroverhead

line

U


line

dielectrical stress at t + ξ

Legend: incident wave

reflected wave

U


line

1

2




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WHAT SURGE ARRESTER


I lightning

V surge arrestor

transmissioncoefficient = 1.5

line

lightningimpulse

cableZc = 20 Ω

risk ofdielectrical

fault

busbarsZc = 60 Ω

Propagation of the voltage wave to a conductor with a higher typicalimpedance, is conducted with a transmission coefficient greater than 1.D ue to this, the permissible d ielectric stress may be surpassed.

Example on an HV network

Changing points are all the more critical, since

the insulation goes from a self-restoring insulation(atmospheric air) to a solid non self-restoring isolation(cable) in which flashovers cannot be allowed.

c If there are no protection and flashover devices, calculationsshow that the voltage in the transformer’s inlet can progressivelyincrease, by increments, up to the theoretical value which is twicethe incident peak value (the increments are linked to the multiplereflections inside the substation).

c When a protective gap is used at the substation’s inlet, stress

is created by the injection of an increment and calculations showpeak voltages that are up to 4 times the flashover voltage.




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WHAT SURGE ARRESTER


This sensitivity comes from their relatively high impedance andfrom their low withstand voltage, which is often lower than that of the line or of the supply cable ( typica lly high voltage motors).If voltage surges on the line are estimated at around 50% of componentwithstands, then the line’s protection devices may be inefficient andthe surge arresters need to be installed directly onto the terminalsof loads that are to be protected.

This mainly concerns installations which are directly subjectedto impulses, and in which there are no other limiting elements(i.e. weak points, such as protective gaps) between the possibleimpulse zones and the installation. Statistical data, such asthe keraunic level, can be obtained from national meteorologicalorganisations. These organisations are usually capable of supplyinginformation concerning current values and the statistical distributionof these values. A choice can then be made in regards tothe acceptable risk.

Keraunic level:number of days per year where thunder was heard in one area:c in France: Nk = 20 (average 10 to 30)c in Indonesia: N k = 180

Lightning stroke density:number of lightning strokes per km 2 and per year in France N = 2 to 6

Example: If for an installation for a given region, statistics show that:c 2% of lighting strokes have a current above 5 kA ;c the probability of receiving a shock is twice a year.

With surge arrester protection, which guarantees that the withstandvoltage of an installation, with a current of 5 kA , will not be exceeded,the yearly probability for installation faults due to a lighting strokeis inferior to 4% .

O ne should always ask oneself if the consequences of a fault areacceptable or not.

Risk assessment greatly depends on the user’s

(or energy supplier’s) policy.

Certain network components should be considered

as particularly sensitive to this phenomenon.



U

t

t

100 µs

Uo

U

100 µs

Uo

Quick wave:the wave may reachvoltage value higher

than U o withoutflashover

Slow voltage wave

Voltage at the terminalsof the spark gap

Voltage wavewithout spark gap

Quick voltage wave

Flashover at 1.6 x U o

Flashover points of the spark gap

Lightning impulse withoutflashover of the spark gap

Lightning impulse withflashover of the spark gap


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WHAT SURGE ARRESTER

FOR WHAT TYPE OF NEEDS?(cont’d) Spark gaps are not always satisfac tory:

c their technical data greatly varies depending on atmosphericconditions, air pollution and electrode settings.

c the level of protection is not precise and the equipment mayundergo severe stress.

c the peak value for a limited wave depends on the rise speed of the voltage wave. This behaviour, due to the air’s ionisation mode,is very different from that of an SF6 or of a solid insulation device that

is often used in switchgear. The drawing below shows the time/voltagedata for a spark gap in the air, with a much higher voltage surgepossibility than the necessary level of protection.

With a slow impulse, the protective gap’s flashover voltageis constant and indicated with the letters U o. This voltage mustalways be 20% lower than the dielectric withstand valueof the equipment to be protected.

If a lighting impulse risk exists, the use of surge

arresters is necessary.

This means that switchgear can be destroyed through internalflashover, if the wave from the lightning impulse has a rapidcurrent rise front, even if the protective gap is in good condition.

Each time such an accident happens, the manufacturer of the

damaged switchgear will be doubted and will be asked to replacethe material free of charge...It is easier to discuss the subject matter before any accidentoccurs.

Spark gap




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WHAT SURGE ARRESTER


5 kA

Using surge arrester is a better answer:

c

zinc oxide ( ZnO ) surge arresters act like strongly non-linear resistors.c under normal service condi tions, the several M egohm withstandand the current, which remains low ( in the realm of mA), limitsdissipated power.

c under voltage surge conditions, the withstand greatly drops(up to roughly 5 or 10 ohms), thus limiting the voltage in surgearrester terminals and therefore in the equipment for the durationneeded for the lightning current to runoff to the earth.

Current throughthe lightning arrester

15 kVMCOV(Maximum Continuous Operating Voltage)

Voltage across the lightningarrestor terminals

Lightning arrester technology

75 kV protection level

Terminal

Internal insulating screen

Active parts(ZnO ceramic blocs)

Conductive spacers(aluminium)

External enclosure(porcelain or synthetic)

Spring to maintaincontact quality




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CHOOSING A SURGE ARRESTERTHREE CHARACTERISTICS ARE TO BE TAKEN INTO ACCOUNT WHENCHOOSING A SURGE ARRESTER:

The MCOV value in kV (greater than the service voltage)

The M aximum C ontinuous O perating Voltage (M C O V) provided bythe manufacturer ensures that the power is maintained within thedesign’s limits and that there is no thermal runaway that canlead to permanent damage of the surge arrester.The surge arrester’s M C O V must be greater than (or equal to)the service voltage of the network in consideration. I t can be muchhigher as long as the voltage level of protection devices suppliesthe remainder acceptable for the installation. T he choice of a surge

arrester should take into consideration the voltage betweenthe conductors and the earth ( phase to neutral voltage) whereasin general the service voltage of the network is given in phaseto phase voltage.

The supplied level of protect ion

The effectiveness of surge arrester protection is measured bythe residual voltage, at the surge arrester terminals, while a givencurrent is flowing through it. Typically, the level of protection is definedby a pair of values (for example 80 kV/10 kA).The voltage value which can be reached when lightning hits mustbe sufficiently low as to reserve a safety margin in regards tothe withstand voltage of equipment.The margin must be at least 20% and should take the surgearrester’s installation mode and cabling into account. I neffectivecabling can lead to voltages in the equipment’s terminals that areconsiderably higher than the surge arrester’s residual voltage, dueto voltage drops in stray impedencies. A compromise can be foundbetween a higher M C O V and a level of protection that is neverthelesssatisfactory.

Thermal withstand

It aims at guaranteeing the non-destruction of the surge arresterin the case of a long impulse (greater than the normalised testimpulse). This performance is given by a current impulse withstand.




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CHOOSING A SURGE ARRESTER

(cont’d)

Example of 3 necessary characteristics for choosing a surge arrester (taken from the specifications of an energy supplier)

c MCOV(M aximum C ontinuous O perating Voltage) = 15 kV

c Level of protection 75 kV/5 kA

c Impulse withstand 65 kA (8/20 µs) to cover the caseof an impulse of 5 kA (200 µs)

5 kA

Current throughthe lightning arrester

15 kVMCOV

Voltage across the lightningarrestor terminals

75 kVlevel of protection

Servicevoltage

100 %

50 %

80 20

I

Time in µs

65 kA

37.5 kA

According to IEC




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WHERE AND HOW TO INSTALL

SURGE ARRESTERS?

Load

V connection

V lightning arrestor

V connection

Dielectricalstress ofthe load

Lightningcurrent

The lighting current runs off to the earth through the surge arrester’scircuit.

The voltage at load terminals is the sum of connection voltages andthe surge arrester voltage.

Example: if the earthing conductor has an inductance of 1 µH/mand if the lightning stroke has a front of 10 kA /µs, then the voltagealong the connection is 10 kV per metre.D epending on the connections’ lengths, the load may rapidly reacha voltage that is greater than the load’s dielectric withstand.

The circuit equiva lent to the connection point for a surge arresteris presented by the diagram below:.




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Rules for installing a surge arrester

Two principles:reduce as much as possible the connection voltages by reducing

the path that the lightning will follow.

Lightningcurrent

cablethe earthing connectionis done close the lightningarrestor terminal

earthingbusbar

connection tothe earthingbusbar

NO YES

NO YES

barre

The surge arrester must be installed very close to critical pointson the network:c junction between overhead and underground lines;c transformer terminals;c substation inlets.. ..

Due to the velocity of this phenomenon, the surge arrester should beas close as possible to the zone that is to be protected. For mediumvoltage, the maximum distance is in the realm of 25 meters.

For additional information, please refer to cahier technique no. 151“voltage surges and insulation co-ordination ”.

WHERE AND HOW TO INSTALL

SURGE ARRESTERS?(cont’d)

connect the load directly to the surge arrester terminals.Since surge arresters set the voltage of their own terminals,the load’s voltage at terminals is thus the surge arrester’s voltage.

surge arrestor in mv networks

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