This document discusses insulation coordination for electrical systems. It defines insulation coordination as selecting suitable insulation levels for system components and arranging them rationally. The goals of insulation coordination are to ensure insulation can withstand normal and abnormal stresses and efficiently discharge over voltages. It also discusses determining live insulation levels, equipment BIL levels, and selecting lightning arrestors. Various insulation levels for lines and equipment are recommended based on system voltage.

1. Welcome
T APR SE AIONON
O E NT T
INSULATION- CO-ORDINATION
BY
A.SAI PRASAD SARMA

2. 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 ensure
3) Insulation shall withstand all normal stresses and
majority of abnormal ones
4) Efficient discharge of over voltages due to internal
/external causes
5) B/D shall be only due to external causes
6) B/D shall be at such places where least damage is
caused

3. 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.

4. 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

5. 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)

6. 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

7. 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

8. 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.

9. LINE INSULATION
• Extra high voltage line can be made lightning
proof by
2 Efficient shielding
3 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°

10. 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.,)

11. OVER VOLTAGE FACTORS
Line Switching Power frequency flash
Voltages Surge flash over (Dry & Wet)
over
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.

12. 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

13. FLASH OVER VOLATAGE(FOV)
OF DISCS 254 X 145 mm
NO DRY FOV WET FOV Impulse
OF ( kV rms) FOV
DISCS (Standard
full wave)
9 540 375 860
10 590 415 945
14 785 565 1265
15 830 600 1345
25 1280 900 2145

14. RECOMMENDED INSULATION
LEVEL OF LINE
Normal Vpn Switching over No of
system In kV volt. (Wet) kV * discs
Voltage (Vph/√3) required
132kV 76 76 x6.5=495 5
220kV 127 127x6.5=825 9
400kV 231 231x5=1755 13
* Compared with Impulse FOV (Value)

15. RECOMMENDED INSULATION
LEVEL OF LINE—contd.
Normal Vpn Power freq. No. No. of As per
system In kV over volt of discs practice
Voltage (wet) discs recom.
(kVrms) req.
132kV 76 76x3=228 6 7 9/10
220kV 127 127x3=381 10 11 13/14
400kV 231 231x3=762 20 22 23/24

16. • 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

17. 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
4 Surge impudence of line
5 Surge voltage arriving over the line

18. 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.

19. BASIC INSULATION LEVEL AS
PER IS (2165 – 1962)
Nominal Highest Impulse withstand One minute power
system system volt kVp for test frequent volt kV (rms)
volt kV volt kV Full Reduced Full Reduced
(rms) (rms) insulation insulation insulation 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.

20. 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.

21. 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 .

22. 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.

23. • 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

24. INSULATION CO-ORDINATION
Over 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

25. 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 interruptions
ii. To reduce component failures
• The selected level of voltage shall be low
enough to be operationally and economically
acceptable

26. • 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

27. Data required:
2. Field data on lightning induced and
switching surges appearing on the
system
3. Establishing insulation strength of
various insulating components of the
system through lab tests

28. 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)

29. 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.

30. 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

31. 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

32. 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)

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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
2
when lightning strikes over earth wire or a tower
Over voltage = Id x Ze + Lc di
dt
Ze = impedance of earth wire
Lc is the inductance of the line conductor

43. 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

44. 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.

45. 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 tower
2. Line conductor and earth
3. 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

46. 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 line
V 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

47. 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.

48. 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

49. 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.

50. 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

51. 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

52. 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

53. 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.

54. Switching Over voltages in Substations
Switching duty of Applications and Phenomena
C.B. Remedial Actions
Opening of capacitor Switching of shunt Re strike in circuit
bank currents, cable capacitor banks used for breakers giving over
charging circuits, filter p.f. correction. voltage.
banks - Use of re strike free C.B.
for capacitor switching
duty.
EHV lines * Long EHV transmission. Traveling waves
* Closing unloaded - Use of pre-closing travel to and fro
lines resistors with circuit giving rise to a
breakers. Use of lightning switching surge.
* Closing charged
lines arresters. Use of shunt
reactors in transmission
* Auto re closing of
C.B. lines.

55. Methods of Reducing Switching
Over Voltages
Switching operation Method to reduce
causing over voltage switching over voltage
Energising an uncharged High voltage shunt reactors
line are connected to line to
reduce power frequency
over voltages.
Elimination of trapped Line shunting after opening
charged on the line by means of earthing switch
Reduction of current Opening resistors
chopping ( Resistance switching with
CB) used only with ABCB

56. Methods of Reducing Switching
Over Voltages
Switching operation Method to reduce
causing over voltage switching over voltage
Reducing the switching over Single stage pre closing resistor
voltages due to closing insertion with CB.
Two stage pre closing resistor
insertion with CB.
Closing resistors in between circuit
breaker and shunt reactor
Reducing switching over voltages Synchronous switching of three
by improved switching sequence poles.
Simultaneous operation of circuit
breakers at both ends of line,
Use of surge arrestors While closing of line
While disconnecting reactor

57. 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.

58. Over-voltage in Network and Remedies
Phenomena Causes Effect Remedies
Surges Lightning strokes on Line insulation flash -Use of Ground
overhead lines or over or puncture. wire
substation The traveling wave - Surge Diverters
reaches substations. -Earthing of
The insulation of towers
equipment is
-Lightning Masts
stressed by impulse
surge
Switching Breaking inductive circuit, Wave travels from -Use of opening
surges the energy stored C.B. to both sides resistors with C.B.
inductance gives rise a Transmission line - Use of restrike
voltage rise across insulator, stressed. free C.B.
capacitor. Terminal apparatus -Use pre-insertion
Switching of capacitive, insulation stressed resistors with C.B.
line charging currents give
rise to a over voltage due
to restrike. Closing of EHV
lines

59. Over-voltage in Network and Remedies
Phenomena Causes Effect Remedies
Resonance The fault causing Very high, voltage Filters to
resonance between surges occur. eliminate
inductance and Insulation failure harmonics
capacitance in a part of likely to occur.
the circuit
Traveling High voltage waves get Reflected waves -Proper
waves reflected – on reaching gets superimposed switching
a junction or end. for initial wave. sequence.
Voltage may rise
to several time the
normal voltage.
Sustained Poor voltage control Failure of -Proper Voltage
Power transformers and control
frequency Rotating Machines
over
voltage

60. Protective Devices Against Lightning Over
voltages
Device Where applied Remarks
Rod gaps Across insulator string, -Difficult to coordinate
bushing insulators -Create dead short
circuit
-Cheap
Overhead Ground -Above overhead lines -Provide effective
Wires (earthed) -Above the substation protection against
area direct strokes on line
conductors towers sub
station equipment
Vertical Masts in -- in sub stations -instead of providing
substations overhead shielding
wires
Lightning Masts/Rods - Above tall buildings Protect buildings
(earthed) against direct strokes.
Angle of Protection
œ = 300

61. Protective Devices Against Lightning Over
voltages
Device Where applied Remarks
Surge Arresters -- on incoming lines in -- Diverts over voltage to
each substation earth without causing
-Near terminals of short circuit
Transformers and -Used at every voltage
generators level in every sub-
-Near motor and station and for each line.
generators terminals
Surge Absorbers -- near rotating machines -Resistance
connected between phase Capacitance
and ground Combination absorbs
the over voltage surge
and reduces steepness
of wave

62. 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.

63. 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

64. 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 neutrals
c) 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.

65. 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

66. 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.

67. 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

68. 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.

69. 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%.

70. 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 system
Discharge voltage of LA = 649 kV
Allowing 10 % margin protection level = 713 kV
BIL of equipment = 900 kVp
Protection margin = 900-713 = 187 kVp
There is more than 20 % of the BIL of 180 kV

71. 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.

72. 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.

73. The L.A. voltage rating
Rated system Highest system Arrester rating
voltage KV voltage KV in KV
132 145 120/132
220 245 198/216
400 420 336

74. 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.

75. 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. BIL KV Max.
voltage KV Peak distance
132kV 550 35.0
650 45.0
220kV 900/1050 Closer
400kV 1425/1550 to
Trans.