voltage surge or transient voltage

Protection against over voltages

By....... K.RAGHAVENDRA REDDY , M.Tech Asst.Prof.(EEE Dept.),CRRCE

Causes of Over voltages

Causes of Over voltages

Internal causes

Switching surges

Open circuited line

Loaded line

Current chopping

Insulation failure

Arcing ground

Resonance

External causes(Lightning)

Direct stroke

Stroke A

Stroke B

Indirect stroke 9/20/2012 2 SIR C R REDDY COLLEGE OF

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Internal causes • These occurs mainly due to oscillations set up the sudden changes in

the circuit conditions. In general insulation is suitably designed to with stand such surges

• Switching Surges:-

• Open circuited line:-

• when the unloaded line connected to a

voltage source, a voltage wave is set up

which travels along the line. on reaching the terminal point A ,it is reflected back to supply end without change of sign

• The voltage at supply end is 2√2E,which is temporary in nature

• Due to the presence of line losses attenuation in the wave takes place and in a very short time ,the line settle down to its normal supply voltage

9/20/2012 3 SIR C R REDDY COLLEGE OF

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• A loaded line:-

• Over voltages will also be produced during switching operations of a loaded line.

• If a loaded line is suddenly interrupted it will set up a voltage of 2Zni across the break.

• Current chopping:-

• This results in the production of high voltage transients across the contacts of ABCB

• This can be avoided by resistance switching.

• Insulation failure:-

• The earthing of line causes two equal voltages of –E travel along XQ & XP containing currents –E/Zn & +E/Zn

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• Arcing ground:-

• In early of transmission, neutral wire was omitted to gain two advantages

In case of LG fault, the line is not put of action

Zero sequence currents eliminated, resulting in the decrease of interference with communication lines

• Insulated neutrals give no.of problems with short lines and comparatively low voltages

• In case of long lines and operating at high voltages ,serious problem of arcing ground sets in, which produces severe oscillations of 3-4 times normal voltage

• The transients produced due to arcing ground are cumulative and may cause serious damage to the equipment in the power system by causing breakdown of insulation

• This problem can be prevented by earthing neutral

• Resonance:-

• Under resonance the impedance of circuit equal to resistance at UPF

• It causes high voltages

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SIR C R REDDY COLLEGE OF ENGINEERING,ELURU

5

External causes: Lightning • An electric discharge between cloud and earth, between clouds (or)

between the charge centers of the same cloud is known as lightning

• Lightning is a huge spark and takes place when clouds are charged to such a high potential (+ve or –ve)with respect to earth (or) to a neighboring cloud, then dielectric strength of neighboring medium (air)is destroyed

• During the up rush of warm moist air from earth, the friction between the air and tiny particles of water causes building up of charges. when the drops of water are formed, the larger drops become +ve charged ones and the smaller drops become –ve charged

• The charge on cloud may become so great that it may discharge to another cloud (or) to earth. we call this discharge as lightning

• The thunder which accompanies lightning is due to the fact that lightning suddenly heats up the air, there by causing it to expand


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Mechanism of Lightning discharge • When a charged cloud passes over the earth, it induces equal and

opposite charge on the earth below

• As the charge acquired by the cloud increases, the potential between cloud and earth increases, and therefore gradient in the air increases

• When the potential gradient is sufficient (5kv/cm-10kv/cm)to break the surrounding air and the lightning stroke starts


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• A streamer called leader streamer (or) pilot streamer starts from the cloud towards the earth, when the air near the cloud breakdown

• The leader streamer will continue its journey towards earth as long as the cloud feeds enough charge to it to maintain gradient at the tip of leader streamer above the strength of the air.

• If this gradient is not maintained, the leader streamer stops and the charge is dissipated without the formation of a complete stroke

• As the streamer continues its journey towards earth until it makes contact with earth or some object on the earth.

• As the leader streamer moves towards earth ,it is accompanied by points of luminescence which travel in jumps giving rise to stepped leaders.

• The distance travelled in one step

is 50m and have sufficient

luminosity to view phenomena

of discharge


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• As the leader streamer reaches near the earth, a return streamer shoots up from the earth to the cloud

• It is similar to closing suddenly the switch between +ve and –ve terminals, a sudden spark occurs which is called as lightning

• To the normal eye lightning appears as a single flash but made up of separate strokes

• It has been found that 87% of all

lightning stokes result from

–ve charged clouds and only

13% originate from +ve charged clouds

• Lighting discharge may have currents in

the range of 10KA to 90KA


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Direct stroke • In direct stroke, the lightning discharge (i.e. current path) is

directly from the cloud to overhead line

• From the line ,the current path may be over the insulators down the pole to ground

• The direct strokes can be of two types (i)Stroke A (ii)Stroke B


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• In Stroke A ,the lightning discharge is from cloud to overhead line. The cloud will induce a charge of opposite sign on overhead line. when potential between cloud and the line exceeds the breakdown value of air ,the lightning discharge occurs between the cloud and the line

• In stroke B,let there are P,Q & R.The charge on Q is bound by the cloud R.If the cloud P shifts too near to the cloud Q,then the lightning discharge will occur between them and charges on both these clouds disappears quickly. As a result charge on cloud R suddenly becomes free and then it discharges rapidly to the earth, ignoring tall objects

• Direct strokes on the power system are very rare


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• Stroke A will always occurs on tall objects and hence protection can be provided against it, But Stoke B completely ignores the height of the object and can even strike the ground. So it is not possible to provide protection against Stroke B

Indirect stroke • These result from electro statically induced charges on the

conductors due to the presence of charged clouds

• A +ve charged cloud above the line and induces a –ve charge on the over head line by electrostatic induction. This –ve charge is confined to the portion right under the cloud only and the portions of the line away from it will be +ve charged as in fig. This induced +ve charge leakes slowly to the earth via insulators


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• When the cloud discharges to earth (or) to another cloud ,the –ve charge on the overhead line is isolated as it can’t flow quickly to earth over the insulators. The result is that –ve charge rushes along the line in both the directions in form of travelling waves. Majority of surges in a transmission line are caused by Indirect Lightning Strokes

Lightning surge A sudden rise in voltage for a very short duration on power system

Surges or transients are of temporary nature and exist for a very shorter duration(few hundred μs) ,but cause over voltages on power system

1/50μs surge means, it reaches Max. value in 1μs and decays to half of its peak value in 50 μs

This may cause damage to line insulators, nearby transformers, generators and other electrical equipments connected to it

Klydonograph is used for the measurement of surge voltage on transmission line caused by lightings of either polarity

)( btat eeEsurge

Advantage of corona • Any over voltage surge appearing on a transmission

line due to internal or external disturbances propagates in the form of travelling waves towards the ends of the line.

• The lightning surge is rapidly attenuated ,partly due to the corona as it travels along the line.

• Corona can be a natural safety valve to some extent



Need for protection • The internal causes produce voltages to twice the normal

value where as the external causes like lightning increase the system voltage to several times the operating voltage.

• It is neither possible nor economical to design the insulation of a transmission system to withstand voltages several times greater than the normal operating voltages.

• It is practicable is to design the insulation of the system to withstand voltages of twice the normal value for a reasonable length of time and to provide the protective devices

• the protective devices are adjusted to breakdown before the insulation of the system fails or otherwise prevent a dangerous voltage form breaking the insulation

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15

Protection against Lightning

Protection against Lightning

Protection against Direct strokes

Power station & substation

Earthing screen

Vertical masts

Over head transmission lines

Ground wires

Protection against Indirect strokes

(or)

Travelling waves

Lightning arresters (or)

Surge diverter

Surge absorber (or) surge modifier used to reduce the steepness of travelling wave Lightning arresters (or) surge diverters protect the station apparatus against both direct strokes and the strokes that come into the apparatus as travelling waves. 9/20/2012 16


power station/sub-station Protection • The power station and sub station can be protected

against direct lightning strokes by providing earthing screen.

• It consists of a network of copper conductors mounted all over the electrical equipment in the substation or power station.

• The shield is properly connected to earth on at least two points through low impedance.

• On the occurrence of direct stroke on the station, screen provides a low resistance path by which lightning surges are connected to ground. This method does not provide protection against the traveling waves which may reach the equipment in the station



Vertical masts or rods

• The radius of base line is ky

• Protective ratio=ky/y=k

• In a substation masts or rods are erected at the corners or over vertical columns, so that the buses and apparatus will fall with in the cone of protection

• A cone may be assumed to have a base radius equal to twice the height of the mast



Over head Transmission lines Protection • Transmission line is protected from direct strokes by running a

conductor known as ground wire over the towers or poles and earthed at regular intervals, preferably at every pole or tower.

• The wire can be run either above or below the line but running of earth wire above the line is better as it provides more effective shield.

• The selection of ground wire should be based on mechanical rather than electrical considerations.

• These should have high strength and non-corrosive. • The degree of protection by ground wire depends on the shielding

angle α



• It the angle α subtended by the outermost line conductors at the ground wire. The lower this angle ,the greater the protection(<30°),for hill-side tower α=45°

• Due to the proper location of ground wires the lightning stroke will not reach the line conductor

• When direct lightning stroke occurs ,it will be taken up by the ground wires. The heavy lightning current (10kA-50KA) from ground wire flows to ground, thus protecting line from harmful effects of lightning

• The voltage of tower rises to Vt= I R where R is tower footing resistance. The tower footing resistance should be low(10Ω-20Ω) to avoid flash over

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Requirements of Ground wire The ground wires should be mechanically strong

This is met by use of galvanized steel wire or ACSR wire

There must be sufficient clearance between power line conductors and ground wire for the given system voltage

This is met by suitable design of cross arm so that value of protecting angle is low (<30°) and by adjusting sag in such a way to avoid midspan flash overs.

The footing resistance of the tower should be as low as possible

This is met by chemical treatment of soil, which is used for improving grounds of sustations,Ground rods and Counterpoises



• The ground wire also provides electrostatic screening by reducing the voltage induced in the conductors by discharge of neighbouring cloud.

• As the ground wire is above the line conductor ,there will be increase in cy which results in reduction of induced voltage

• With a single ground wire the induced voltage is reduced to half of that without ground wire

• With two ground wires ,the reduction

is about one third and for three ground

wires, it is one fourth



Electrostatic shielding by earth wire

VCC

C

yx

x

is ground and line theacross Voltage

Merits & Demerits of Ground wires Ground wire gives protection against direct lightning strokes on

transmission lines

There is damping effect of the ground wire on the disturbance travelling along the line as it acts as a short circuited secondary

For external fields, it provides electrostatic shields to certain extent which reduces the voltage induced in the line conductors due to discharge of a neighbouring cloud

It requires additional cost

It may be possible that due to breaking of ground wire and falling on the line conductors can cause a direct short-circuit fault.

Ground wires are used for protection against direct lightning strokes for voltages ranging from 110kv to 500kv.



Ground rods • It is used for lowering the footing resistance of the tower

• Theses rods are put into the ground surrounding the tower structure

• The size of the ground rod has no effect in reducing the ground resistance

• The length of the rod plays major role

• The long rods which are thin or many small rods may be used



Counterpoise • It is a galvanized steel wire which either run in parallel, radial or

combination of two, with respect to overhead line • The corners of the squares represent the location of tower legs. When the

lightning stroke appears on tower top, it discharges to ground through the tower and then through counterpoises.

• The leakage resistance of counterpoise should be less than the surge impedance to avoid the raising of tower voltage rather than lowering due to positive reflections of the surge

• It is recommended to have more short counterpoises than a single continuous counterpoise which can discharge the surge to ground more quickly.

• But while designing short ones its surge impedance should not be smaller than leakage resistance to avoid raising of tower voltage



Lightning Arresters • It consists of a spark gap in series with a non-linear resistor.

One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded.

• The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc.

• The property of the non-linear resistance is that its resistance increases as the voltage (or current) increases and vice-versa.

Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting

On the occurrence of over voltage, the air insulation across the gap breaks down and an arc is formed providing a low resistance path for the surge to the ground.

After the surge is over, the resistor offers high resistance to make the gap non-conducting.


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Types of Lightning Arresters

Types of Lightning Arresters

Rod gap

Horn gap

Multi gap

Expulsion type (protector tube)

Value type

Metal oxide(Zno) 9/20/2012 27


Rod gap (or) Spark gap L.A • It is a very simple type of diverter and consists of two 1.5 cm rods,

which are bent at right angles with a gap in between

• One rod is connected to the line circuit and the other rod is connected to earth.

• The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it.

• Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators.

• Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly conducted to earth


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After the surge is over, the arc in the gap is maintained by the normal supply voltage, leading to short-circuit on the system. The rods may melt or get damaged due to excessive heat produced by the arc. The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester. The polarity of the f the surge also affects the performance of this arrester.

Due to the above limitations, the rod gap arrester is only used as a back-up protection in case of main arresters.

Horn gap L.A • It consists of a horn shaped metal rods A and B separated by a small air gap. The horns are

so constructed that distance between them gradually increases towards the top

• The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and choke coil L while the other end is effectively grounded.

• The resistance R helps in limiting the follow current to a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be protected.

• The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the gap

• On the occurrence of an over voltage, spark-over takes place across the small gap G. The heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves progressively into positions 1, 2 and 3


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• At some position of the arc (position 3), the distance may be too great for the voltage to maintain the arc; consequently, the arc is extinguished. The excess charge on the line is thus conducted through the arrester to the ground

Time of operation is large

The bridging of gap by external agency can render device useless

The setting of horn gap is likely to change due to corrosion

Multi gap L.A

• It consists of a series of metallic (generally alloy of zinc) cylinders insulated from one another and separated by small intervals of air gaps. The first cylinder (i.e. A) in the series is connected to the line and the others to the ground through a series resistance.

• The series resistance limits the power arc. By the inclusion of series resistance, the degree of protection against traveling waves is reduced.

• In order to overcome this difficulty, some of the gaps (B to C) are shunted by resistance. • Under normal conditions, the point B is at earth potential and the normal supply voltage is

unable to break down the series gaps. On the occurrence an over voltage, the breakdown of series gaps A to B occurs.

• The heavy current after breakdown will choose the straight – through path to earth via the shunted gaps B and C, instead of the alternative path through the shunt resistance.

• Hence the surge is over, the arcs B to C go out and any power current following the surge is limited by the two resistances (shunt resistance and series resistance) which are now in series.

• The current is too small to maintain the arcs in the gaps A to B and normal conditions are restored. Such arresters can be employed where system voltage does not exceed 33kV.

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Expulsion type(or) protector tube L.A • It is used on system operating at voltages up to 33kV. • It essentially consists of a rod gap AA’ in series with a

second gap enclosed within the fiber tube. • The gap in the fiber tube is formed by two

electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth.

• One expulsion arrester is placed under each line conductor. Fig shows the installation of expulsion arrester on an overhead line.


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• On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is stuck between the electrodes in the tube. •The heat of the arc vaporizes some of the fiber of tube walls resulting in the production of neutral gas.

• In an extremely short time, the gas builds up high pressure and is expelled through the lower electrode, which is hollow.

• As the gas leaves the tube violently it carries away ionized air around the arc. This de ionizing effect is generally so strong that the arc goes out at a current zero and will not be re-established

They are not very expensive. They are improved form of rod gap arresters as they block the

flow of power frequency follow currents They can be easily installed.

An expulsion type arrester can perform only limited number of operations as during each operation some of the fiber material is used up. This type of arrester cannot be mounted on enclosed equipment due to discharge of gases during operation. Due to the poor volt/am characteristic of the arrester, it is not suitable for protection of expensive equipment



Value type L.A • Valve type arresters consists of two assemblies (i) series spark gaps

and (ii) non-linear resistor discs in series.

• The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container.

• The spark gap is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing.


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• The voltage distribution across the gap is line raised by means of additional resistance elements called grading resistors across the gap.

• The spacing of the series gaps is such that it will withstand the normal circuit voltage.

• However an over voltage will cause the gap to break down causing the surge current to ground via the non-linear resistors.

• The non-linear resistor discs are made of inorganic compound such as thyrite or metrosil. These discs are connected in series.



CNK

KII

K

I

K

I

KI

KIV

RI i.e.

NKIR discs thyriteN haveL.A caseIn

I

VRdiverter of Resistance

-

1

• They provide very effective protection against surges and operate very rapidly

They fail to check the surges of very steep wave front from reaching the terminal apparatus

Their performance is adversely affected by the entry of moisture into enclosure. This need effective sealing all the time

Station type Valve L.A can be used for large station power equipment in the circuits of 2.2kv to 400kv

Line type Valve L.A are used for sub-station power equipment for voltages up to 66 kv

Distribution type Valve L.A are used for pole mounted power distribution Transformer 2.2kv to 22kv

Secondary Valve L.A are used for low voltage apparatus 120 to 750 V

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35

1conditionfrequency lowunder gedown voltaBreak

condition surgeunder gedown voltaBreak ratio Impulse•

• A line of 500 KV having Surge impedance of 500 is terminated through a Thyrite Surge Arrester. The Diverter has a characteristic RI0.72=72000.Caluclate surge current and Resistance of Diverter

• Sol:



49872000

Rdiverter of Resistance

.10500

10*500

line of impedance surge

voltagesurgecurrent surge

72.0

3

3

I

Amp

Metal Oxide L.A • This arrester has its element with non-linear resistance

characteristics. • The main constituent of arrester element being Zinc Oxide ,it

is economical as compared to thyrite arresters • The degree of non-linearity of this arrester element is much

more than that in the thyrite arrester. • No series gap is required. • Zno metal type is of simple construction, better surge

protection and more stable protective characteristics • It is important for the test personnel to be aware that when a

metal oxide arrester is disconnected from an energized line a small amount of static charge can be retained by the arrester. As a safety precaution, the tester should install a temporary ground to discharge any stored energy.

9/20/2012 37


Ratings of L.A



Installation of Arrester • The arrester should be connected to ground to a low resistance for effective

discharge of the surge current.

• The arrester should be mounted close to the equipment to be protected & connected with shortest possible lead on both the line & ground side to reduce the inductive effects of the leads while discharging large surge current



Lightning Arrester vs. Lightning conductor

• It is a device for limiting surge voltage near an equipment by discharging or by passing surge current

• It is located very close to the equipment to be protected

• Used for protection of Transformers, Transmission lines etc.

• It consist of different parts • Prevents damage against

indirect lightning strokes i.e. travelling waves

• Lightning conductor or lightning rod is a shielding device used for protection of buildings

• It is located at a point above the structure to be protected with a ground connection so that lighting gets diverted into the ground

• Used for buildings, storage tanks for petroleum products

• It is a single rod made up of good conducting material

• Prevents damage against direct lightning strokes



Surge Absorber (or) Surge Modifier • The travelling waves set up on the transmission

lines by the surges may reach the terminal apparatus and cause damage

• The amount of damage caused not only depends upon the amplitude of surge but also upon the steepness of its wave front


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• The surge absorber absorbs the surge energy • A condenser connected between line and earth acts as a surge absorber

to protect transformer winding. The reactance Xc low at high f and high at low f

• Since surges are of high frequency; Xc is low, so capacitor acts as short circuit and passes them directly to earth .For power frequency Xc is high, so no current flows through ground

• The choke offers high reactance to surge frequency so surge forced to flow through resistance R,and made to dissipate

• The steeper the wave front of surge, the more damage caused to equipment. So in order to reduce the steepness of wave front of surge, we use surge absorber or surge modifier

Ferranti Surge Absorber

• Ferranti surge absorber consist of an air cooled inductor connected in

series with the line • The inductor is surrounded by but insulated from an earthed metallic

sheet called dissipator • This is equivalent to a transformer with short circuited secondary • Inductor form primary where as dissipator forms short circuited

secondary • The energy of surge is used up in the form of heat generated in the

dissipator due to transformer action



Complete protection of power Transformer



Surge Arrester vs. Surge Modifier • It provides the protection

against travelling waves reaching the station equipment

• In this surge is diverted to the ground

• In essential parts are spark gap and non-linear resistor

• It should be connected close and across the incoming terminals of the equipment to be protected

• It reduces the steepness of the waves reaching the station equipment

• In this the energy of the surge is absorbed i.e. dissipated as corona loss

• The essentials parts are choke or capacitors

• It should be connected close and either across or in series at the incoming terminals of the equipment to be protected



Insulation Coordination • It is correlation of insulation of electrical equipment and lines with

the characteristics of protective devices such that the insulation of the whole power system is protected from over voltages

• Suitable value of insulation level for different components is chosen keeping in the view overall power system protection

• The insulation strength of various equipment like transformers, circuit breakers etc. should be higher than that of lightning arresters and other surge protective devices

• Proper insulation coordination ensures

that voltage-time curves of equipment B

will lie above those of protecting devices A

such as LA



• The common insulation level for all the insulation in the station is called Basic Impulse Insulation Level (BIL)

• According to Indian standards BIL is expressed in impulse crest voltage with a standard wave not longer than 1.2/50μ sec

• Apparatus insulation by suitable tests should be ≥ BIL

• A lightning arrester is required to protect a 5MVA 66kv/11kv transformer. The transformer is connected to a 66kv system and the voltage fluctuations are of the order of 10%.The neutral of the transformer and that of the network are directly and solidly grounded. Choose a suitable arrester rating from the table

Take the following into consideration: B.I.L for 66kv :350kv Dynamic over-voltage:1.3 time system operating voltage Power frequency breakdown voltage of arrester: 1.5 times rating of arrester in kv Allow 5% margin for lower limit of arrester rating calculation. Check the margin available on

the arrester chosen with reference to BIL Sol: Voltage rating of arrester=1.1*66*0.8*1.05 (5% margin in calculations) =60.98kv choose the arrester rating=60 kv The discharge voltage of 60 kv arrester from table=220kv protective margin=BIL-220kv=350-220=130kv Power frequency breakdown voltage of arrester=1.5 *60=90kv Dynamic over-voltage=1.3*66/√3=50kv Power frequency breakdown voltage is > Dynamic over voltage that 60 kv arrester rating is chosen 9/20/2012


46

Summary



voltage surge or transient voltage

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