6654882 Presentation on Insulation Coordination

WelcomeWelcomeTO A PRESENTATION ON

INSULATION- CO-ORDINATION

BY

A.SAI PRASAD SARMA

INSULATION CO-ORDINATION

• It is selection of suitable insulation levels of various components in any electrical system and their rational arrangement.

• It is required to ensure1) Insulation shall withstand all normal stresses and majority of

abnormal ones2) Efficient discharge of over voltages due to internal /external

causes

3) B/D shall be only due to external causes4) B/D shall be at such places where least damage is caused5) Safety of operating personnel and public.

Determination of Insulation coordination – contd.

Steps in the determination of Insulation coordination

• Determination of live Insulation

• Selection of BIL and Insulation levels of other equipment

• Selection of Lightning Arrestors.

Definition:- Flash over voltages

• Dry flash over voltage (Dry for) Power frequency voltage. Which will cause flashover of the Insulation.

• Wet flash over voltage:- Power frequency voltage. Which will cause flash- over when sprayed with water of a resistance 9000-11000 ohm-cms drawn from a source of supply at a temp within 10°c of the ambient temperature in the neighbour- hood of insulation under testing and directed at an angle of 45° the volume of water being equivalent to precipitation of 0.305 cm /min

Definition:- Flash over voltages

Impulse flash over voltage:-

• The voltage which will cause flash over of an Insulation When subjected to a 1.2x50µs impulse

• (British standards1x50µ sec)• (American standards 1.5 x 40µsec)

Definition:- Flash over voltages

• Basic Insulation level :-The crest voltage of standard wave that will not cause flashover of the insulation is referred to as “Basic insulation level”(Basic impulse insulation voltages are levels expressed in impulse crest voltage with a standard wave not longer than 1.2x50 µs (Indian standards)Equipment insulation as tested shall be equal or above the BIL

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 movement of spark over (i) Occurring on the wave front

(ii) Occurring on the wave peaks (iii) Crest of the voltage for spark over

occurring on the wave tail

Impulse spark over volt- time characteristic -contd.

• This characteristic is established by means of a 1/50 impulse wave

• A line drawn meeting the three B/D values is the characteristic

• Proper insulation co-ordination will ensure that the voltage time Curve of any equipment will lie above the volt -time curve of the protective equipment, say, Lightning arrestor.

LINE INSULATION

• Extra high voltage line can be made lightning proof by

1 Efficient shielding

2 Low tower footing resistance equal to or less than 10 ohms

shielding angle

Transmission lines up to 220kV 30°

400 kV at and above 20°

Line insulation -contd.

• Line insulation shall be sufficient to prevent a flashover from the power-frequency over voltages and Switching Surges.

• It shall take into consideration the local un favourable circumstances which decrease the flash over voltage (rain, dirt, Insulation pollution etc.,)

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.

OVER VOLTAGE FACTORS—Contd.

-To take care of one disc in the string becoming defective.

-Facilitate hot line maintenance

Up to 220 kV Line – 1 disc for each string

400 kV Line – 2 discs for each string

FLASH OVER VOLATAGE(FOV) OF DISCS 254 X 145 mm

NO OF DISCS

DRY FOV WET FOV ( kV rms)

Impulse FOV (Standard full wave)

9 540 375 860

10 590 415 945

14 785 565 1265

15 830 600 1345

25 1280 900 2145

RECOMMENDED INSULATION LEVEL OF 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 9

400kV 231 231x5=1755 13

* Compared with Impulse FOV (Value)

RECOMMENDED INSULATION LEVEL OF LINE—contd.

Normal system Voltage

Vpn

In kVPower 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

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

Insulation=√2x50x10³x10=700kV• As per the table for 7 discs, the impulse

FOV ( kVp =695kVp)• For better performance tower forting

resistance shall be brought down.• For 132kV best is 7 ohms

Co-ordination of line Insulation and Sub-Station Insulation

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

• Impulse flash over voltage of line Insulation determine the highest surge voltage that can travel into the sub-station.

• Current through lighting arrestor can be calculated from

1 Surge impudence of line

2 Surge voltage arriving over the line

Co-ordination of line Insulation and Sub-Station Insulation

• Discharge voltage of the LA on that current is the basic protective level of the substation equipment.

• Discharge voltage across LA varies with surge current.

BASIC INSULATION LEVEL AS PER IS (2165 – 1962)

Nominal system volt kV (rms)

Highest system volt kV (rms)

Impulse withstand volt kVp for test

One minute power frequent 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 630

Reduced insulation is used where system is effectively earthed.

INSULATION LEVELS OF EQUIPMENT

• Transformers, Isolators, Instrument Transformers are manufactured for the standard Insulation level.

• Some times transformers, are manufactured for one step 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 the transformer, some of the equipment may lie well out side the protective zone of the LA.

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 step

higher Bill.• Generally BILL of substation equipment other

than transformer are designed for10% higher BIL than that of Transformer .

INSULATION LEVELS OF EQUIPMENT

• BIL of Open poles of a disconnect switch shall be 10 to 15% higher than that provided between poles and earth.

• EHV system must be designed to operate under stresses associated not only with normal operating power frequency voltage but also those caused by transient over voltage.

• These transient over voltage rise principally from lightning over voltage and switching operations

• The former is predominant in system at 100 kV and below.

• Switching over voltage are of concern in system at 220 kV and above

INSULATION CO-ORDINATIONOver Voltage

• Let Un = line to line normal RMS voltage

• Let Um = Rated highest system voltage rms line to line

• √2 Un / √ 3 = Peak of rms voltage phase to ground for nominal system voltage

• √2 Um / √ 3 = Peak of rms voltage phase to ground voltage for highest system voltage

• Any voltage higher than √ 2/ √ 3 Um is called over voltage

Over voltages

• In addition, temporary over voltages also occur at power and harmonic frequencies at times for considerable time under certain conditions.

• The insulation strength and characteristics of various components of a system (including those of voltage limiting devices) must be selected relating to those stresses.

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

• The selected level of voltage shall be low enough to be operationally and economically acceptable

• IEC 71 covers “ Insulation Co-ordination”• IEC -71- Part-I definition, principles• IEC 71- Part – II Guidance for selection of rules

(i) electric strength of the plant, (ii) electric strength of LAs or protective spark gaps

IEC 71-3• Phase to phase insulation co-ordination• Complimentary to part I & II• Standard phase to phase insulation level for voltages up to

and above 300 kV• Voltage stresses In service and clearances in air

Data required:

1. Field data on lightning induced and switching surges appearing on the system

2. Establishing insulation strength of various insulating components of the system through lab tests

Causes of over voltage:

• Phase to earth faults ( it is assumed that resulting temporary voltages will not exceed

–1.4 Pu for solidly earthed networks–1.7 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 a long cable or over head line)

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

Causes of over voltage: contd.

• Ferranti effect: (receiving end voltage greater than sending end voltage under no load or light load conditions)

• By care full design and natural earthing sustained over voltages involving resonance and arcing ground faults are eliminated

• Below 145 kV method of earthing will normally determine the level temporary over voltages.

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 way)• 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

striking the switching surges will rarely exceed 3 pu• 2.5 pu would be typical maximum based on which the

discharge duty of LA is assessed• However in systems above 300 kV, it may be necessary

to suppress maximum switching surges to 2 pu or less by the installation of a shunt reactor and/or closing resistors on the circuit breakers

Resonance effects

• For voltage level below 300 kV. Resonance effects occur

i. When switching transformer

ii. When switching cable and overhead line combination

iii. Between lumped capacitive and reactive elements and over head lines

iv. Charging long lines without shunt reactor compensation

Resonance effects-- contd

• Ferro resonance encountered on a transformer feeder 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 coupling from the parallel line circuit which resonates with transformer impedance at a sub harmonic frequency

• (operation procedure such as opening the line isolator at the transformer end on the disconnected circuit will eliminate the problem)

Mode of action of flash over on a line

• A lightning flash can impress over voltage on a 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

Induced Voltage Surge

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

– The highest amplitude normally associated is in the region 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

Direct stroke

• A direct stroke can be to the earthed tower

top or on phase conductor• Stroke on earthed lower top, for

transmission of shielded design, is innocuous

• Raise in potential caused by passage of current through tower impedance to earth will be less than with stand strength of line

Direct stroke—contd.

• However the rise in potential can be severe and 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 the frequency of flash over

Direct stroke—contd.• Direct stroke on phase conductor• May occur if there is a shielding failure i.e. stroke avoids

earth wire 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

Z0

VI0 = minimum flash over voltage for 1/50 Wave

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

Surge propagation:

• Surge waves are propagated at the velocity of light along the conductor

• On arrival at substation, equipment there in get stressed.

• Rod gaps and surge arrestors provide necessary protection

• Waves are subjected to considerable attenuations due to losses both in the conductor (ohmic losses) and corona losses

Lightning discharges

• Clarification of lightning discharges stroke (A)stroke (B)Stroke (A) : produced by the charged cloud which induces a charge on the stationery objects such as high buildings etc.

• Charge distribution causes concentration of potential at the top most point

• Electro static stress being great at that point ionization of surrounding atmosphere takes place

• Dielectric strength of surrounding air decreases giving an easy path to lightning stroke.

• Decrease in dielectric strength of surrounding air takes considerable time

Lightning discharges

Stroke B:• A, B & C are three clouds with A and C positively

charged and B negatively charged• When there is a stroke between (A) and (B) the charge

on (C) becomes free and immediately and indiscriminately strikes on 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 charge

through a lightning conductor placed on the top of the building

Static induced charges

• An over head conductor accumulates statically induced 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

Lightning strokes• Over voltage due to lightning strokes

surge impedance of the line = Zs

Discharge current = Id

Over voltage due to direct stroke = Vd = Id x Zs

However current travels in both directions over voltage = Vd = Id x Zs

2when lightning strikes over earth wire or a tower

Over voltage = Id x Ze + Lc di dt

Ze = impedance of earth wireLc is the inductance of the line conductor

Protection against lightning

1. Protection of transmission lines from direct strokes

2. Protection of power stations and substations from direct strokes

3. Protection of electrical equipment from traveling waves

Protection of transmission lines

Against the direct strokes :

• Most harmful

• Effective protection required shielding to prevent lightning from striking the electrical conductors.

• There shall be adequate drain facilities so that the charge can be grounded without affecting Insulators or line conductors.

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 non –corrosive Resistance of ground wire shall be low for better protection

against direct stroke. (b) Adequate clearance between

1. Line conductor and tower2. Line conductor and earth3. Clearance between line conductor and ground wire all

through the 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 supported center points of outer conductor and ground wire. It shall be 30° for 132 & 220 kV lines 20 ° for 400 kV lines

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 C1

Induced voltage on the lineV L1 = C1 x Ec

C1+C2

• When ground wire is present it increases capacitance between conductor and earth i.e. C1 Decreases induced voltage on the line.

• It is observed that presence of a ground wire reduces induced voltage on line to half.

• For two ground wires the induced voltage comes down to one third

• Presence of two ground wires also provides better shielding

Earth wires

• Disadvantages with ground wire:

(a) higher line cost

(b) Probable direct shorting between line conductor and ground wire when the later gets cut

In 400kV system transmission line towers will have twu earth wires.

Alternative method of line protection

• Even after providing ground and reducing the likely induced voltages, harmful voltages can still develop

• Lightning arrestors act as additional protective devisees by by-passing the surges to ground

• Protector tube is a fiber tube with electrode at earth end.

• Fitted directly below the conductor• The arc type electrode on the top of the tube

forms a series gap with conductor

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 the fiber of tube wall to emit gases which will quench the arc

• This tube successfully prevents re-striking• The break down voltage of tube shall be less

than flash over voltage of the insulation.

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 the terminals of the equipment.

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

iii. Steep wave front resulting into resonance and high voltage may cause internal or external flash over causing building up of oscillations in the equipment

• Protective equipment : LAs and Surge diverters

• They are connected between line and earth

Action of the Surge diverter• A traveling wave reaches surge diverter and

attains a prefixed 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 of the current flowing to earth to prevent break down of insulation

• Important aspect is that the surge diverter shall provide low impedance path to earth only when traveling surge reaches the surge diverters

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 power

frequencies but function only when abnormal frequencies are applied

• When there is a discharge through them they shall be capable of carrying the discharge current for some time interval.

• After the over voltage discharge it must be capable interrupting normal frequency current from flowing to earth as soon as the voltage reaches below the break down value

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 ½ LI2 and system capacitance ½ CV2

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

• Most severe over voltages occurs during the closing unloaded transmission 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.

Switching Over voltages in SubstationsSwitching 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 circuit breakers. Use of lightning arresters. Use of shunt reactors in transmission lines.

Traveling waves travel to and fro giving rise to a switching surge.

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 to reduce power frequency over voltages.

Elimination of trapped charged on the line

Line shunting after opening by means of earthing switch

Reduction of current chopping

Opening resistors ( Resistance switching with CB) used only with ABCB

Methods of Reducing Switching Over Voltages

Switching operation causing over voltage

Method to reduce switching 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

Rod gaps or coordinating gaps• They are used on insulators, equipment and

bushings• Conducting rods are provided between line

terminal and earth terminal with an adjustable gap ( Air insulation)

• Rods are of 12mm dia approx.• The gap is adjusted to break down at about 20%

below the 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 to

development of surge arrestors.

Over-voltage in Network and RemediesPhenomena Causes Effect Remedies

Surges Lightning strokes on overhead lines or substation

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.

Over-voltage in Network and RemediesPhenomena Causes Effect Remedies

Resonance The fault causing resonance between inductance and capacitance in a part of the circuit

Very high, voltage surges occur. Insulation failure likely to occur.

Filters to eliminate harmonics

Traveling waves

High voltage waves get reflected – on reaching a junction or end.

Reflected waves gets superimposed for initial wave. Voltage may rise to several time the normal voltage.

-Proper switching sequence.

Sustained Power frequency over voltage

Poor voltage control Failure of transformers and Rotating Machines

-Proper Voltage control

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 substation area

-Provide effective protection against direct strokes on line conductors towers sub station equipment

Vertical Masts in substations

-- in sub stations -instead of providing overhead shielding wires

Lightning Masts/Rods (earthed)

- Above tall buildings Protect buildings against direct strokes. Angle of Protection œ = 300

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 phase and ground

-Resistance Capacitance Combination absorbs the over voltage surge and reduces steepness of wave

Lightning arrester selection• 1. To determine the magnitude of the power frequency phase to ground

voltage expected at the proposed arrester location during phase to ground fault, or other abnormal conditions which cause higher voltages to ground than normal.

• 2. To make a tentative selection of the power frequency voltage rating of the arrester. 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

current and 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 full wave impulse, withstand level of the equipment insulation to be protected, by adequate margin.

• 7. To establish the separation limit between the arrester and the equipment to be protected.

Types of Earthing

• For purpose of selection of voltage rating of a LA three types 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 to line voltage

• If in a system all transformers have star connected winding 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

Types of Earthing - conted.

(II) Non effectively earthed system: a) if the line to earth voltage in healthy phases in

case of a fault exceed 80% of the line to line voltage but does not exceed 100% of it, the system is called non effectively earthed system

b) System with few solidly earthed neutralsc) Systems with neutral Earthed through resistors or

reactors 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

healthy phases exceed 100% of the line to line voltage.

Selection of lightening arrestors

• Tentative selection of arrestor Voltage:

• Arrestor Voltage rating shall not be less than product 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

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 (part –I) 1965 the rating is

198 kV

• By going for a higher voltage rating for a surge arrestor, the degree of protection for equipment gets reduced.

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 LA

Ia = 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

Selection of arrestor discharge current

• In a 220 kV system using 11 insulators Transmission line will not permit a traveling wave of a value more than 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.

Selection of arrestor discharge Voltage• Most important characteristic of LA determining the

protection level being offered• The arrestor discharge voltage shall be less than BIL of

equipment 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. But increase is much restricted due to non –linear resistance property.

• Increase in discharge from 5 kA to 20 kA produces only 25% rise in discharge voltage.

• Increase in rate of current from 1000 to 5000 Amps per micro second increases discharge voltage by only 35%.

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 manufacturing tolerance shall be allowed.

• Protective margin shall be 20% of the BIL of the equipment when closely located

• In a 220 kV systemDischarge voltage of LA = 649 kVAllowing 10 % margin protection level = 713 kVBIL of equipment = 900 kVpProtection margin = 900-713 = 187 kVp There is more than 20 % of the BIL of 180 kV

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.

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.

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

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 in open position in grater than between terminals and ground.

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

. 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.

Location of Lightning Arresters:

• The electrical circuit length between L.A. and the transformer bushing terminal (inclusive of lead length in metes for effectively earthed) should not exceed the limits given below:

Rated syst. voltage KV

BIL KV Peak

Max. distance

132kV 550

650

35.0

45.0

220kV

400kV

900/1050

1425/1550

Closer to Trans.