Over voltage protection.pptx

7/27/2019 Over voltage protection.pptx

1/65

Over voltages

The insulation strength and characteristics ofvarious components of a system (including those ofvoltage limiting devices) must be selected relatingto those stresses.

i. To reduce frequency of supply interruptionsii. To reduce component failures

The selected level of voltage shall be low enoughto be operationally and economically acceptable


2/65

Causes of over voltage:

Phase to earth faults ( it is assumed that resulting temporaryvoltages will not exceed

1.4 Pu for solidly earthed networks1.7 Pu for resistance earthed networks

2.0 Pu for reactance earthed networks Load rejection (supplying capacitive current through a large

inductive reactance ex. A smaller generator connected to along cable or over head line)

Ferro resonance ( inter change of stored energy for series orparallel combination of inductive and capacitive reactance)


3/65

Causes of over voltage: contd.

Ferranti effect: (receiving end voltage greater thansending end voltage under no load or light load

conditions)

By care full design and natural earthing sustainedover voltages involving resonance and arcing ground

faults are eliminated

Below 145 kV method of earthing will normallydetermine the level temporary over voltages.


4/65

Switching surges

They are of short duration and irregular form Typical switching impulse standard form is the 250/2500 sec.

( time to crest/ time to half value wave)

The magnitude of internally operated switching surges is

related to the system operating voltage In a system where CBS are not subjected to multi re strikingthe switching surges will rarely exceed 3 pu

2.5 pu would be typical maximum based on which thedischarge duty of LA is assessed

However in systems above 300 kV, it may be necessary tosuppress maximum switching surges to 2 pu or less by theinstallation of a shunt reactor and/or closing resistors on thecircuit breakers


5/65

Resonance effects

For voltage level below 300 kV. Resonanceeffects occur

i. When switching transformer

ii. When switching cable and overhead line

combination

iii. Between lumped capacitive and reactive

elements and over head linesiv. Charging long lines without shunt reactor

compensation


6/65

Resonance effects-- contd

Ferro resonance encountered on a transformerfeeder greater than 5 to 10 Km in length

When one feeder/transformer on a double circuit is

switched out but parallel feeder remains energized,the dead circuit draws energy by captive couplingfrom the parallel line circuit which resonates withtransformer impedance at a sub harmonic frequency

(operation procedure such as opening the lineisolator at the transformer end on the disconnectedcircuit will eliminate the problem)


7/65

Mode of action of flash over on a line

A lightning flash can impress over voltage ona over head line by

a) Induction when it discharges to earth close to

line

b) By direct contact on the line either to the

earthed structure or to the phase conductor


8/65

Induced Voltage Surge

A close flash to ground up to about 14 m away caninduce a voltage rise on phase conductors

The highest amplitude normally associated is in theregion of 200 kV

Significant in case of low voltage lines

At 11 kV estimated that it accounts for some 90%of all faults

Little significance on lines of 275 kV and above


9/65

Direct stroke

A direct stroke can be to the earthed tower topor on phase conductor

Stroke on earthed lower top, for transmission ofshielded design, is innocuous

Raise in potential caused by passage of current

through tower impedance to earth will be lessthan with stand strength of line


10/65

Direct strokecontd.

However the rise in potential can be severeand exceed with stand capability, if

Tower footing resistance is high

Rate of rise of current exceeds a certain level

Flashover may occur

Through the system voltage, losses is thefrequency of flash over


11/65

Direct strokecontd.

Direct stroke on phase conductor

May occur if there is a shielding failure i.e. stroke avoids earthwire and lands on line conductor. Discharge current flows equally in both directions. Impedance to earth is half the surge impedance (Z0) of the

conductor. IN a 400 kV line Z0

= 175 ohms

Voltage rise is sufficient to cause failure of line insulation Minimum critical current for flash over Ic = 2 V I0

Z0VI0 = minimum flash over voltage for 1/50 Wave

At flash over the impedance through which the dischargecurrent flows drops abruptly from Z0/2 to impedance of tower, x-arm, tower footing


12/65

Surge propagation:

Surge waves are propagated at the velocity of lightalong the conductor

On arrival at substation, equipment there in getstressed. Rod gaps and surge arrestors provide necessary

protection

Waves are subjected to considerable attenuationsdue to losses both in the conductor (ohmic losses)and corona losses


13/65

Lightning discharges

Clarification of lightning dischargesstroke (A)stroke (B)

Stroke (A) : produced by the charged cloud which induces acharge on the stationery objects such as high buildings etc.

Charge distribution causes concentration of potential at the topmost point

Electro static stress being great at that point ionization ofsurrounding atmosphere takes place

Dielectric strength of surrounding air decreases giving an easypath to lightning stroke.

Decrease in dielectric strength of surrounding air takesconsiderable time


14/65

Lightning discharges

Stroke B:

A, B & C are three clouds with A and C positively charged andB negatively charged

When there is a stroke between (A) and (B) the charge on (C)

becomes free and immediately and indiscriminately strikeson any object on the ground

For stroke (B) there is no time lag Stroke (B) may completely ignore highest building and strike

bare ground.

No protection can be arranged against stroke `B` Stroke `A` can be made safe by channelising the chargethrough a lightning conductor placed on the top of thebuilding


15/65

Static induced charges

An over head conductor accumulates staticallyinduced charge when a charged cloud comes above

When the cloud is swept away charge on the

conductor is released The charge travels on either side giving rise to

two travelling waves

The earth wire does not prevent such surges


16/65

Lightning strokes

Over voltage due to lightning strokessurge impedance of the line = ZsDischarge current = IdOver voltage due to direct stroke = Vd = Id x ZsHowever current travels in both directions

over voltage = Vd = Id x Zs2

when lightning strikes over earth wire or a tower

Over voltage = Id x Ze + Lc didt

Ze = impedance of earth wire

Lc is the inductance of the line conductor


17/65

Protection against lightning

1. Protection of transmission lines from

direct strokes

2. Protection of power stations and

substations from direct strokes3. Protection of electrical equipment

from traveling waves


18/65

Protection of transmission lines

Against the direct strokes : Most harmful

Effective protection required shielding to

prevent lightning from striking the electricalconductors.

There shall be adequate drain facilities so that

the charge can be grounded without affectingInsulators or line conductors.


19/65

Design of transmission line against

lightning

Design shall consists of(a) General wire of adequate mechanical strength to provide shielding

for line conductor. They shall also be noncorrosive

Resistance of ground wire shall be low for better protection againstdirect stroke.

(b) Adequate clearance between1. Line conductor and tower

2. Line conductor and earth

3. Clearance between line conductor and ground wire all throughthe span including mid Span or point of lowest sag.

(c) Tower footing resistance shall be low(d) Angle of protection (shielding angle) angle between the normal

passing through the ground wire and line joining the supportedcenter points of outer conductor and ground wire. It shall be 30for 132 & 220 kV lines 20 for 400 kV lines


20/65

Effect of number of earth wires In the absence of a ground wire: When there is a charge cloud over a transmission line without

any ground wire

There will be two capacitances(1) Between cloud and conductor C2(2) Between conductor and earth C1Induced voltage on the line

V L1 = C1 x EcC1+C2

When ground wire is present it increases capacitance betweenconductor and earth i.e. C1 Decreases induced voltage on theline.

It is observed that presence of a ground wire reduces inducedvoltage on line to half.

For two ground wires the induced voltage comes down to onethird

Presence of two ground wires also provides better shielding


21/65

Earth wires

Disadvantages with ground wire:

(a) higher line cost

(b) Probable direct shorting between lineconductor and ground wire when the later

gets cut

In 400kV system transmission line towers will

have twu earth wires.


22/65

Alternative method of line protection

Even after providing ground and reducing the likelyinduced voltages, harmful voltages can still develop

Lightning arrestors act as additional protectivedevices by by-passing the surges to ground

Protector tube is a fiber tube with electrode at earthend.

Fitted directly below the conductor

The arc type electrode on the top of the tube forms aseries gap with conductor


23/65

Alternative method of line protection

The lower electrode is solidly grounded In case of surge on the conductor, an arc develops

between conductor and top electrode of the tube.

Arc shifts within the tube and vaporises some of thefiber of tube wall to emit gases which will quench thearc

This tube successfully prevents re-striking

The break down voltage of tube shall be less thanflash over voltage of the insulation.


24/65

Protection against traveling waves

The traveling waves cause the following damages:i. High peak voltage of surge may cause flash over in the

internal winding or external flashover between theterminals of the equipment.

ii. steep wave front may cause internal flash over betweenturns of the transformer

iii. Steep wave front resulting into resonance and highvoltage may cause internal or external flash over causingbuilding up of oscillations in the equipment

Protective equipment : LAs and Surge diverters They are connected between line and earth


25/65

Action of the Surge diverter

A traveling wave reaches surge diverter and attains aprefixed voltage

A spark is formed across the gap

The diversion provides a low impedance path to earth

The surge impedance of the line limits the amplitude ofthe current flowing to earth to prevent break down of

insulation

Important aspect is that the surge diverter shallprovide low impedance path to earth only when

traveling surge reaches the surge diverters


26/65

Action of the Surge diverter It shall absorb any current during normal operation

for over voltage surges.

It means that it shall not function at powerfrequencies but function only when abnormalfrequencies are applied

When there is a discharge through them they shall becapable of carrying the discharge current for sometime interval.

After the over voltage discharge it must be capableinterrupting normal frequency current from flowing toearth as soon as the voltage reaches below the breakdown value


27/65

Switching over voltage protection in a

substation

Operation of breakers causes transient over voltages Over voltage value varying between 1.1 Pu to 6 Pu based on

switching duty and the type of circuit breaker Over voltage occurs mainly due to exchange of energy

between system inductance LI2 and system capacitance

CV2

Over voltage occurs during the opening of circuits and closingof long EHV lines

Most severe over voltages occurs during the closing unloadedtransmission line

Preventive measure Provision of Pre insertion resistors ( 400 to 800 ohms per

phase) Simultaneous closing of lines at both ends Using shunt reactors, surge arresters etc.


28/65

Switching Over voltages in Substations

Switching duty of

C.B.

Applications and

Remedial Actions

Phenomena

Opening of capacitor

bank currents, cable

charging circuits, filter

banks

Switching of shunt

capacitor banks used for

p.f. correction.

- Use of re strike free C.B.

for capacitor switching

duty.

Re strike in circuit

breakers giving over

voltage.

EHV lines

* Closing unloaded

lines* Closing charged

lines

* Auto re closing of

C.B.

* Long EHV transmission.

- Use of pre-closing

resistors with circuitbreakers. Use of lightning

arresters. Use of shunt

reactors in transmission

lines.

Traveling waves

travel to and fro

giving rise to a

switching surge.


29/65

Methods of Reducing Switching Over

Voltages

Switching operation

causing over voltage

Method to reduce

switching over voltage

Energising an uncharged

line

High voltage shunt reactors

are connected to line toreduce power frequency

over voltages.

Elimination of trapped

charged on the line

Line shunting after opening

by means of earthing switchReduction of current

chopping

Opening resistors (

Resistance switching with

CB) used only with ABCB


30/65

Methods of Reducing Switching Over

Voltages

Switching operationcausing over voltage

Method to reduceswitching over voltage

Reducing the switching over

voltages due to closing

Single stage pre closing resistor

insertion with CB.

Two stage pre closing resistor

insertion with CB.

Closing resistors in between circuit

breaker and shunt reactor

Reducing switching over voltages

by improved switching sequence

Synchronous switching of three

poles.Simultaneous operation of circuit

breakers at both ends of line,

Use of surge arrestors While closing of line

While disconnecting reactor


31/65

Rod gaps or coordinating gaps

They are used on insulators, equipment and bushings

Conducting rods are provided between line terminaland earth terminal with an adjustable gap ( Airinsulation)

Rods are of 12mm dia approx.

The gap is adjusted to break down at about 20% belowthe flash over voltage of the insulation. Spark over causes dead Short circuit Voltage of phase with respect to ground falls very low

The rod gaps are no more used consequent todevelopment of surge arrestors.

O lt i N t k d R di


32/65

Over-voltage in Network and Remedies

Phenomena Causes Effect Remedies

Surges Lightning strokes on

overhead lines orsubstation

Line insulation flash

over or puncture.The traveling wave

reaches substations.

The insulation of

equipment is

stressed by impulse

surge

-Use of Ground

wire

- Surge Diverters

-Earthing of

towers

-Lightning Masts

Switching

surges

Breaking inductive circuit,

the energy stored

inductance gives rise a

voltage rise across

capacitor.

Switching of capacitive,

line charging currents give

rise to a over voltage due

to restrike. Closing of EHV

lines

Wave travels from

C.B. to both sides

Transmission line

insulator, stressed.

Terminal apparatus

insulation stressed

-Use of opening

resistors with C.B.

- Use of restrike

free C.B.

-Use pre-insertion

resistors with C.B.


33/65

Over-voltage in Network and Remedies

Phenomena Causes Effect Remedies

Resonance The fault causingresonance between

inductance and

capacitance in a part of

the circuit

Very high, voltagesurges occur.

Insulation failure

likely to occur.

Filters toeliminate

harmonics

Travelingwaves

High voltage waves getreflected on reachinga junction or end.

Reflected wavesgets superimposed

for initial wave.

Voltage may rise to

several time the

normal voltage.

-Properswitching

sequence.

Sustained

Power

frequency

over

voltage

Poor voltage control Failure of

transformers and

Rotating Machines

-Proper Voltage

control


34/65

Protective Devices Against Lightning Over voltages

Device Where applied Remarks

Rod gaps Across insulator string,bushing insulators -Difficult to coordinate-Create dead short

circuit

-Cheap

Overhead Ground

Wires (earthed)

-Above overhead lines

-Above the substationarea

-Provide effective

protection againstdirect strokes on line

conductors towers sub

station equipment

Vertical Masts in

substations

-- in sub stations -instead of providing

overhead shieldingwires

Lightning Masts/Rods

(earthed)

- Above tall buildings Protect buildings

against direct strokes.

Angle of Protection

= 300


35/65

Protective Devices Against Lightning Over voltages

Device Where applied Remarks

Surge Arresters -- on incoming lines in each

substation

-Near terminals of

Transformers and

generators

-Near motor and

generators terminals

-- Diverts over voltage to

earth without causing

short circuit

-Used at every voltage

level in every sub-station and for each line.

Surge Absorbers -- near rotating machines

connected between phaseand ground

-Resistance

CapacitanceCombination absorbs

the over voltage surge

and reduces steepness

of wave


36/65

Lightning arrester selection

1. To determine the magnitude of the power frequency phase to ground voltageexpected at the proposed arrester location during phase to ground fault, or otherabnormal conditions which cause higher voltages to ground than normal.

2. To make a tentative selection of the power frequency voltage rating of thearrester. This selection may have to be reconsidered after step (6) is completed.

3. To select the impulse current likely to be discharged through the arrester.

4. To determine the maximum arrester discharge voltage for the impulse currentand type of arrester selected. 5. To establish the full-wave impulse voltage withstand level of the equipment to

be protected.

6. To make certain that the maximum arrester discharge voltage is below the fullwave impulse, withstand level of the equipment insulation to be protected, byadequate margin.

7. To establish the separation limit between the arrester and the equipment to beprotected.


37/65

Types of Earthing

For purpose of selection of voltage rating of a LA threetypes of earthing are considered

(I) Effective earthed system: a system is effectively earthed

if under any fault condition the line to earth voltages of

healthy phases do not exceed 80 % of the system line toline voltage

If in a system all transformers have star connectedwinding with neutrally solidly earthed then the system

is effectively earthed

However if only few transformers are earthed like that,it is not effectively earthed system


38/65

Types of Earthing - conted.

(II) Non effectively earthed system:

a) if the line to earth voltage in healthy phases in case of afault exceed 80% of the line to line voltage but does not exceed100% of it, the system is called non effectively earthed system

b) System with few solidly earthed neutrals

c) Systems with neutral Earthed through resistors orreactors of low ohmic value or arc suppression coil

(III) Isolated or un earthed neutral systems :-

system neutrals are not earthed. Line to earth voltage of healthyphases exceed 100% of the line to line voltage.


39/65

Selection of lightening arrestors

Tentative selection of arrestor Voltage:

Arrestor Voltage rating shall not be less thanproduct of system highest voltage x co-

efficient of earthing

Co-efficient of earthing : Effectively earthed system 80%

Non effectively earthed system - 100 %

and isolated earth system


40/65

Selection of lightening arrestors

In a 220 kV effectively earthed system Highest system voltage = 245 kV Co-efficient of earthing = 80%

Arrestor voltage rating >= 245x0.8 = 196 kV As per IS 3070 (partI) 1965 the rating is 198 kV

By going for a higher voltage rating for a surge

arrestor, the degree of protection forequipment gets reduced.


41/65

Selection of arrestor discharge current

This can be calculated from(a) Spark over voltage of transmission line insulation

(b) Surge impedance of the line

(c) Residual discharge voltage of LAIa = 2E- Ea

Z

Ia = Arrestor discharge current

E = Magnitude of incoming surge voltage

Ea = Residual discharge voltage of an arrestor

Z = Surge impedance of the line


42/65

Selection of arrestor discharge current

In a 220 kV system using 11 insulators Transmissionline will not permit a traveling wave of a value morethan 1025 kVp

As per IS 3010 (Part 1) -1965 the residual voltages of

LA at a discharge current of 10kA is 649 kV. Considering the surge impedance as 450 ohms Maximum value of discharge current of LA =

2(1025000)-649000 = 3100 Amps

450 The LAs normally in 200 kV system have a discharge

current rating of 10 kA.


43/65

Selection of arrestor discharge Voltage

Most important characteristic of LA determining the protectionlevel being offered

The arrestor discharge voltage shall be less than BIL ofequipment for effective protection

Discharge voltage depends on(I) discharge current

(II) rate of rise of current applied(III) Wave shape of current applied

Discharge voltage of LA increases with discharge current. Butincrease is much restricted due to nonlinear resistanceproperty.

Increase in discharge from 5 kA to 20 kA produces only 25% risein discharge voltage. Increase in rate of current from 1000 to 5000 Amps per micro

second increases discharge voltage by only 35%.


44/65

Protective margin of LA

Protective margin of LA = BIL of the equipment---maximum discharge voltage of LA

While determining protection level offered by a LA 10%allowances towards drop in lead length and manufacturingtolerance shall be allowed.

Protective margin shall be 20% of the BIL of the equipmentwhen closely located

In a 220 kV systemDischarge voltage of LA = 649 kV

Allowing 10 % margin protection level = 713 kVBIL of equipment = 900 kVp

Protection margin = 900-713 = 187 kVp

There is more than 20 % of the BIL of 180 kV


45/65

Protective margin of LA-Continue.

In American system

Average discharge voltage x 1.25 +40 kV = BIL

protected

When adequate margin is not available LAs

with lower rating shall be chosen taking risk.


46/65

Insulation Co-ordination Scheme

For 220 KV system. L.A. Voltage rating=system highest voltage x co-efficient of earthing =245x.8=196Kv. Selecting standard rating from Table 12.1 column 1,L.A. voltage rating=198 Kv Discharge current rating= 10KA (assumed) Residual voltage, from column 3 of table 12.1,=649Kv (peak) Protection level of the L.A. =649x1.1=714Kv For a margin of 20% between the B.I.L. and the protection level of L.A., the B.I.L. should be

=714x1.2=857Kv.

Choose standard B.I.L. Table 14.3 (b) Col. 4=900 Kv, The corresponding power freq. I minute test voltage =395kv Switching surge flashover voltage =220 x6.5=825kv 3 Check it is less than B.I.L. of 900kv.

Power frequency over voltage=220x3=228kv rms 3 This is less than 395kv. B.I.L. of CBs, instrument transformer, disconnect switches etc,.=900x1.1=990kv. Choose standard B.I.L.=1175kv.


47/65

The L.A. voltage rating

Rated system

voltage KV

Highest system

voltage KV

Arrester rating

in KV

132 145 120/132

220 245 198/216

400 420 336


48/65

Establishment of Separation Limit

When arrestor are to be located away from equipment. A traveling wave coming into the station to location to the discharge voltage of the arrestor. Proximity to transformer or breakers.

- Transformer is most expensive price.

- Repair to transformer is costly and with higher revenue loss.

- Transformers are always at the end of a circuit where voltage regulation.

. For circuit breakers and disconnecting switches flash over distance between terminals when inopen position in grater than between terminals and ground.

. Surge in excess to insulation strength will flash over to ground with out damaging theequipment.

. At best there can be only outage .

. By reducing BIL of transformer savings in the cost of insulation can be obtained.

. Not possible incase of CB or disconnections switches.

. Hence a set of LAS shall be closer to transformers.


49/65

Location of Lightning Arresters:

The electrical circuit length between L.A. and the transformer bushingterminal (inclusive of lead length in metes for effectively earthed) should

not exceed the limits given below:

Rated syst.voltage KV BIL KVPeak Max.distance

132kV 550

650

35.0

45.0

220kV

400kV

900/1050

1425/1550

Closer

to

Trans.


50/65

Definition:- Flash over voltages

Impulse flash over voltage:-

The voltage which will cause flash over of anInsulation When subjected to a 1.2x50s

impulse

(British standards1x50 sec)

(American standards 1.5 x 40sec)


51/65

Definition:- Flash over voltages

Basic Insulation level :-The crest voltage of standard wave that will notcause flashover of the insulation is referred to asBasic insulation level

(Basic impulse insulation voltages are levelsexpressed in impulse crest voltage with a standardwave not longer than 1.2x50 s.

Equipment insulation as tested shall be equal or

above the BIL


52/65

Impulse spark over volt- time characteristic

This characteristic is obtained by plotting--Time which elapses between the moment the voltage wave

is applied and the moment of spark over -- on abscissa

-Voltage at the moment of spark over

(i) Occurring on the wave front(ii) Occurring on the wave peaks

(iii) Crest of the voltage for spark overoccurring on the wave tail


53/65

LINE INSULATION

Extra high voltage line can be made lightning proofby

1 Efficient shielding

2 Low tower footing resistance equal to or less than10 ohms

shielding angle

Transmission lines up to 220kV 30400 kV at and above 20


54/65


55/65

Line insulation -contd.

Line insulation shall be sufficient to prevent aflashover from the power-frequency over

voltages and Switching Surges.

It shall take into consideration the local unfavourable circumstances which decrease the

flash over voltage (rain, dirt, Insulation

pollution etc.,)


56/65

OVER VOLTAGE FACTORS

Line

Voltages

Switching

Surge flash

over

Power frequency flash

over (Dry & Wet)

220kV 6.5 V pn 0.3

400kV 5.0 V pn 3.3

Vpn = Phase to Neutral Voltage (rms)

Add one or two more Insulators for each string.


57/65

OVER VOLTAGE FACTORSContd.

-To take care of one disc in the string

becoming defective.

-Facilitate hot line maintenanceUp to 220 kV Line 1 disc for each string

400 kV Line 2 discs for each string

RECOMMENDED INSULATION LEVEL OF


58/65


LINE

Normal

system

Voltage

Vpn

In kV

(Vph/3)

Switching over

volt. (Wet) kV *

No of

discs

required

132kV 76 76 x6.5=495 5

220kV 127 127x6.5=825 9400kV 231 231x5=1755 13

* Compared with Impulse FOV (Value)



59/65


LINEcontd.

Normal

system

Voltage

Vpn

In kV

Power freq.

over volt

(wet)

(kVrms)

No.

of

discs

req.

No. of

discs

recom.

As per

practice

132kV 76 76x3=228 6 7 9/10

220kV 127 127x3=381 10 11 13/14

400kV 231 231x3=762 20 22 23/24


60/65

Tower forting resistance 10ohms severest lightning discharge 50kA (rms) Impulse strength of

Insulation=2x50x10x10=700kV

As per the table for 7 discs, the impulse FOV (kVp =695kVp)

For better performance tower fortingresistance shall be brought down.

For 132kV best is 7 ohms

C di ti f li I l ti d S b


61/65

Co-ordination of line Insulation and Sub-

Station Insulation

Line Insulation is not directly related to theInsulation of equipment within the Sub-Station.

Impulse flash over voltage of line Insulation

determine the highest surge voltage that can travelinto the sub-station.

Current through lighting arrestor can be calculatedfrom

1 Surge impudence of line

2 Surge voltage arriving over the line

Co ordination of line Ins lation and S b


62/65

Co-ordination of line Insulation and Sub-

Station Insulation

Discharge voltage of the LA on that current isthe basic protective level of the substation

equipment.

Discharge voltage across LA varies with surgecurrent.

BASIC INSULATION LEVEL AS PER IS (2165


63/65

BASIC INSULATION LEVEL AS PER IS (2165

1962)

Nominalsystem

volt kV

(rms)

Highestsystem

volt kV

(rms)

Impulse withstandvolt kVp for test

One minute powerfrequent volt kV (rms)

Full

insulation

Reduced

insulation

Full

insulation

Reduced

insulation

132 kV 145 650 550 275 230

220 kV 245 1050 900 460 395

400 kV 420 1550 680

1425 630Reduced insulation is used where system is effectively earthed.


64/65

INSULATION LEVELS OF EQUIPMENT

Transformers, Isolators, Instrument Transformers aremanufactured for the standard Insulation level.

Some times transformers, are manufactured for onestep lower insulation level for the sake of economy.

(LAs will be designed for a still lower level)

Where LAs are provided right on the top of thetransformer, some of the equipment may lie well outside the protective zone of the LA.


65/65

INSULATION LEVELS OF EQUIPMENT

Protective zone is determined based on

A With stand level of equipment

B Discharge volt of LA

C Distance between LA and equipment.

Such equipment shall be designed for one stephigher Bil.

Generally BIL of substation equipment other thantransformer are designed for10% higher BIL than thatof Transformer .

Over voltage protection.pptx

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