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Indian StMard
High Voltage Techniques Sectional Committee,
Repwlding
San: V. R. NARASIMHAN
DEPUTY Dramxoa
( Alkrnak
V . R . Narasimhan
K. S. Bix*anwAJ
Sum A. K. CwoPaA ( Alkrnak )
Siemens India Ltd, Bombay
Da S. C. BAATSA
Da D. P. SAHGAL ( Alkrnak )
Da K . DAS GUPTA
Smar A. K.
SERI v.
Da P. SATYANARAYANA
Smr V. S. MANX
Alkmatc )
&RI K. S. MADAAVAN ( Alkmak
.Smu S. K. MtxHEaJEE
National Test House? Calcutta
bharat Heavy Electncala Ltd, New Delhi
D. V. Nnrure
.%a1 B. N. Geosa
( Alkmak )
!&ax P. N. SRalvAsrAvA
The Bombay Electric Supply C Transport UnderSRRI V. H. NAVKAL
Saat M. L. DoNoRE ( Alkmak )
G. R. GOWNDA RAJU
DR B. I. GURURAJ ( Altemak )
Kamataka Electricity Board, Bangalore
.%iRlK. S. SWAPRAKASAM
Swru H. M. S. LXNQAIII ( Afkrnafe )
V. SamvAsAN
Smu K. THIRUvENKADATXAN
Sxax T. V.
%WZAMANIAK
SHar S. KR~PANIDHX(
Alknuk
@ Cbpyrfght
,I983
fbis publication is protected under the fn4m Copyrf&
Acr (XIV of 19S7 ) Rnd
mprodu~ti~tt in whole or in part by my m~aarexcept with written pamissiot~ of tba
pub&her nbaIl be deemed to be an infriorwot
of copyripbt uodar the rid Act.
fSr3716-1978
( c5nriRwd~om pugc1 )
@menring
SHnI SURENDXASINQH
Sam KOMAL SINGH ( Alfemak )
Crompton Greava Ltd, Bombay
DR G. PARTHASARATIIY ( Altwnafc )
The Tata Hydra-Electric
Pow.er SUPPlY Co Ltd,
SHRI P. J.* WADIA
DR R. RANJAN ( Alkmah )
Director General, IS1 ( &~fi&
Mnnbn)
Srmx S. P. SACHDEV,
Sanx c. R.
s#cY&qY
Sxnr M. N.
( Elec tesb )
I. SCOPB
3. VOLTAGE STRESSESINSERVICE
4. INSULATIONWITHSTAND
BETWEENSTRESSES
AND WITHSTANDCONSIDERA31
TIONSCOMMON
TO RANGESA, B AND C ...
BETWEEN STRESSESAND WITHSTANDVOLTAGE
FORRANGE A
VOLTAGEFOR
RANGBB
9. ~~;~TION
SURGETRANSFERENCE
THROUGHTRANSFORMERS . . .
VALIDITY OF SWITCHINGAND LIGHTNING IMPULSE
TESTS,50 PERCENTDISRUPTIVE
TEST AND 15 IMPULSES
EVALUATIONOFTHE PROTECI.IVEEFFECT
OF SPARK GAPS . . .
APPBNDICES
E_SAMPLES
OF APPLICATION . . .
CLEARANCES IN AIR BETWEEN LIVE CONDUCTIVE
PARTS AND EARTHED STRUCTURESTO SECURE A
SPECIFIEDIMPULSEWITHSTANDVOLTAGE FOR DRY
RELATION BETWEEN POLLUTXONLEVPIS,
TEST LBVBLSAND CREEPAOB
SCALE OF NATURAL POLLUTIONLSVELS
TABLU 3
CNOIOB OF THE INsuLAm
LBVEL 01 CA1 I BQuxPHuaT
(FOR &AMPLE Tkmmmeua hBY fik?RoB
I)IVSRTRRS)
op Chs~ IIl!&ume~~
OTRDDXSOONNROTINOS~)...
TABLB 4 CNOICBOFTHEI~mum~NLava~
(ICBRfiSAMPLRU-
Rrsr:OFFAIL~RR OF hsu~&moN
%VITCHl
NO hPUISE
WlTHSTAND
hNCTIOX8
VOLTAOES
TABUZ6A ~RRfCLA~IONS BMWEXN
hWLATION
~~R?~R~~&A$E-T~-EARTHAIR~~
TADLE6B CoRRmmoss
L.lWEts AND
I~svnlron
Am CL.EARAN~S
PIUSE-TO-EARTB
EQUIPWMT OF TABLR 4
O~RVOLT~~ES
PROBABIUTY OF DISRUFFIVEVOLTAGE
UNDRRI~
Fxca. 4
hhXIMUM
htHIMBI.JZ
ARMSTBR
OF LIN+ABIJE
!SIXPL~~~
hNC3TH
Msrmto~
&JR=
PURER OF
CORRRLA~RS
I~TwRRN
FAUWUZ
RxmorF~itune
(8) AND STAGmm~~~~aBansoar
TE62IOALm
h&TC3X (y) Ff3RVARIOUS~W~~
(R) AWD STA-
V~IO~~SWMUUNG
CORRBLA~ONS
BETWBEN RISKOFFAMJRE (R) AND STII
'ITIRCALSAPE~Y FACTOR (y) FOR VAWWS Swrmmn,
SUR~BDmRIBuTIoNs
TISTIOAL~A~~YFA~~R
soaot Drmmmmxom
Fro.- 8
FAILUREOF A
SURGE DrmuewHoNs
Fxo. 7
Jtnvorro~ONLY ...
op INSULATION
CABXS
OF TXE
k3ULATION
ISr3716.1976
RISK OF FAILURE (R) AND STATISTICALSAFETY FACTOR (y) FOR VARIOUS LIOHTNING
Suxmu D~~~IBUTI~NS
CORRELATIONSBEIWEEN
RISKOF FAILURE (R) AND STAT~SI~CALSAFETY FACTOR (y) FOR VARIOUS LIGHTNING
SURGE DISTRIBUTIONS
CORRELATIOIUS
BETWEXNRISKOF FAILURE (R) AND STAT~~CAL SAFXTY FACXOR(~) mxt VARIOUS LIGHTNING
SuRoBIhTR1zwTxoNs
BLOCK-DIAORANOF INSULATION Co-ORDINATIONAND
INITIALCAPACITIVE VOLTAGL SPIKB
VALUESOFFACWR I
DEFINITIONOFTHE INSULATION STRENGTH OF A PIE~R
AT TBB TIME t AS A Fmmm
OF&WMXXUT
DENST-
OF THE MEAWRED
w OF A POPWATI~N OFAPP;ARATW~... 75
QBAN l!&IPMBNT
To PASS TEIE DIFFEBEHT
TYPE OF Tzsr AS A FUWXION OF ITS Iracwma;
cnAltAmwrncs
PROBABiLXl'T~~OFT~~AlLURE
thmomds
thTOMER8
P~TOFAXLTHSTEST
FUNCTION cm Ik-rIY
FtnmrxoaOF bthNUFAm=RS
lS:3716-1978
Standards Institution on 19 April 1978, after the draft finalized by the
High Voltage Techniques Sectional Committee had been approved by the
Electrotechnical Division Council.
0.2 This standard was first issued in 1966 and covered recommended
practices for the co-ordination of the insulation of electrical equipment
located in electrically exposed situations.
The insulation levels recommended in the 1966 edition were on the basis of IS: 2165-1962;.
revision has been undertaken with a view to bring this guide in line with
the latest technological developments taking place in the field of insulation
co-ordination like the switching overvoltages.
This revision is aligned with
the revision of IS : 2165 which has been issued in 1977 wherein proper
emphasis on the switching overvoltages has been covered.
0.3 Probabilistic concepts and probabilistic language have also been
introduced in this revision for the procedure of insulation co-ordlnatiori.
This revision acknowledges that the engineers, particularly those who.
work with extra high voltage field equipment, with the help of powerful
0.3.1 The traditional approach to insulation co-ordination was and
still is, to evaluate the highest overvoltage to which an equipment may be
submitted at a certain location on a system, and select from a table of
*Guide for insulation coordination.
IS : 3716- 1978
The object of this standard is not to give strict rules for insulation
co-ordination and design, but to provide a guidance toward rational and
economic solutions. Therefore, it is intended to consider only a few basic
cases, it being evident that stations constituting exceptions to normal
design, or included within systems having exceptional characteristics, will
require special study by experienced engineers.
0.3.2 In a more elaborate process, it is recognized that over-voltages are
random phenomena and that it is uneconomical to design plants with such
a high degree of safety that they can sustain the most infrequent ones. It
is also acknowledged that tests do not ascertain a withstand level with a
100 percent degree of confidence.
In consequence, it is realized that
insulation failures can occur occasionally in well-designed plant, and that
the problem is to limit their frequency of occurrence to. the most economical value, taking into account equipment cost and service continuity.
Insulation co-ordination should be more properly based upon an evaluation and limitation of the risk of failure than on the u priori choice of a
0.4 In accordance with the latest decision taken at the international level,
it has been decided to use the term surge arrester in place of lightning
0.5 In the preparation of this standard assistance has been derivedfrom
IEC Pub 71-2 ( 1976 ) Insulation co-ordination - Part 2 : Application
guide issued by the International Electrotechnical Commission.
0.6 A typical example for 220 kV transformer has been included in
Appendix A to make the guide more useful since 220 kV system is more in
vogue in our country.
0.7 For the purpose of deciding whether a particular requirementof this
standard is complied ivith, the final value, observed or calculated, expressing the result of a test, shall be rounded off in accordance with IS:
2-1960*.
1.1 This standard provides guidance on the selection of the electric
strength of equipment, of surge arresters or protective gaps, and of the
most suitable degree of switching overvoltage dorrtrol. The rated withstand voltages of the equipment are covered in IS : 2165-1977t which forms
a necessary adjunct to this standard.
Non - This guide is based on apparatus types and ratings in use at present. As
new equipment and equipment characteristics are developed and proved, this guide
should not be interpreted as a limit to their adoption.
*Rules for rounding off numerical values (mid).
thdatiou
co-ordination ( sucendretin ).
1.2 Thisguide covers_only phase-to-earth insulation and deals separately
with the three following ranges of the highest voltage for equipment:
a) Range A : above 1 and less than 52 kV.
b) Range B : from 52 to less than 300 kV.
1.3 It covers installations of all kinds and in all situations involving
voltages higher than 1 kV, whether they are exposed to lightning or not,
with the exception of overhead lines. However, the test procedures apply
to the latter also.
2.1-For the purpose of this standard the definitions given in IS : 2165-1977*
3. VOLTAGE STRESSES IN SERVICE
3.1 General -
Dielectric stresses on insulation may be classified as follows:
a) Power-frequency voltages under normal operating conditions,
b) Temporary overvoltages,
c) Switching overvoltages, and
In IS : 2165-1977*,
the overvoltages ar,n classified with reference to
the shape of the voltage wave which determines their effect on insulation
and on protective devices, without reference to the cause of the
3.1.1 The term temporary overvoltages refers to sustained overvoltages,
or to overvoltages having several successive peaks, with a decrement of the
amplitude such as to be comparable with a sustained voltage at power
frequency or at harmonic frequency.
3.1.2 The term lightning or switching overvoltages refers to overvoltages for which only the highest peak value has to be considered and
which can be represented, with regard to their effects on insulation and
protective devices, by the steep front standard lightning impulse or the slow
front standard switching impulse used for test purpose. The foregoing
names have been chosen because such overvoltages often, but not always,
originate from lightning discharges or switching operations.
3.1.3 For example, the energization of a transformer-terminated line
gives rise to an overvoltage that may be regarded as a switching or temporary overvoltage depending on the decrement of the successive peaks
&ulation co-ordination ( second m&ion ).
(that is, depending on the circuit constants).
As another example, a lineto-earth fault, although actually a switching operation (the same phenomenon would arise if a phase conductor was connected to earth by a
circuit-breaker operation) may give rise to steep-front overvoltages, similar
to those due to lightning; on.the other hand a lightning surge transferred
through a transformer by inductive coupling between windings may
produce on the secondary side of the transtoettd
similar to those due to switching operations.
3.2 Power Frequency
Voltagea - In insulation co-ordination
since overvoltages and impulse voltages are defined in terms oft Reir peak
vah.uzs to earth, it is also convenient to make use ofthe phas&to-earth peak
value of the system voltage, which is &/ q/3
I a816 times the usual
rms phase-to-phase voltage.
Under operating conditions, power-frequency voltagecan be expected
to vary somewhat in magnitude and may be described by means of a
probability distribution about the average operating value. This distribution
will differ from one point of the system to another.
For purpose of insulation design and co-ordination, it should, however, be considered as constant and equal to the highest voltage for
eqmpment, which in Range C does not materially differ from the highest
system voltage, with a peak phase-to-earth value of Urn d/2
1 d/3.
Range A and in Range B up to 72.5 kV the highest voltage for equipment
may be substantially higher than the highest system voltage, as given
in 2.3 of IS : 2165-1977*.
For the sake of standardization it is, however, assumed that equipment insulation will always be .able to operate satisfactorily at the -highat
voltage for equipment immediately above, if not equal to, the highest
- The severity of temporary overvoltages
is mainly characterized both by their ampIitude and duration.
The importance of temporary overvoliages in insulation co-ordination
a) on the one hand the characteristics of temporary overvoltages at
the surge arrester location are of great importance
arrester selection; and
b) 011 the other hand, the successive repetition of overvoltage peaks
of opposite polarity, even if of lower amplitude than soxix other
overvoltages may determine the design of both the internal insulation of equipment as well as the external insulation
( surfaces
exposed to contamination ) .
*Insulationco-ordination( smndratsbn).
I6 I 3716- 1970
overvoltages generally arise from:
a) earth faults,
b) load rejection,
c) resonance and ferro-resonance.
3.3.1 Earth Faults - The overvoltage at power frequency on the sound
phases when another phase is accidentally earthed depends, at a given
point of the system, on the treatment of the system neutral with respect to
earth, as characterized by its earth fault factor at that point.
3.3.1.1 In the evaluation of the earth
remarks should be considered :
In general, in order to evaluate this factor at a given location,
it is assumed for simpli@ty that the fault is located at the point
for which the factor is desired; but, in some special cases, it may
be desirable to investigate the effect of other locations on the
highest value of the voltage to earth.
In principle, there are as many particular values of the earth
fault factor at a given location as different possible configurations
of the system. The factor which characterizes the location is the
highest of the values that correspond to the different system configurations which may occur in practice.
The system configurations which have to be considered are those
which exist during a fault; thus one should take into consideration
those changes in the system which may be produced by the fault
itself, for example, on account of the operation of circuitbreakers.
For many systems, it will be sufficient to consider only one value
of the earth fault factor which covers all the locations on the
Attention is drawn to the fact that the highest voltage at system
frequency which may appear on a sound phase during a particular
earth fault does not depend only on the value of the earth fault
factor but also on the value of the phase-to-phase voltage at the
time of the fault. This phase-to-phase
voltage will generally
be taken at the highest system voltage, as given in 2.2 of IS :
21651977*;
but, in some cases, in order to predict the operation
of protective devices and specify their characteristics,
necessary to take into account the increased value of the
voltage that may appear at the selected location
under the abnormal conditions not covered by this definition.
lI~ulationio-ordination
( secondrevision).
33.1.2Within Range A and in some cases within range B many
systems or installations are operated with their neutral earthed through a
high impedance; an arc-suppression coil or with their neutral isolated. For
the purpose of insulation co-ordination particular attention shall therefore,
be paid in these cases to the earth fault factor.
3.3.13 Independently
of the earth fault factor particularly
overvoltages may arise in Range A and Range B systems in the case of:
a) earth faults in a system the neutral of which is earthed through
an arc-suppression coil when the circuit is under-compensated.
b) arcing earths in a system, the neutral of which is isolated and
in many cases in a system the neutral of which is earthed through
an arc-suppression coil.
3.3.2 Sudden Changes in Load - In the usual conditions of operation the
phase-to-phase voltage does not exceed the highest voltageof the system
( see 2.2 of IS : 2165-1977*);
but higher values may temporarily be reached
in the case of sudden disconnection of large active and reactive loads; they
depend on the system layout after disconnection and on the characteristics
of the sources (short-circuit power at the station, speed and voltage regulation of the generators, etc).
This voltage rise may be specially important in case of load rejection
at the remote end of a long line ( Ferranti effect ). It affects mainly the
apparatus at the station connected on the line side of the circuit-breaker.
the point of view of overvoltages, a distinction should be made between
NOTE -From
various types of system layouts.
Extreme cases, namely, those with relatively ahort
lines and high value of the short-circuit power at the terminal stations; and those with
long lines and reduced value of the short-circuit power at the generating site may be
With the latter layouts, as are usual in an extra-high voltage system in its
initial stage, much higher overvoltages at system frequency may result when a large
Ioad is suddenly disconnected.
Due to the characteristics of the systems, overvoltages of this kind are
more severe in voltage Range C than in voltage Range B, overvoltages of
this kind, in voltage Range A, may occur in generator transformer-circuits.
33.3 Resonanceand Ferro- Resonance-Temporary
overvoltages due to these
effects may generally arise when circuits with large capacitive elements
( lines, cables, series compensated lines ) and inductive elements ( transformers, shunt reactors) having non-linear magnetizing characteristics, arc
These situations are generally found for systems in Ranges C and B
a ) A lightly loaded line, fed or teminated by a transformer may show,
for example, harmonic oscillations and pronounced overvoltages
if the natural frequency of the linear part of the system corresponds
to one of the harmonics of the transformer magnetizing current.
l&mllationco-ordination( MCOnd
&tioll
b ) Subharmonic odlations
and ovcrvoltagcs may occur in series compensated systems terminated by lightly loaded power transformers
or shunt reactors if the impressed voltage, the effective circuit
resistance, which is strongly influenced by synchronous machines,.
and the kircuit capaciknce fall between certain Ii&its.
c ) If harmonic filters are connected to a system containing saturable
elements, oscillations due to resonances between these elements and
the filter capaciton can develop.
These f-o-iesonance
efTects following energization processes are either
stationary or last several cycles of the power frequency being related to the
time constant of transformer in-rush currents.
Causes of resonance
and ferro-resonance
in voltage Range A
resonance between inductive
example, ivhen capacitors for power-factor correction are used.
W ferro-resonance
which may occur when a tran&rmer
secondary is loaded by a small capacitance only, is switch& in or
out, with an appreciable time between operatSons on each pk
ferro-resonance which may occur where there. is an iron-cored
inductive load such as a voltage transfokner ahzrdy connected or
being switched in.
Ovrrvokagea
3.4.1 For the purpose of this guide switching overvoltages are, as stated
above, of a m-which
may be simulated by a standard switching impuk,
ihat is, an aperiodic wave with a front duration of the order of humkda-qf
microseconds and a tail
the order of thousands .of microseconds. They
stress the various parts of an insulation in about the same proportions as
power-frequency voltages but arc not repetitive and only one peak of either
normally significant.
3.43 Lightning overvoltages are thosq which may be simulated by a
standard lightning impulse, that is, an.aperiodic wave with a front duration
the order of one microsecond and a tail durition of the order \of several
Due to the front steepness they strew more than the
tens df microseconds.
former the longitudinal insulation
inductive w,
shorter duration, generally a somewhat higher stress may bi withstood by
a given insulation.
3.4.3 These overvoltagea generally arise from:
ia) line energization and raenergization,
b) faults and fault clearing,
I3:3716-1978
ofcapscitive
and of small or moderate inductive
loadrejtcti~and
li&ing
strokes (first or subsequent components of a lightning
Linemrgidon and rs-snnrgiration oven&ages - Overvoltages
due to closing, and to single and threophase reclosing are of great importance in the selection of system insulation in Range C. In this voltage
ranp;mstrike-fke
breakers are in general use.
Oven&ages
particular importance in the other ranges of voltages.
3.4.3.2 Szvi&ag oruvoitag~ dut to faults and fault &wing - In Range A
and, in relatively kw cases, in Range B under the conditions listed in items
in 3.3.l.3 high switching overvoltages
can arise at the
&tLZr
(o?! fault .
In all the voltage ranges, high overvoltages may arise due to taults to
earth in composite circuits (overhead lines and cables as transformer terminated f&n).
At the highest voltages of Range C a high degree of control of
overv&ages caused by Ike energizatiou and re-cnergieation is normally
attempted. Forthis mason switching ovcrvoitagesdue to faults and firult
&wing ( single and daubie liitiearth
faults and their clearance) need
cvehrl co&&ration.
3.4.3.3 &#&@ps
dyr b sunkhing of hdwtivr ad cqgdeitioc currents In Range A the switching tif inductive or capacitive currents may give rise
w&h may raqt+e attei+n,
both in high voltage d&t+-.
b&r& *~*$&$a ;rp$ ia i*
m$&&&o ns and power stationa.
casic 4 L e timer,
high overvoltagea
may ark
if the c&u&breaker
deiakea
so rapidly ti to f&ma &a current prematu$y
t.9 26x0, so-called
current+pping.
a) Iritaruption of thr Wig
currenzs of motor&
_diaductk
cummt, for example, when .interrupting
ZW@Et=lngcwFeatOfatr~~Or~Or;
cr Sting
and operation of arc funuces
and their tranakmers
vlhich may lead to current chopping;
J8 : 3716 - 1978
d) Switching of unloaded cables and capacitor banks; and
of currents by high-voltage fuses.
In Range B overvoltages due to the interruption of capacitive currents
(switching off unloaded lines, cables or capacitor banks ) may be particularly dangerous since the use of restrike-free breakers may not alway be
3.4.3.4 Load rejection - Overvoltages due to load rejection may start
with a high switching surge followed by a temporary overvoltage.
Overvoltages of this kind are particularly important in Rang C at
the highest voltages where a high degree of control of reclosing surges is
3.4.3.5 Lightning overvoltages - Lightning overvoltages are caused either
by direct strokes to the phase conductors, back-flashovers, or as a result of
earth flashes in the proximity of the line which produce indirect lightning
surges. The overvoltages by which subs:ation insulation is stressed are a
The confifunction of the line construction and the system configuration.
guration of the station itself has a great influence if the travelling time of
surges within the station is not negligible in relation to the front time of the
Depending on the system configuration, overvoltages with time
parameters in the range of switching surges may also arise as a result of
Lightning discharges which produce significant overvoltages, in Range
B and C, are confined to direct strokes to phase conductors or strokes to
towers or earth wires with subsequent back-flashovers.
In Range A indirect lightning surges shall also be considered.
Furthermore in this range surges transferred through transformers from
a higher voltage system need careful consideration.
3.3 Determination of the Expected Overvohage
3.5.1 Range A -For
voltages of less than 52 kV switching overvoltages
constitute generally no serious problem for overhead supply systems and
insulation co-ordination is based on lightning overvoltages.
Switching overvoltages transferred from an overhead line into a plant
through transformers or lengths of cable-may, in general, be ignored for
the same reason. An exception is the case of an insdation
the lower-voltage side of a high-voltage transformer feeder, particularly if
resonance occurs between the two systems during ane or two-phase
IS t 3716- 1978
In industrial plants and power stations, the amplitudes and
wavcshapes of switching overvoltages generated within the installation
vary over a very wide range.
In the great majority of cases they are
innocuous; in some, serious overvoltage magnitudes and rates of change may
Thus sudden voltage swings may be caused when a switching
device re-strikes; the resulting rate-of-change of voltage may equal that
caused by a severe close lightning strike.
A very large amount of practical operational experience is available
from Werent
industrial plants and power stations and the most severe
overvoltages or voltage swings may usually be avoided by eliminating
resonance and by the correct choice of switching device to be employed.
Detailed representation of the system under consideration on the
digital computer or on the TNA may not be economically justified at this
voltage level since accurate representation is needed to obtain accurate
results and a complex plant frequently consists of many pieces ofequipment
Furthermore the operation of some types of
switching device and arcing earths arc difficult to simulate with a sufficient
Experience is often the -best guide and in exceptional
cases, deliberate switching tests with simultaneous recording ( both highspeed and low-speed ) will produce the most valuable information so that
remedialmeasures may be taken as the result of subsequent calculations
and confirmatory tests.
The amplitudes, waveshapcs and frequency of occurrence of lightning
overvoltages on systems in Range A may be estimated with a reasonable
As the impulse flashover voltage of insulators used on
overhead lines in this rauge is quite low as compared with the potential
impressed on such a line by a direct lightning stroke, the stresses to which
substation quipment
is liable to be subjected are primarily determined
Thus .careful protection of substation
by the type of line construction.
equipment is required if this is connected to a wood- ole line with unearthed
crossarms. Reduced protection is needed where Jl e lines are erected on
steel masts, reinforced concrete poles or where metal crossarms are
otherwise earthed.
Apart from this important difference, the amplitude and waveshapes
are affected by the following factors which characterize the constitution of
the system and arrangement of the station.
35.1.1 Surge im_b&nce of those lines or ccblts which are conneclrd to thr
- For example, when only one line is connected to a terminal transiz
the surge is reflected at the termination and is doubled in voltage
amplitude; when n lines of the same surge impedance are connected to the
busbars of ziitation and if no lightning stroke to the line occurs near the
station, the voltage at the busbars becomes 2 u/n, where u is the amplitude
of the surge voltage transmitted along the line on which the lightning surge
I8 i 3716- 1978
35.12? Cables with an earthed metallic sheath in serus with the line or connected
between the station bwbars and apparatus to be protected - A short cable mainly
reduces the steepness of most of the waves entering the station; a cable of
one or a few kilometrcs length may also reduce the surge amplitude.
the GIX of a direct lightning stroke to the last span in front of a station, a
cable section between overhead line and station affords practically no
relief to the station equipment.
Protective earth wires on the overhead lines extending up to a fm
k&n&es ahead of the station - These are effective against close lightning
strikes to the line which are the most dangerous.
shielding by the earth wires is sufficiently well designed to prevent direct
strokes reaching the phase conductors, and that the earthing resistance
the tower is sufliciently low to reduce the risk of back flashover, for
example, 10 ohms for a 36 kV line.
35.1 .d Protective spark gaps or protective earth wires extending over one OT two
front of the sta1ion - These may materially reduce the amplitudes of
incoming surges on lines with high insulation to earth, for example, /on
fully insulated wood-pole lines.
spans in
3.5.1.5 Earthing resistances and i&lances
of the down leads of towaz,
particulars ciose to the station - In the cases of high values of earthing resistance or the inductance of the down lead of the tower or pole, a lightning
strike to such a tower or pole or to an earth wire may cause high overvoltages on the phase conductors by back flashover across the line insulators
to one or more phase conductors.
3.5.1.6 In Range A lightning surges transferred through transformer.
expressions for the electrostatic
electromagnetic terms of the transferred voltage are derived in Appendix A.
In this range of voltage4 also the insulation levels are
3.52 Range Bgenerally such that switching overvoltages are seldom a major problem
and that insulation co-ordination
is still mainly based upon lightning
overvoltages in overhead line systems.
Furthermore, also in this voltage range, there is usually no decisive
economic incentive towards a detailed study of overvoltage stresses.
Thus the considerations
in 3.5.1 also apply to Range B.
3.5.3 ZZmrgeC- Although a switching impulse test has been substituted
in this range, as more realistic, for the traditional one minute power frequency test, it is only for the highest values of the range that switching
overvoltaga become the predomiit
factor in insulation co-ordination.
then compels consideration of less
liberal designs of insulation co-ordination, while, in turn, the serious comequenca of a failure necessitate a more precise estimation of the ovcrvoltageo
These have to be evaluated for each type of significant
overvoltage in the particular system considered.
Because of the extensive computational requirements, virtually all
practical overvoltage predictions must be made using transient network
analyzers or digital computers.
Experience with studies of a wide variety of Fystcms has shown that
development of generalized formulae for expected overvoltages is difficult
because of the large number of parameters affecting the overvoltage value.
Both analogue and digital techniques of transient solution require a
reasonably highlevel of skill in problem-solving.
These skills are principally useful in the selection of significant cases ( it being impractical to study
all possibilities ), in the reduction of the system to a reasonable number of
busbars and lines ( it is not practical to represent the entire system on
either TNA or digital solutions ) and in the description of system constants
and apparatus characteristics.
Whenever possible, field tests to check the validity of the parameters
used are recommended.
In the sophisticated approaches to insulation coordination now which
are increasingly used for the highest values of voltage, the amplitudes of
the overvoltages to be expected at a given location due to a given type
of event, may not be defined by a single value ( see Fig. 1 ). It is only
possible to state what is the probability f, ( U) dU that an overvoltage
value comprised between U and U + dU may occur, f. ( U ) being the
overvoltage probability density. The probability Fo ( U ) that the value
U may be exceeded is then given by:
Fo(u> =
~(U)dUjirF,(U)=
( U ) dU.
. ..(l)
4.1.1 Self-R&wing
and .Non-Self- Restoring Insulation - Clauses 2.8 and 2.9
of IS : 2\6,65- 1977 subdivide insulation into self-restoring and non-selfrestorin8insulation
according to its behaviour in case of the occurrence of
a disruptive discharge during a dielectric test. On the former kind of insulation it is possible to carry
under conditions that imply an
appreciable risk of such discharges, for example, by applying a large number
of impulses at the rated impulse withstand voltage, or even in conditions
with deliberately applied discharges as in a 50-percent disruptive dipcharge test carried out at voltages above the rated impulse withstand level.
lxnmll8tion co-oKlh8tion
( umd
rmisior,
lS : 3716 - 1978
Line Encrgization
Across an Insulator String Due to Lightning Strokes to the Tower
f. ( IJ 2 = overvoltage probability density
( cumulative ) probability
me ( u ) = overvoltage
1;:~.
ERVOLTAGXS
IS : 3716 - l!B%
On non-self-restoring insulation a disruptive discharge destroys the
insulating property ,of the insulation and a large number of impulses at
rated withstand voltage may result in a gradual deterioration
insulation. Non-self-restoring insulation is for these reasons tested by application of a limited number of impulses at rated withstand voltage.
The degree of information on the dielectric strength of the equipment
directly obtainable may thus be much higher for self-restoring insulation.
However, in the case of non-self-restoring insulation, the economic importance for the manufacturer of the risk of having the equipment rejected
tends to oblige him to design the equipment for a very low probability of
failure under test. Taking these two factors together, no difference is made
in IS: 2165 - 1977* between impulse withstand levels, in relation to the
kind of insulation or the nature of the test.
While self-restoring insulation does not lose or modify its insulating
ability following a disruptive discharge in a dielectric test, it should not
be inferred that damage may not occur in service if the disruptive discharge
is followed by an intense power arc.
possible damage to equipment is not the only
consideration to be introduced in the selection of an acceptable risk of
discharge in service, as the effect on continuity of supply also has to be
considered. For example, a much lower probability of insulation failure is
required in the case of bus-bars than on Ldividual lines.
It shall be emphasized that the insulating structures of a piece of
equipment are always made up of self-restoring and non-self-restoring parts.
Generally it may not, therefore, be stated that the insulation of an apparatus
is self-restoring or non-self-restoring. But the probability that discharges
may occur across or through non-self-restoring parts irk the presence of selfrestoring rarts may, for different types of equipment, be negligible or not.
Due to the different voltage-time discharge characteristics of solid and air
insulations, this probability
tends tc increase with increasing impulse
voltage amplitudes: thus it may be negligible at the rated withstand voltage
but may become appreciable around the 50 percent disruptive discharge
4.1.2 SeLxtion
cf the Type of Test- For some types of apparatus, within
the range of overvoltages that tests have to simulate, the probability that a
In this cask
discharge occurs across a non-self-restoring part is negligible.
the discharge probability coincides with that of the self-restoring parts of
the apparatus and its insulation may be called essentially self-restoring; or,
for the sake of simplicity, self-restoring.
Disconnecting switches may be
considered an example of this type; in fact even when applying impulses
well above the 50 percent discharge voltage during a 50 percent discharge
test, sparkover takes place usually in air without any puncture of the porce*Insulation co-ordination( scrond
IS : 3716 - 1978
lain. For this type of equipment
is possible and recommended.
the test given in 7.3
of IS : 21651977*
Insulation of other pieces of equipment, for instance bushings, behaves
like self-restoring
insulation up to the rated impulsS withstand
level, but this implies an overinsulation
of the non-self-restoring
compared with the self-restoring
ones, and this overinsulation,
permissible, hasto be limited to an acceptable
above this value a non-negligible
exists that discharge takes
place on non-self-restoring
of this type of equipment,
that could be called combined,
should be tested according
to 7.4 of
IS : 21651977*.
Finally, the cost of the non-self-restoring
parts of some equipment,
for instance power transformers,
may be so high as to make overinsulation
of this kind of equipment
is called essentially
non-self-restoring
or, for the sake of simplicity,
non-self-restoring.
kind of insulation is verified by means of the test described
in 7.5 of
IS : 2165-1977*.
4.2 Insulation Behaviour at Power-Frequency
Voltage and Temporary Overvoltages-In
general, discharge under power-frequency
in normal operating conditions and under temporary
caused by progressive
of the insr:lating
or by exceptional
severe ambient conditions.
In the latter case, the concept
of ccntamination
( see 3.2 ).
Because of the difficulties involved, no use of statistical concepts will
be made in these specifications
in respect of insulation
voltage and temporary overvoltages ( see also 6.1 and 6.2 ).
of Insulation Under Impulse VoltagesThe ability
a given insulation to withstand the dielectric stresses caused
of an impulse of given waveshape and peak value U is,
in most cases, a random phenomenon,
even if w-e consider a time interval
so small ( such as that needed to carry out a dielectric test on equipment )
that the ambient and insulation conditions may be considered
least in respect of quantities such as pressure, temperature,
which mav be measured and which are used to define the ambient and
insulationconditions
The discharge probability
of an insulation to an impulse of given
waveshape and polarity, and to aspeak value U in a short time interval as
lEmulation co-ordination
( secot.~!mision ).
IS:3316-1978
defined above ( for example, in a dielectric test ) may be determined,
the insulation ia self-restoring,
by applying the impulse U .iV times within
this time interval, and counting the number n of discharges.
will be-the more accurate
for this probability
the value is of N.
of the discharge probability
of a given piece of nonself-restoring insulation caused by the application of a very small number of
voltage impulses of given waveshape and peak value U, which shall be
withstood without any failure is, obviously not possible; but this should not
prevent us from regarding it as existing in theory and studies are continuing
aspect of non-self-restoring
insulation ,failure.
If we consider either switching or lightning impulses of different peak
values U, we shall be able to associate with every possible value of U a
probability Pt, thus establishing
P, ( U) for a
given insulation in a short time interval at or for the sake of simplicity at
a time t ( see Fig. 2A ) .
The values of P, ( U) increase from near 0 to near 100 percent
probability in a more or less narrow band of voltage values.
the resulting curve may be defined by a biparametric
law, one parameter
being associated with the position of the band and giving an indication
of the withstand level, and the other associated with the bandwidth
giving an indication
of the scattering of the voltage values which give
appreciable proportions of both discharges and non-discharges.
Generally in a laboratory the parameter that defines the position of
the probability curve is taken as the voltage
which corrcspJnds
the 50 percent discharge (withstand ) probability.
(ct ), which corresponds to half the difference between
the voltages that give discharge probabilities
of 16 percent and 84 percent,
is usually taken as the parameter which expresses the scattering.
In the service the ambient and insulation conditions
constant. Therefore, the discharge probability
curve of insulation, as defind
above for the time t, is bound to change from one moment to another (P,, ,
P t... . ..) ( see Fig. 2A ). The variations are determined, as regards external
insulation, mainly by atmospheric
Taking the ambient and insulation conditions
as random, it will be
necessary to consider for each insulation,
P, ( U), as defined above, also a discharge
insulation PT ( U) to overvoltages of amplitude U liable to !?occur at any
For the purpose of insulainstant of a long time interval T of operation.
tion design, it is this second distribution which is of interest to the engineer
( see Fig. 2C >.
IS:3716 -1978
FIQ. 2 PROBABILITY OF DISRUPTIVE DISCHARGE ( FOR GAUSSIAN
DISTRIBUTION) OF INSULATION UNDER IMPULSE VOLTAGE
rs 13716- 1978
Similarly to P, ( U),. Pr ( U ) nmy be. d&necI by the voltage Urss
which corr
onds to the 50,percent discharge ( withstand ) probability and
by the stan?z rd deviation er of the distribution.
The variations of Pt( U) within the time interval AT may
conveniently defined by the probability density Pn which is a function
of VW taken as a random variable ( scc Fig. 2B ). This latter function
may be characterized by the 50 percent discharge voltage uTjo
and by its standard deviation an.
On the simplifying assumption that the standard deviation ut of
Pt ( u) is constant within the time interval Ar, the following relation
In clause 2.22 of IS: 21651977* the parameter that defines the
p&ion
of the probability curves P (v) is taken as the voltage which
corresponds to a withstand probability of 90 percent although the 50
percent discharge voltage which was referrcd to above is a convenient
measure for piecesof insulation that may be submitted to a 50 percent
disruptive discharge tests.
The reason for this choice is that the 50 percent disruptive dischagc
test may not generally be applied to all kinds ofinsulation. Thus, in order
to have the same value of the rated impulse withstand voltages for all
types of equipment, whatever its insulation, and to use these values
in the definitions of the statistical distributions, it has been deemed
appropriate to refix to a higher value ( 90 percent) of the withstand
probability, the rated impulse withstand voltage URw being identical
with the lowest permissible value of the statistical impulse withstand
voltage under specified test conditions ( Ut,)).
For risk-of-failure evaluations it is, however, convenient to express
the probability curves of insulation discharge in terms of their 50 percent
discharge voltages and standard deviations.
Assuming for Pt (U) a Gaussian distribution with standard deviation
et, the difference between the 59 percenh e&barge ( withstand ) vohgc
and the statistical or 90 percent wi&stand w
at is taken as PO3 or Q-66 dependin & the type of impulse, lightning or switching, unlessanother value has %een speci&d for the relevant
equipment (see 7.3 of ISt2165-1977*).
The probability of discharge Pt ( U) of a piece of equipment which
during the test is confbrrning
to what is specified in IS: 2165-1977. may
lIesuWon ~Miation
( rrcond
rmirion
PIt3716-1376
then be defined in terms of its 50 percent
standard deviation, as follows:
voltage and its
1.3 ut
The Pt ( U), defined by the above parameters, refers to the moat
severe test conditions for the equipment, since URw is the rated switching
or lightning impulse wrthstand voltage. Therefore, if impulse tests have
to be made with the equipment both dry and wet, Pt ( U) refers generally
The probability of discharge in service PT ( U) of a given piece of
equipment may be deduced from field tests only, depending on the site of
Iiowever, as a broad indication the PT ( U) of a piece of equipment
conforming to what is specified in IS : 2165-1977* may bedeflned, recalling
equation 2, in terms of its 50 percent discharge voltage and its standard
deviation, as follows:
where k is the ratio between the 50 percent discharge voltage of a given
equipment in service during a time interval A r and the 50 percent
discharge voltage under the most severe impulse test for the equipment
( wet or dry, positive or negative polarity ).
For switching impulses U of positive polarity, the values ofk and an
relevant to time intervals A I of fine dry weather or various bad weather
conditions do not show appreciable differences.
concerning the degree of ambient pollutron, at least in the range from clean
conditions to lightly polluted conditions.
For switching impulses U of negative polarity, the values of k and
on are highly dependent on the type of weather within the time interGal
Ar under consideration.
Concomitance of bad weather (rain, snow,
fog, mist etc) and not negligible pollution leads to a low value ofk; bad
weather also increases the value of on.
Values of k - 1 and un = 5 percent are suggested in these recommendations for normal conditions and a time interval Ar equal to the
seasonal cycle to cover the worst polarity impulse,
This value of aa
results in a value of or a little lower than 8 percent.
*Jnrulation
(second n&ion j.
I!%:371611376
The same values of A and an are also suggested for lightning
This gives a value of eT equal to 6 percent approximately.
The information given above is to be considered merely as broadly
indicative and it is recommended
that use shall be made of more detailed
data derived from field tests if available.
with Windings -An
windings, such as a transformer or reactor designed to withstand only fullwave tests, is vulnerable to a certain extent to a surge of high amplitude
chopped in its vicinity, because higher internal stresses than under full-wave
All flashovers
conditions may be developed across adjacent turns and coils.
to earth in a substation result in chopped waves of various degree ofampliIf because of the use of protective spark gaps these are
tude and steepness.
liable to occur frequently
in service the strength of the windings against
surges shall be determined by testing with a suitable chopped wave.
provision for such a test is left to the relevant equipment specification.
resistor-type
waves are less likely
chopped wave tests are usually not required.
to arise and
For all types of apparatus having windings, such as rotating machines,
transformers and reactors, rapidly changing voltages due to the restriking
of switching devices may also produce
similar to those caused by lightning overvoltages.
that such equipment, irrespective of whether or not it is to
be used in installations subjected to lightning overvoltages, should be tested
with a lightning-impulse
voltage to check the winding insulation for voltage
withstand across turns and coils.
devices fall under three classes:
b) expulsion-type
A only) ; and
c) spark gaps.
The choice between these three devices, which do not provide the
same degree of protection,
depends on various factors, for example,
of service, etc.
In the following clauses,
the point of view of insulation
5.2 Non-linear
Surge Arresters - These protective
devices should be designed and installed to limit the magnitudes
of over25
voltages against which they have to protect equipment so that the total
surge-arrester voltage during operation does not exceed an acceptable
value. They are defined and their characteristics are given in IS: 3070
( Part I )-1974*. Their rating is defined as the designated maximum
permissible rms value of power-frequency voltage between their terminals
at which they are designed to operate corre~fly; this voltage may be
applied to surge arresters continuously without changing their operating
characteristics. In addition to this defined capability, some types of surge
arresterst may successfullywithstand either (a) higher than rated voltage
for a specified short duration or (b) a specified number of successive
In either case a controlli
factor in the selection of the surgearrester
is its ability to interrupt powers ollow current at either the rated voltage or
at the higher temporary overvoltages.
A primary poitit is that the total voltage produced across the
terminals of the arrester at any moment during operation shall be considered
in the determination of the switching impulse protective level and the
lightning impulse protective level.
5.2.1 Lightning Impulse Protective Lwel - The lightning impulse protective
level of a surge arrester is characterized by the following voltages:
a) The sparkover voltage for a standard full lightning impulse wave
[ seeTable 3 of IS : 3070 ( Part I)-1974; 1;
b) The residual ( discharge ) voltage at the selected standard
nominal current [ seeTable 4 of IS : 3070 ( Part I )-1974* J; and
c) ihe front-of-wave sparkover voltage
(Part I )-1974; 1.
[ seeTable 3 of IS : 3070
Nary.-- The tables mentioned here give for each surge arrester voltage rating the
upper limit for each of the above voltages. If better characteristics than those zqcc&d
in IS : SO70 ( Part I )-1974* are available, the actual voltages for the specific surge
arrester will be obtainable from the mannfWurcr. IX?. it is recommended that the
actual voltages for the surge arrester protective characteMtia be used for co-ordination
studic3.
The protective level under lightning impulses is taken for insulation
co-or&nation
purposes as the highest value among either (a), (b) or (c)
divided by l-15 ( see 2.29 of IS: 2165-1977x j. This evaluation of-the
protective level gives a conventional value representing a generally acceptable approximation. For a better definition of wave-front protection by a
surge arrester, reference should be made to IS : 3070 ( Part I )-1974*.
*Specification for lightning arresters for alternating current systems: Part I Non-linear
resistor type lightning arresters (first n&.&a ).
tTbcu qxcial types of surge arresters arc at present applicable to Range C only.
$Insnlation co-ordination ( secuadr&a
I6 t 3716 - 1!978
5 9.2 Switching
Impulse Prot&vc Level - The switching impulse protective
level of a surge arrester is charauterized by the following voltages:
a) The maximum sparkover voltage for the standard impulse shapes
specified in 7.7.4 of IS : 3070 ( Part I )-1974*; and
b) The total surge arrester voltage exhibited by the surge arrester
when discharging switching surges.
The protective level for switching impulses is the higher value of (a)
or (b). Until astandard test for (b) is devised, reference should be made
to the surge arrester manufacturer.
5.5 Espdsion-Type
Surge Arresters - These protective devices operate
to limit overvoltages and interrupt follow currents within their rating.
They have low residual voltages. The characteristics of these devices are
given in IS : 3070 ( Part II )-1966t.
The impulse sparkover characteristics resemble those of protective
spark gaps but are in general lower and flatter for the same sparkover
These arresters may not appreciably limit the amplitude of the follow
current before interrupting it and may have current-interrupting ratings
which must be compared with the prospective fault current and the prospective transient recovery voltage at the point of installation.
5.4 Spark Gaps - The spark gap is a surge protective device which
of an open air gap between an energized electrode and an earth
On supply systems operating at voltages up to 245 kV spark gaps have
proved satisfactory in practice in some countries with moderate lightning
The adjustment of the gap settmgs shall often be a compromise
geFCei: perfect protection and service continuity but this difficulty may
be largely overcome by the use of rapid automatic reclosing.
The sparkover voltage and the time-to-sparkover of the gap depends
on the distance between the electrodes; they are influenced by
the shape of the electrodes and also by their disposition and distance
to the neighbouring live and earthed parts.
wentidly
In order to improve the operation of a spark gap under steep-fionted
surges and to provide a flatter impulse sparkover-voltage time characteristic,
the geometrical conf&uration of the simple rod to rod electrode arrangement
may be mod&d, for instance by appropriate shaping of the electrodes and
lSpccifieationfor lightning arresters for alternating eurrcnt systems: Part I Non-linear
m&or type lightning arredtcn (Jrd ~&&II ).
$3p&kation for lightning arrcatcrs for alternating current ryrtcms : Part II Ex@aio~
type lightningarresters.
I8:3716-1978
by provision of a central auxiliary electrode. In Range A duplex-type gaps
have also proved advantageous in regions where birds or small animals are
5.4.1 Protective C%amctit&s of u @ark Gap -The
protection obtained
by means of spark gaps is less precise and the protective level may not be
given as precisely as the protective level of non-linear surge arresters for the
following ,reasons:
a) The dispersion of the sparkover voltage of a gap; and
b) The increase in the sparkover voltage with increasing amplitude
of the applied wave when sparkover takes place on the front of the
The performance of a gap under impulse (switching or lightning )
is characterized by the 50 percent value and the standard deviation of its
Since spark gaps
discharge voltage under standard laboratory conditions.
constitute typical self-restoring insulation, the contents of 4.3 apply to them
as well. Furthermore, because of the reasons given in item (b) above,
knowledge of the times-to-sparkover of the gap for values of the applied
impulses w& aboyt the 50 percent sparkover value is often needed (see
tive Gaps
5.4.2.1 When the.gl$-operates
on a voltage surge and a power-arc
results, it frequently persists until disconnected by a fault protective device;
this constitutes a short-circuit in the case of a system with directly earthed
neutral, entails mechanical stresses on the various parts of the system equipment and may cause disturbances to consumers.
should therefore, be considered in relation to its effect on the system
protection and operation.
5.4.2.2 The gap is unacceptable from the point of view of service
countinuity if its presence noticeably increases the number of circuit
outages provided these flashovers are neither self-extinguishing
interrupted by means of high-speed tripping followed by high-speed
5.4.2.3 Spark gap operation causes chopping of the wave, thus
increasing the probability of producing chopped waves close to the termiThis has to be taken into consideration for
nals of protected apparatus.
insulation of high voltage windings ( see 4.4 ).
5.4.2.4 Damage to the apparatus may be caused by the power arc
across the gap if this is not installed in a suitable position.
a spark gap is fitted to a bushing, for example, of a transformer or circuitbreaker, its distance from the bushing surface shall be sufficiently large to
prevent a power arc being blown against the insulator,
Is t 3716- 1378
of DiGrent
Protoctivc
5.5.1 Protection with .Non-linear Resistor-Type
Surge Arresters - In order to
ensure that an apparatus is not subjected to a surge voltage exceeding that
which appears across a surge arrester,
rule t,> locate
the arrester as close as possible to the apparatus.
arresters should preferably
be either installed on the transformer
or its high-voltage and earth terminals should be connected
to the transformer by the shortest possible connections.
Similarly surge arrc&s
should be fitted close to cable terminations,
if they need protection, with the shortest possible connections
of the surge arrester and the phase conductor and the cable
sheath respectively.
NOTE - Jn the case of surge arresters close to the apparatur
following conventional safety factors are recommended:
a) Range -4 -A
safety factor of approximately
1.4 should be provided between
the rated lightning impulse withstand
level of the apparatus to be protected
and the impuhe protective level of the surge arrester.
b) Rongcz B and C- Conventional
for lightning overvoltages.
c) RcrngcC-Conventional
switching ovcrvoltagcs.
safety factors of I.2 - 1.4 are normally provided
of 1.1 - 1.2 are normally
IS : Qc)04-1978.
of surge arresters close to the apparatus
protected may be achieved more easily in Range A than in Ranges B and C.
When the surge arrester is separated from the apparatus to be protected the apparatus is subjected to a surge voltage which exceeds the
level of the arrester.
The excess voltage is due firstly to the
inductive voltage drop in the connecting
leads of the arrester itself and
those connecting
it to the apparatus to be protected. Secondly, if the time
of surge propagation
compared with the front duration of the incoming surge, the effect is a
short-time increase of the voltage at the terminals of the apparatus to bc
protected over the protective level of the surge arrester.
increase due to both these factors depends on a number of
conditions, namely, distance of the surge arrester and its location ahead of,
or behind, the apparatus to be protected; characteristics
of the !ine;.capacimnce of the apparatus to bs protected;
may be limited by all
steepness of the incoming wave.
arrangemeut.s which limit the steepness of the surge arrivmg at the station
guide for non-linear
systems ( second rcGsion ) .
ISr37l6-1978
[ extension of shielding wires, local&d capacitance, cable ( even short )
large number of connected lines]. The adoption of a reduced protective
level is another help.
5.5.2 Protection with Expulsion-ljpe
Surge Arresters - These amaters
sometimes used on high-voltage distribution circuits where shielding against
lightning is not provided ( Range A ).
The impulse sparkover voltage time characteristics of such an arrester
is flatter than that of a rod gap of the same sparkover distance, but not
quite as flat as that of soAM type of equipment, for example, a transformer
winding or a cable.
For this reason, an adequate margin of safety is required, not only
for the lightning impulse sparkover voltage of the arrester and of the equipment to be protected but also for the corresponding front-of-wave sparkover voltage. These conditions are assisted by the usual practice of
installing these types of diverter close to the equipment to be protected.
For further specification concerning the applications of these devices,
reference should be made to IS : 3070 ( Part II )-l!Wj*.
5.5.3 Protection with Spark GapsThe impulse sparkover voltage-time
curve of a spark gap is usually much more curved than those of some of the
types of apparatus to be protected, particularly those of transformers and
Due to the curved shape of the voltage-time characteristic of a spark
gap, the distance over which protection is given for all surgesis very small,
usually not more than a few metres. If a spark gap is applied for protection
against surges of a limited front steepness ( considerably lower than the
steepness of the standard lightning-impulse test voltage wave), a distance of
several tens of metres between the gap and the object to be protected does
not appreciably modify the conditions for the protection provided again&
such surges.
A spark gap is, therefore, liable to operate not infrequently when
stressed by lightning surges, and occasionally when stressed by switching
surges, the amplitudes of which are below the lightning-impulse withstand
voltages of the apparatus to be protected. In a large number of cases the
operation ofthe spark gap causes a circuit outage if the gap is on the
supply side of the opening switch. If the supply may be restored quickly
by high-speed automatic reclosing the setr;ng of the spark gap may be so
m to provide an acceptable degree of protection to the apparatus
without causing an excessive number of troublesome supply interruptions
Nom- Salty factors of the order of thoJt giVm for surge arrcdtcn secure
ge~ally
satisfactory protection provided the oeeurrencc of very steep fronted rurga
is cx&dcd
*Specification for lightning
:ype lightning am&em
arruterS for alternating
current rystems
: Part II Expulsion
CtxumDfNAmN
BammEzu
sTB%ssES
TO RANGkS A, B anil C
6.1 hsah&a
Design to Power lkqmmcy
Upewing
Tempmaq
Overvekages-Pa
amd AgeingIn 3.4 and 35 of IS : 2165-1977* it has been specified that the relevant
equipment specScation shall prescribe the long duration power-frequency
tests intended to demonstrate the behaviour of equipment with respect to
internal insulation ageing or to external pollution. Only general guidance
is given to the equipment committees; it is indicated that as regards the
voltage under normal operating conditions, the insulation shall withstand
permanent operation at the highest voltage for equipment.
6.2 PoUutiam
6.2.1 For insulation susceptible to contamination, the problem ofspecif$ng a suitable test method and pollution severity levels is at present under
consideration for various relevant equipment specifications. When contamination tests are established, it is anticipated that the system engineer will
specify a degree of pollution severity level in relation to the pollution of
the ambient in which the equipment is installed.
6.2% Table 1 gives a provisional basis to the system engineer for establishing a qualitative degree of pollution severity.
6.2.3 A scale defined in quantitative terms with reference to a test method
should be associated with each of the qualitative levels ofnatural pollution
severity for various type of insulators.
6.2.4 Besides being reproducible, a test method should, as far as possible,
satisfy the requirement of validity, that is, d a satisfactory simulation of
the natural conditions in which the equipment is to be in:talled. Therefore,
the most satisfactory tests, among those presently adopted, may vary from
case to case .
6.2.5 It shall be stressed that Table 1 does not cover some environment
at situations such as desert areas, where long dry perio.ds are followed by
condensation or light rain.
6.2.6 As an example, for line insulators, rather than a specific apparatus,
Table 2 gives an indication of the possible requirements corresponding to
the various types of tests.
- 6.2.7 An indication of the required creepage distance is also given,
although it is recognized that the performance of surface insulation is
greatly affected by insulator shape.
6.2.8 The data in Table 2 are intended to cover the behaviour of equipment at the appropriate voltage, that is, either UrJJ/J3 or U, in case of a
system which may operate with a phase earthed for long durations.
lInsadatiorl co-ordination( second rmhion ).
II) I 3716- 1978
( Clauses 6.2.2 and 6.2.5 )
PERPORMANCE OF EXISTING
No faults are observed in high
( fog,
mist, etc ) on 145 kV lines even
when equipped with less than
9 to 10 insulators of the
normal type*, nor on 245 kV
lines even when equipped with
less than 15 such insulators.
Areas without industries and
with low density of houses
equipped with heating plants;
areas with some density of
industries or houses hut subjected to frequent winds and/
or rainfalls. Ail areas shall be
situated far from the sea or at
a high altitude, and shall in
any case not be exposed to
winds from the sea.
Areas with industries not producing particularly polluting
smokes and/or with average
density of houses equipped
with heating plants; areas
with high density of houses
and/or industries but subjected to frequent clean winds
and/or rainfalls; areas exposed
to winds from the sea but not
too close to the coast ( at least
about 1 km).
Faults occur in fog conditions on
143 kV lines with leas than
9 to 10 insulators of the normal
type* and on 245 kV lines
equipped with less than 15
such insulators.
Areas with high density of industries and suburbs of large cities
with high density of heating
areas close to the sea or in any
strong winds from the sea.
Faults occur in foe conditions.
or when the wind blows from
the sea, on HV lines equipped
with normal-type insulators*
( unless the number of units
per string is exceptionally
large : more than 11 to 12
units on 145 kV lines and more
than 18 units on 245 kV lines).
V-y Herrvyt
Areas, generally of moderate
extension, subjected to industrial smokes producing particularly thick conductive deposits; areas, generally of moderate extension, very close to
the coast and exposed to very
strong and polluting winds
Faults occur in fog conditions or
during salt-storms on HV
lines, even when quipped
with antipollution-type insulatorst ( unless the number
of units per string is exceptionally high; more than 1 l-12
antipollution units on 145 kV
lines and more than 18 antipollution units on 245 kV
lines ) .
*Reference is made to normal-type ins~tlators with
-__ --the following characteristic spacing:
146 mm: diameter : 255 mm, creepage dutance : XXI mm.
tAreas of moderate extension very close to highways where a mixture of salt and bitumen may cause severe deposits on the insulators may be subjected to a high pollution level.
$The reference to antipohutiofftype i.mulators is somewhat vague. due to the great
variety ofantipolhrtion-type insulators whrch are praently in servrce on HV lines.
Is a3716- 1978
BETWE&
TEST LEVELS AND CREEPAGE
IJZVRX.S,
( Clauses6.2.6 and 6.2.8 )
POLLUnON
r--__--Salt Fog Method
Solid LayerMethod7
Withstand Salinity
No sign&ant
(cmlkV_1
NOTEI - The value reportedin the table were cs@blished on the basis of normal cap
and pin insulating For other types of insulator and pawcularly for very large~insulators
in substations, the correlation with present test methods, between test I*
creepage distance and sexvice experience is not yet sticient
to give more defimte indication&
The creepage diitances given in the table are those recommended
f& ihe
variour pollutions
leveb and do not necessarily agree with the creepage distances mved
from thccol3 of Table 1 which refer to existing lines whose behaviour to power frerlucncy
voltage may or may not be satisfactory.
65.9 If temporary overvoltages are frequent and severe, it may be
necessary to take them into account in prescribing the pollution test.
6.2.10 In the case of stations with a high degree of pollution, when it
may be impossible, or extremely expensive, to ask for the necessary performance of equipment under pollution coziitions, the alternatives of
greasing OI washing the insulating surfaces.should be considered.
For insulation susceptible to ageing, the problem
6.3 Ageiagspecifying suitable test methods are also at present under consideration.
VOLTAGE FOR RANGE A
7.1 Selection of the Rated Power FreqnesU?y WfthM8ad VO&agOTable 1 of IS : 2165-1977* indicates for each system voltage &, one value
of rated Gower-frequency withstand voltage only.
*Insulationco-ordination( &ond fruition
.,,33
l8 I 3716 - 1978
7.2 Selection of the Rated Lightning-Impulse Withstand VoltageTable 1 of IS : 2165-1977*
leaves open the choice between two corresponding rated lightning-impulse withstand voltages, according to List 1 and List 2.
Reduced rated lightning-impulse withstand voltages have been used with
good results and for a wide range of equipment over long periods of time.
Comprehensive tests have also been performed on different types of equipment for this voltage range to determine their impulse withstand voltagesi
both for standard lightning impulses and representative switching impluses.
It has been found, in particular, that the breakdown voltage of insulation
under typical switching impulses is always higher than that of the peak of
the power-frequency test voltage. This is one of the reasons why it was not
found necessary to introduce a separate switching-impulse withstand in
The choice between List I and List 2 is to be made in accordance
with 4.2 of IS : 2165-19778 and the following considerations relevant to the
equipment installations :
4 Equipment with no connection to an overhead line,
bl Equipment connected to an overhead line through a transformer,
4 Equipment connected to an overhead line either directly or
73.1 Equipment with No Connection to an Overhead Line-A
wide variety of installations is covered by this category, for example large
underground cable networks in cities, many industrial installations, power
Equipment in such positions is not subject
stations and ship installations.
to any external ( lightning ) overvoltages but may be subjected to switching overvoltages ( se6 3.4.3.3 ).
In 4.2 of IS : 2165-1977*, the conditions are specified under item 1
in which equipment to List 1 may be used in such installations.
other cases, equipment to List 2 should be used and, with few exceptions,
no mrge protection is required.
One such exception is an electric arcfurnace installation where high overvoltages are liable to develop due to
current chopping by a circuit-breaker.
Protection by special surge
arresters may be required in such a case both between phases and between
phases and earth.
7.2.2 F&ulpment
Traosfmer
to an .Overhead
7.2.2.1 Gene*1
conrlderatlona - Equipment
lower-voltage side of a transformer the higher voltage, side of which is
supplied from an overhead line is not directly subjected to lightning or
*Insulation co-ordination
( sswnd
switching overvoltages originating on the overhead line. However, due to
electrostatic and electromagnetic transference of such overvoltages from the
higher-voltage winding to the lower-voltage winding of the transformer,
such equipment may be subjected to overvoltages which, in certain
circumstances, may exceed its breakdown voltage.
Analytical expressions for the electrostatic and electromagnetic
of the transferred voltage are given in Appendix A.
For a given transformer the magnitudes and waveshapes of these
transferred overvoltages are mainly dependent on the nature of the lowervoltage circuit and, for this rewon, it is convenient to consider the selection
of the rated lightning-impulse withstand voltage of the equipment and its
protection separately for the two basic categories of installation as follows:
Category I : Equipment connected through transformers to highervoltage overhead lines, and incorporating connections of
moderate length, say up to 100 m, between the lowervoltage side of the transformer and the equipment, such
as the main supply switchgear of a cable distribution
network or an industrial installation.
C&gory 2 : Generator-transformer
a) Category 1 equipment - Factors which tend to increase the
magnitude of transferred overvoltages for such equipment are:
a transformer having a high voltage ratio and high capacitance
betwan windings;
2) a transformer disconnected from its load on the lower-voltage
connections between a transformer and its
4) a higher-voltage winding which is not earthed ( for example
delta or unearthed star ), or having a star point which is
earthed through a high reactance ( for example arc-suppression
coil ) ;
5) surges having steep wavefronts and surges having long
durations; and
surges due to energizing a transformer from a
remote point on an overhead-line system ( that is energizing a
transformer feeder ).
Estimates of the_ magnitudes of transferred overvoltages may
be made and methods of calculation, with examples, are described
Category 1 equipment may usually be protected by surge
arresters, and where such protection, is provided as a normal
practice it is not necessary to make these calculations. For other
cases, basic guidance is given below on the nature of the
tranrferred voltages, the general influence of circuit ~~nditi~n~
and the criteria which may be used to determine whether
NOTBI -For rurge voltages between phases, transferredaurge voltaga
bchvecnphases are often higher than those between phase and earth. Corn--
pared with the voltage to earth due to a surge on one phase of an overhead
line, t&e phase-twphase
voltage may theoretically approach twice as much for
the same 8urgc or three times as much for equal surges of opposite polarity on
two &arcs.
should be given to the probability of this occurring in service and hence the precautions
which are necessary to PmteCt
againstit.
resortant voltagea, a condition of resonance
systems connected by a transfbrmer may cause abnormally large voltpga to
that an examjnaby transfrmd
through the trasx+forma.
ticm of the circuits for pol$Mt resonance should be made, and modifications
shobld be made as necessary to avoid WSXIUIC~.
The application of short duration, or steeply rising, voltage
surges to the higher-voltage side of the transformer, for example,
a lightning stroke to the transmission line very close to the
transformer may through capacitive coupling, give a short duration voltage spike on the lower-voltage side. This may exceed
the impulse test voltages given inTable 1 of IS : 2165-1977*. On
the other hand, the %%test possible front time, determinedby
wave impedance of the line and transformer input capacitance,
is often so long that the capacitively transferred voltage may be
For Category 1 equipment the most onerous condition rises
when the load circuits are disconnected,
that is, on the transformer side of the lower-voltage switcbgear, since with the load
connected its capacitance is ufl;Lafy suflicient ts~ reduce the
amplitude of the initial voltage spike to a safe value.
If the capacitance of the cosmeetionsbetween the transformer
and lower-voltage switchgear is not sufficient to reduce the
amplitude of the initial voltage spike, either additional capacitance may be connected between tbe transformer terminab and
earth, or equipment in accordance with Table 1, List 2 of
IS : 2165-1977* shall be wed.
It may also be desirable to consider adding surge arresters.
Attention is also drawn to the possibility of increasing inductively
trPnsfared overvoltagcs by additional capacitance. This increase
lIilWktioa
co-o&nation
( SocQDd
may be reduced by a series damping resistor of carefully adjusted
The application of a longer duration voltage surge to the
higher-voltage side of the transformer, for example, lightning
stroke to the transmission line some distance from the transformer or a switching surge, will, through inductive coupling, give
a voltage surge on the lower-voltage side of the transformer having
a longer duration and an amplitude which shall be compared
with the peak of the power-frequency test voltage given in Table 1
of IS: 2165-1977*.
Dangerously high over-voltages can be transferred to the lower-voltage side of a transformer through capacitive coupling from the higher-voltage winding when an earth fault
exists on the higher-voltage system and when the neutral of the
system is earthed through an arc-suppression coil or when it is
Recommendations on the need for surge
Catego~ 2 r&mreat-protection of generator-transformer installations and on suitable
types of protective equipment need to be based on consideration
of overvoltages of atmospheric origin only since studies have not
revealed any more onerous condition likely to arise from Pransfenmce of switching surges. Corresponding to the front of an
incident lightning surge or to the collapse of voltage due to chop
ping there may be a capacitively transferred voltage of short
duration ( initial voltage spike ). This is independent of the
longer-duration voltage which is usually transferred by the
combined effect of inductive and capacitive couplings.
The maximum amplitude of the initial voltage spike is
highly dependent on details of the design of the installation.
Where these are such as to assist capacitive transference there
may be justification for making a low-voltage surge-injection test
on the installation or on the generator-transformer connected to
a circuit simulating the generator and its external connections.
Factors which tend to increase the magnitude of transferred
over-voltages for such equipment are :
1) high capacitance between the transformer windings,
2) low-capacitance
3) high voltage r&o of transformer,
4) 3 Iamzr-voltage transfbrmcr winding not cnw
5) surgia having steep wavefronts and surges having iong
lrmBul8tioll co-ordin8tion ( d
IS13716-1978
If there are indications that the amplitude of the initial
voltage spike should be reduced, this may be done effectively by
connecting capacitors from each phase to earth by means of lowinductance connections, preferably at the lower-voltage terminals
Attention is drawn, however, to the possibility of increasing inductively-transferred
overvoltages by additional capacitors.
The longer-duration transference generally takes the form of
a unidirectional voltage with superimposed oscillations having a
frequency of several kilohertz and if reduction of this is necessary,
consideration should be given to the addition of surge arresters.
However, voltage division between the reactances of the
generator transformer and the generator normally ensures that
the amplitude of the longer-duration
transference does not
warrant the use of surge arresters.
If the generator transformer
can be energized from the high-voltage system when the generator is disconnected, this voltage division does not occur and
consideration should be given to the higher amplitude of the
longer-duration transference affecting that part of the lowerFor
voltage circuit which remains connected to the transformer.
large generator installations, surge arresters located on the
generator are not considered as protecting the low voltage side of
a connected transformer and calculations should be made. In
so far as surge arresters can readily be applied to small installations there is no necessity in these cases for calculations of
transference to be made.
The effects of the application of longer duration voltage
surges to the higher-voltage side of a transformer and their transference to the lower-voltage side when an earth fault exists on the
higher-voltage system when the neutral of that system is earthed
through an arc-suppression coil, or if it is isolated, are subject to
the same considerations as described for Category 1 equipment.
7.2.2.3 Selection of insulation level - The choice of whether to use List
1 or 2 of Table 1 of IS : 2165-1977*
and whether additional overvoltage
protection is necessary should be based in the first place on service experience with similar installations. It may also be useful to make measurements on an existing similar installation, using a low-voltage impuheinjection method.
For a large installation, and where the necessary data concerning the
transformer and the protective equipment are available, it will be useful
to calculate the overvoltages liable to be transferred and to compare the
results with the appropriate withstand voltages of the equipment to be
*Insulationco-ordination( sued mui*
IS t 3716 - 1978
This is normally advisable only for direct connections between
generator and transformer and for low-voltage tertiary windings on large
system transformers.
If a circuit-breaker is installed between a generatortransformer and its associated generator, consideration should be given to
the cases when the breaker is closed and when it is open, although a load
is usually connected to the lower-voltage side of the transformer by which
transferred voltages may be reduced even in the latter case.
Several methods of calculation have been published and, on the
whole, these seem to give similar results. Although no absolute accuracy
may be claimed for any method of calculation, comparison between calculation and experimental results on a variety of installations has shown
It is therefore deemed appropriate to illustrate a
method- of calculation by reference to two numerical examples, covering
respectively Categories 1 and 2. Examples are given in A-2.
7.2.3 Equi/munt Connected Directly to an Overhead Line - Equipment
installed in a substation connected directly to an overhead line is subject
to direct or indirect lightning overvoltages. Such equipment should, as a
general rule, comply with the rated lightning-impulse withstand voltages
specified in List 2 (Table 1 ) of IS : 2165-1977*.
All equipment and, in particular, transformers in such positions
require protection by surge arresters or spark gaps. Having regard to the
flat impulse-breakdown time characteristic of a transformer winding, transformers should preferably be protected by non-linear resistor-type surge
arresters in areas of intense lightning activity.
In areas of moderate
lightning activity, expulsion-type surge arresters may be used. Where
lightning activity is slight, protective spark gapiZave proved adequate,
particularly where the transformer is connected to a line with earthed
crossarms or where the transformer is designed to withstand steep-fronted
chopped waves.
The bushings of circuit-breakers, instrument transformers and substation insulators having curved impulse flashover voltage time characteristics,
may be effectively protected by existing protective devices on the
In areas of moderate or low lightning activity, equipment having
rated lightning-impulse withstand voltages in accordance with List 1 of
Table 1 of IS : 2165-1977 may be used but, in that case, careful attention
has to be paid to adequate covervoltage protection. In systems the neutral
of which is earthed through a low resistance, surge arresters or spark gaps
may be used for this purpose. In systems the neutral of_which is earthed
through an arc-suppression coil, adequate overvoltage protection shall be
provided; if surge arresters are used, those which can withstand repeated
operations during the persistence of arcing grounds are recommended.
lIslsulUionco-ordination( UC& wvi#i8a).
I8 t 3716- 1978
In the absence of any overvoltage-protective device, lightning surges
impressed on an overhead line are limited only by sparkovers on the line
at the weakest points which the surges meet during their propagation. If
not correctly localized, such sparkoven may cause damage to equipment
as a result of surge r&ections between the point of sparkover and a
vulnerable apparatus, such as a transformer winding.
In the case of a substation with a number of lines normally connected
to the busbars, the surge voltage arising at the busbars is likely to be
sticiently
reduced (stt 3.5.1 ) not to overstress apparatus in the
However, such a solution ( no overvoltage-protective device ) may be
acceptable in practice on overhead supply systems in regions of very low
lightning activity, at least if equipment to List 2 of Table 1 of IS : 21651977*, is used.
7.24 Equij?mtnf Conntcted to an Ovtrhtad Line Through a Cable - Insulation
co-ordination in this case is not only concerned with the protection of the
substation equipment but also with that of the cable. In this respect the
two ends of the cable may be assumed to be subjected approximately to
the same overvoltage amplitudes.
When a lightning surge propagated along an overhead line impinges
on a cable, the latter acts, substantially, like __a capacirance. Thus the
front steepness of the original surge is reduced as the surge voltage enters
the cable. The amplitude of Us of the surge entering the cable is given
u, = amplitude of surge voltage on overhead line;
surge impedance of overhead line, in practice & = 400
to 500 ohms; and
,& = surge impedance of cable; in practice & = 25 to 50
ohms but for some types of cable it may be as low as 5
This initial surge is reflected at the station end of the cable in
accordance with the effective surge impedance at the station busbar.
Subsequent refleetions along the cable continue to be governed bv the
above equation with due regard to the fact that VI and & invakably
refer to the wave which impinges on a point of reflection while U, and 2s
refer to the reflected wave.
( smnd r&.kn
I6 I 3716 - 1978
Provided at least one further cable of a few hundred metres length
is permanently connected to the busbar, the surge voltage to which the
cable and the station equipment aresubjected is notably lower than that
on the line on which the surge originated and this reduction is all the
greater, the lower the surge impedance of the cable.
However, if the cable only supplies a terminal transformer the
incident wave is doubled in amplitude at the transfbrmer.
successive reflections at both cable terminations, this voltage build-up increases towards twice the amplitude until no further energy is supplied by
the original surge.
For a station to which at least two cables are connected, a decision
on the adequacy of equipment to List 1 of Table 1 of IS : 2l65-1977*
the need for overvoltage protection may be determined from the above
However, in the case of a terminal station, the ultimate surge-voltage
amplitudes developed at the cable terminations as a result of successive
reflections are a function of the amplitude and duration of the original
lightning-surge voltage on the line, the length of the cable, and if the
stroke is fairly close to the cable, also the reflections from the point of
strike. For lines with fully insulated crossarm s, the resulting voltage ampli- tudes are so high that, even using substation equipment and a cable with
lightning-impulse withstand voltages to List 2 of Table 1 of IS : 2165-1977+,
surge arresters shall be used at the line/cable junction.
the maximum cable lengths are plotted in Fig. 3 for which the cable and
the substation equipment may be protected by surge .arresters at the line/
cable junction only; the figure demonstrates the considerable benefit of-the
The protection is fully effective
use of a cable of low surge impedance.
against direct and indirect lightning surges and surges due to back flashover provided these originate a few spans away from the line/cable
If the cable length exceeds the values indicated in Fig. 3
additional surge arresters are required at the substation end of the cable.
If surge arresters with lower sparkover voltages than specified in IS : 3070
( Part I )-1974t are used the cable lengths indicated in Fig. 3 may be
increased in proportion to the differences indicated by comparing say a
10.5 kV with a 12 kV surge arrester.
For lines with earthed crossarms feeding a cable with terminal
transformer the impulse flashover voltage to earth of the line insulation i;
only slightly hi her than the corresponding value in List 2 of Table 1 of
IS : 2165-1977 # . In such a case, surge arresters may be required at the
linelcable junction and it may also be necessary to use these at the station
*Insulationco-ordination( s8cond
rroirion
*Lightningarreatcn for altcmatiug eumzntsystems: Part I Non-linearr&or
lightningarresters.
c8blc surge inlpcdaMxin ohmr
L - cable length in metrea
~&UUMUMPERMISSIBLECABUS LENOTIS WITIX
OF LINE/CABLE JUNC~ON ONLY
away from the cable termination.
strokes is generally not possible.
flmum~~
apply to direct stroker a few spana
Full protection against very close
In areas of moderate or low lightning activity, protective spark gapa
may be used in place of surge arresters. However, if the spark gaps at the
line/cable junction are earthed through a low resistance ( the usual case )
and if the cable is terminated in a transformer, dangerous surge voltages
can be developed across the transformer winding. The spark gaps at the
line/cable junction should therefore be earthed through a resistance of
several tens of ohms, ideally equalling the surge impedance of the cable.
Greatly improved protection may be achieved by installing additional
spark gaps-across he line insulators on the first and second poles in front
of the line/cable termination and, in this case, the earthing resistances of
these additional spark gaps are immaterial.
FOR RANGE B
8.1 Selection of the Rated Power Frequency Withstand
the Rated Lightning Impulse Withstand Voltage
8.1.1 Many considerations concerning voltages in Rahge A still apply to
Range B. However, the variety of equipment and situations is not so great
as in Range A.
8.1.2 In Table 2 of IS : 2165-1977*,
one value of rated lightning-impulse
withstand voltage only is associated with each value of rated powerfrequency withstand voltage. Therefore, there will be a unique choice
for the rated power-frequency withstand voltage and the rated lightning
8.1.3 With each value of U, are associated one to three values of rated
power-frequency withstand voltage with a corresponding rated lightningimpulse withstand voltage.
8.1.4 The choice between
72.5 kV shall take account of:
a) the neutral earthing conditions; and
b) the existence of protective devices, their characteristics and their
distance from the equipment considered.
8.1.5 The conventional safety factors normally employed in the application of surge arresters in Range B are given in 5.5&
VOLTAGE FOR RANGE C
$I Insulation Design to Power-Frequency Voltage and Temporary
Overvoltages - For this range of voltage, power-frequency tests are to
be specified in the relevant equipment specifications, in accordance with
the considerations in 6, and taking intoaccount that the temporary phaseto-earth overvoltages will not usually exceed 1.5 p.u. ( per unit ) for one
second on each occasion.
IS: $716 - 1978
9.2 Insdation Dcsigp to Switchin and Li&tninS OvervoltagesIS: 2165-1977*
proposes two mcth J s for coordination of insulation in
respect of switching and lightning overvoltages, namely, a conventional
and a statistical method.
9.2.1 Conurational Method - This is based upon the established concepts
of maximum overvoltages stressing insulation and of minimum strength of
the insulation (2.20 and 2.23 of IS : 2165-1977* ).
The statement of the minimum strength and that of the maximum
overvoltages are rather arbitrary since a rigorous rule may seldom be
followed in the evaluation of the upper and lower limits of the insulation
strength and overvoltage value, which are intrinsically random variables.
Insulation is selected in such a way as to achieve a sufficient margin
between the maximum overvoltage and the minimum strength.
margin is intended to cover the uncertainties of the designer in the evaluation of the maximum overvoltage and of the minimum strength, and no
endeavour is made to assess quantitatively the risk that in&&ion may
The conventional safety factors normally employed in the application
of surge arresters in Range C are to be found in 55.1.
9.2.2 Stutirticuf Method - This attempts to quantify the risk of fhilure for
use as a safety index in insulation design.
Rational insulation design of a transmission system should be based
on the minimum of the installation cost plus the capitalized yearly operational cost and yearly cost of failure, the latter being calculated as the
estimated cost of failure of insulation multiplied by the average expected
number of insulation failures per year.
In order to evaluate the average expected number of failures per
year of a piece of insulation located at a given point of the system in consequence of overvoltages, all the events giving rise to overvoltages whieh
Then tbr
may affect insulation design should be taken into consideration.
each type of event considered the frequency of occurrence during the year
and a separate diitribution of the overvoltage amplitude would be
It b evidqt
that the amplituda of all the overvoltaga
NFayater# can not be combined in one distribution but that only overvoltagea identified by
the same location and cause may bc considered as statistically hemogeneow.
Actudy,
since the ovcrvc+gc
severity differa for waveshapeswhich are compar@le
impulseand with a switching impulse ( w Note under
tively with a ligh
tnmV 977. ) the kervoltage
&:F&d
2.18 of IS: 2165amplitude could be eaid to he
h0m-u
only if i&Mied by the same location, cause and shape.
overvoltrga due to the lsune es*= at a given locationhave broadly a imik
%l8ulation
co-ordktion
( su#nd &
Is ; 3716 - 1976
same cause and lotition may, for the rake of
al4ttlcnfaclhouidalti6edbytlu
If problems of rtandardization of the UVdPsimpkity,be~alhomogalann.
mcnt of an attire network are to bc dult with, ~II extion
of the concept ofa
*luJmogencouSgcuqJof oxrvoltagunceda
to lx considered. In tbii caac a vouP of
ovcrvoltagcS may bc said to be haarogmcous if the 0~er~01tages occur in Similar
locationa of the q&m due to the same arue
For acam~le the rcclosiog ovcrvoltages
on the bus-but ( sending-end ) of any cobjtation of tbc q~tem may be considered a a
bomogcneo~ gtoup ofo==ltag=
a process of insulation design and coordination as outlined
above involves too many difficulties. The statistical approach considered
here is thvefbre restricted to checking that the risk of insulation failure
due to any foreseeable type of event causing overvoltages in the system is
within acceptable limits. These limits depend on the frequency of occurrence of the type of event and on the consequences of the failure of the
piece of insulation under consideration.
Fortunately, the types of event which are significant in insulation
design are generally a few in number to allow an analytical approach.
For instance, the insulation withstand to switching surges of many pieces
of equipment. in a system is, in general, determined primarily by reclosing
If the frequency distribution of overvoltages caused by a given type
of event and the corresponding insulation strength are known, thi risk of
failure ma-? be expressed numerically, as will be shown below.
Let the withstand strength of a given piece of insulation
given time interval Al be defined by the probability PT ( U) of discharge
of the insulation when it is subjected to an overvoltage of value U
( sac Fig. 4 j. Furthermore let the distribution of the over-voltages stressing
the same piece of insulation for the specific type of event considered be
defined by rhe probability density fO( U).
Then the probability that an
~~LQ!&+Q~ of value comprised between U and U + dU may occur is
fo( C ) d4f: %eprobab&tv density-of i&:urc of the insulation due to an
value U is, therefore, the product of the probabihty dens*
that an overvoltage of value u may occur and the probability that the
insulation may fail under an overvoltage of value U that is:
The probability of lirilurc for a value of U at random, that is the risk
of failure R for an event of the type considered, will then be:
fo ( u).pT
( U).dU
ISr3716-1978
---------_-a--
= P,(Uj*fdU~ -J
(U) fo(U)dU
R I #hadedarea
OF THE RISK OF FAILURE OF A
This expression shows the general principle of the method by which
the probability of failure may be assessed. It assumes that_& ( U) and
PT ( U) are uncorrelated.
Non! -In principle formula (7) applies to a single-phase piece of insulation only.
If several pieces of equipment, connected in parallel on the nave phase, are subjected
to the same overvoltage then it may be assumed that the overall risk is equal to that of
a single piece of equipment multiplied by the number of piccu in parallel.
valid if we take into consideration the fact that the risk of failure acceptable for subrtation insulation ia usualiy very low.
it is necessary to evaluate the risk of failure of at least one
phase of a three-phase section of the system following a switching operation
( for example a closing operation ). This risk may be obtained by multiplying by 3 the risk evaluated according to formula (7) if the probabil&y
density f. ( U) of the overvoltages may be assumed to be equal on all
An alternative method is to establish the overvoltage probability
density f.(LJ)by considering only the highest value of the overvoltages
caused on the three phases by a switching operation.
Then the risk of
failure ia evaluated by making use of formula (7).
The former approach gives risk values higher than the actual ones;
the latter lower. Obviously the two approaches give results differing by
let43 than 1 to 3.
overvoltage in formula (
a) Peaks other than
model chosen for defining the severity of an
7 ) is based upon a few simplifications. In fact
the highest one in the wave-shape of overvoltages
b) The wave-shape of the highest peak is assumed to be equal to that
of the standard switching or lightning impulse.
c) The highest overvoltage peaks are assumed to be all of the same
polarity; to be on the safe side the more severe polarity will be
As regards switching overvoltages, which are the overvoltages of
predominant importance in insulation design of EHV systems, assumption
(a) is such as to give a calculated risk of failure lower than the actual
risk. Assumptions (b) and (c) result in a calculated risk higher than
the actual one, since the standard wave-shapes are so chosen as to establish
the lowest withstand of apparatus (see 7.2 of IS : 2165-1977* ).
In general, considering the opposite effects of the assumptions made,
the risk of failure calculated by means of formula (7) gives risk values
greater by about O-5 to 1 decades ( 5 to 10 times ) than the actual values.
Normally formula (7) is, therefore, conservative.
As said above, formula
(7) may be applied for all the specific types of events significant in
Furthermore, it is clear that the accuracy in
risk failure greatly depends on the accuracy in the
overvoltages and of the impulse discharge probability
accuracy of thesa is seldom satisfactory, the accuracy
of failure can be correspondingly poor.
of insulation. Since
of the calculated risk
However, the risk of failure has a precise physical meaning ( contrary
to the conventional safety factor ). By making use of statistical methods
it is therefore, possible to coordinate the security levels of the various parts
of system according to the consequences of a fault. Furthermore,
possible to carry out sensitivity analysis ( for example, the effect of a change
in the overvoltage severity or insulation withstand capability on the
probability of faults ). Statistical methods do, therefore, enable the
engineer to take a decision on a rational basis.
According to the statistical method, insulation is selected in such a
way as to obtain a calculated probability of failure lower than, or qua1
*Insulationco-e&nation ( suond mmon ).
to, a predetermined value that characterizes the required safety, level.
Referring to Fig. 4 a change in the insulation level shifts the curve reprcscnting the discharge probability of the insulation PT ( U) along the
U axis with a consequent modification of the shaded area A which represcnts the probability of failure R for U at random.
The statistical approach may require successive series of tentative
designs and evahlations of risk, until a design that satisfies the risk prerequisites is found.
Formula (7) may also be applied to determine the probability of
failure of an insulation protected by spark gaps or surge arresters, if
PT ( U) is taken as the discharge probability of the insulation in presence
of the protective device. If the time-to-discharge of the protective device
can be_ considered always shorter than that of the insulation to be protected an equally valid and simpler method is to use formula (7) and to
take&( U) as the overvoltage probability density modified by the protective device ( see Appendix C ).
The use of digital computers makes it easy to evaluate the risk of
failure, and therefore the insulation design, once the overvoltage distribution and the discharge probability curves of insulation are known.
9.29 Simprified Statisticul A&hod - Sensitivity analyses and ready
evaluations of the risk of failure may be made on the basis of simplified
statistical methods in which the calculations are performed once andfor
all hy making some generally acceptable assumptions concerning the
mathematical laws by which the actual distributions of the overvoltages
and of the discharge probability of insulation are represented for example
by assuming them to be Gaussian with known standard deviations.~~With these assumptions the complete distribution of overvoltage and
the discharge probability of insulation may be defined by one point only
corresponding to a given reference probability and called in IS : 2165-1977*
statistical overvoltage (~6.219)
and statistical impulse withstand voltage
( see 222)
The risk of failure may be correlated with the
margin between these two values, so that the approach becomes rather
similar to that in the conventional method.
Figure 5 gives a graphical expl
-on of the method. Figure 5A
shows frequency distributions of wervo $
and insulation strength, where
the statistical overvoltage is indicated by
?? s and the statistical withstand
In Fig.SB, the overvoitage/distribution
voltage by V;.
strength distribution are superimposed for three values l-0, 1.2 and 1.4 of
the statistical safety factor ( y ) relating U, and Ur. The correlation
between statistical safety/factor and risk of failure is given in Fig. 5C.
EB : 3716 - 1376
3 IU)
l=d!!d
US = statistical
overvoltage R placed
on the probability density curve
[ the shadad area (2%)
reprarentsthe rofermce probability].
UW = statistical
Irced on the discharge probaEIlity curve (90% represents
the reference probability ).
Three attempts at determining
risk of failure [ area A) for statistical
Iy=l*O*
i-2 and I.4
rtatlsticai
safety fmor
( y ) mnd the riJr of
failure R ( nnrrurrd by +ru A).
~IYPLIPIBD
MTitOD
ts I 3716 - 1978
The reference probability of the overvoltages is chosen in this
standard equal to 2 percent.
The reasons of this choice are discussed
later in this clause. As regards the reference probability of the withstand
voltage the 90 percent value was chosen in IS : 2165-1977* for the reasons
given in 4.3 of this standard.
As regards switching surges, Fig. 6, 7 and 8 illustrate the rehitionship
between risk of failure and the statistical safety margin for air insulation
The discharge probability curve of insulation was assumed to be
Gaussian as stated in 4.3 with k ( formula 5 in 4.3 ) equal to 1 and er
( formula 2 in 4.3) equal to 6 percent, 8 percent and 10 percent ( see Fig.
6, 7 and 8 respectively ). If t were to be taken as differing from 1, the
statistical safety factor given for k = 1 would have to be multiplied
Figure 6 is applicable to laboratory conditions, while Fig. 7 is
generally applicable to service conditions. Figure 8 may be used for particularly severe conditions (high values of an in formula 2 >. In all the
three cases the overvoltage distributions were assumed to be Gaussian
truncated at three and four times the standard deviation aBr or not truncated and with standard deviation Usequal to 10 percent, 15 percent and 20
percent. Figures 6 to 8 give the average correlation between statistical safety
factor and risk of failure as well as the upper and lower enveIopcs of the
correlations obtained when considering the nine overvoltage distributions
resulting from all the possible.combinations of values of standard deviation
and upper truncation point.
The choice of a Gaussian distribution to define the overvoltage
severity does not mean that other distributions ( for example extreme value
distribution) may not give better approximations,
but that Gaussian
distributions match actual distributions reanonably well over the range of
The correlation between the statistical safety factor and risk of failure
appears to be only slightly affected by changes in the shape of the ovcrThis is due to the fact that the 2 percent value
chosen as a reference probability of the overvoltages falls in that part of the
overvoltage distribution which gives the major contribution to the risk of
If, on the contrary, a much lower
f:$lure$n the range of risk considered.
or hi her value were chosen, the influence of the shape of the overvoltage
distri %ution would have been very pronounced.
Figures 6 to 8give the risk of failure of a piece of single-phase equipment ( for example a post insulator ). If the risk of failure of several pieces
of equipment is required reference may be made to the Note under 9.2.2.
*Insulationco-ordination( second revisiin ).
Dlrttlbutlon truncated at 3 *
Dlstribullon truncated at
Dlrtrlbutlon not truncated
Upper mvrlopo
Standard deviation of ov~rvoltagc distribution w, Standard deviation of inrulation q - 6 percent
FIO. 6
10, 15 and 20 percent
BETWEEN RISKOF FAILURE(R) AND STATISTICAL
SAFETYFACTOR (y )POR VARIOUSSwx~cxmo SURQEDISTRIBUTIONS
Stamiard deviation of insulation a~ - 8 percent
FIG. 7 CORRELATIONS
BETWEEN RISK OF FAILURE ( R ) AND
STATISTICAL SAFETY FACTOR ( y ) FOR v~Rr0u.s
!hlTCliINt3
!bROa,
hTJUBUTIONS
For example, if the number of single-phase pieca of equipment at a
line entrance is equal to 21 ( seven on each phase ) and the risk of &Sue
of each for a &ret-phase reclo&g is IV, then the risk of firilun fbr the
whole line entrance will be clou to 21 .lW.
Extension of this method to ove&ead lizua ir p&ble
problems particularly for long lines with which the present guide iETn not
I6 I 3716 - 1918
Standrrd deviation of insulation UT = 10percent
Fm. 8 CORRELATIONS BETWUIU RISK OF FAILURE ( R ) AND STATISTICAL
SAFETY FACTOR ( y ) FOR VAR~US SWITCHING SURGE DLWRIBUTIONS
As regards lightning surges, analogous correlations between statistical safety factor and risk of faihrre are given in Fig. 9 to 11 for
comparison purposes always for air insulation.
Assumptions similar to those of the previous paragraphs were made
as regards the discharge probability curve of insulation to lightning surges.
The lightning overvoltage distributions were assumed to be Gaussian
and not truncated, with standard deviations equal to 40 percent
percent. It is thought that such di&ributions approximate actual ligI?tning
overvoltage distributions quite well around the 2 percent value.
I8 83716 - lmb
Figures 9, 10 and 11 give the correlation between statistical safety
factor and risk of failure for both overvoltage distributions and for
standard deviations of insulation equal to 3 percent, 5 percent and
7 percent respectively.
NOTII- The correlations between statistical safety factor and risk of failure given
above apply to relf-restoring insulation only. However, they may be considered
acceptable for the entire equipment in most cases for the following reasons:
The tats da&bed in 7.4 and 73 of IS : 2165-1977. are intended to ascertain
the impulse insulating ability of self-restoring equipment and combined equipment
respectively. Such tats do not allow any discharge on the non-self-restoring
parts of the apparatus. Consequently, when both the self-restoring pieces of
insulation of the same apparatus are designed on the basis of the same risk of
failing the test, the non-self-restoring parts will have an inherently,. lower discharge
probability than the self-restoring parts in respect of overvoltages of the same
amplitudes as those of the impulses applied during the test.
Consequently, it may be stated that risk failure of the non-self-restoring parts
of self-restoring equipment ( tested according to 7.4 of IS : 2165-1977 ) is lower
than tbat ofthe self-restoring parts if the major contribution to the overall risk
of fiilure .ir given by overvoltages in the range of U56y0 * 20.
For combined insulation equipment the non-self-restoring parts will be
designed for a low risk of failure at test voltage. In the cases where the major
contribution to the failure risk ( calculated on the basis of the statistical withrtand
voltage ) is given by overvoltages around the test voltage the evaluation of the
risk of failure may be carried out as for self-restoring insulation.
The foregoing considerations naturally presuppose that wave-chopping by a
self-restoring piece of insulation does not cause serious stresses in the non-sclfrestoring insulation of equipment and does not cause ageing of insulation.
9.3 Block Diagram of the Insqlation Design and Coordination
Installation - Most predictions or analyses of system overvoltage levels assume that a piece of equipment ( for example, a circuit
breaker ) will operate as designed. In other cases, an arbitrary limit may
be placed on the credible severity of surges, as is often done in the case
of lightning. It is obvious that surge levels based on such assumptions
will sometimes be exceeded.
Whether or not it is necessary to take this into account, depends
largely on the consequence of failure resulting from such abnormally high
overvoltages. For example, the consequences of transformer or reactor
failure are so serious that there insulation co-ordination must usually
provide for even extreme contingencies.
This is achieved by applying
surge *arresters at their terminals.
There are other types of equipment
such as post insulators, disconnecting switches, etc, where the consequence
of failure is not so serious as to warrant application of surge arresters.
In developing a squence of insulation design and co-ordination of
an electrical system in the form of a block diagram, it is convenient to
lInrJation co-ordination( sacodrmirion
IS:37169,I978
0.5 0.6 0.7 (I-80.9 1-O 1.1 1.2 1.3y
o8 = 40 aqd 60 percent
Fro. 9 CORRELATIONSBETWEEN RISK OF FAILURE (R) AND STATISTICAL
SAPETY FACTOR (y) FOR VARIOUS LIGHTNING SURGE DISTRIBUTIONS
differentiate between a Gas:: I ( surge arrester protection) and Case II ( no
surge arrester or remote surge arrester protection).
illustrating the method to be adopted is shown in Fig. 12.
The first step in insulation co-ordination ( Block 5 ), common to all
types of equipment, has the purpose of ensuring the eqtlipment ability of
withstanding poweti-frequency voltage under normal conditions and under
tempdrary overvoltages.
The system engineer will specie an equivalent pollution severity test
level for insulation susceptible to contamination,
however, no special
specifications will be given for insulation susceptible to ageing ( see 9.1).
If the expected phase-to-earth temporary overvoltages ( Block 3 ) are
more severe than the overvoltages taken into consideration in the relevant
cquipmznt specification in specifying the power-frequency tests discussed
ISt3716-1978
O-5 O-6 $7
0.j 69
l-0 l-1 I.2 1.3'f
Stmdard deviation of imuhthn a-r - 5 percent
Standard deviation of overvoltage distribution e, - 40 md 60 percent
Fxa. 10 CORREWTION~BSTWG~NBUK OF FAIL( R ) AND
STATISTICALSAFETY FACTOR ( y ) FOR VARIOVS LIOHTMNO
thltOE hWUlWTIONS
in 3.6 of IS :2?65-1977*,
it will be necessary to specify different voltage
levels or durations of the test or to ado t suitable means or operational
procedures to reduce temporary overvo Ptages in the system (feedback
from Block 5 to Block 2 dashed line).
Insulation dosign as regards operating
voltages and temporary
over-voltages leads to a certain withstand of the equipment to both switching
For instance, if a given withstand salinity is
and lightning impulses.
required for a post insulator, a minimum distance in air is obtained which
varies according to the post insulator type (see Appendix E ). The equipment will, therefore, exhibit a certain withstand to switching surges due to
requirements imposed by the operating voltages and temporary
in Fig. 12 by dotted lines
( for example, from Block 5 to Block 7 and Block 8 ).
*Insulation co-ordination (second reuisioh).
O-5 O-6 0;7 O--80;9
1;l 112 1.3 I'
Standard deviation of insulation OT= 7 pacat
Standard deviation of ovcrvoltagc distribution ea - 40 and 60 percent
BETWEEN RISKOF FAILURE(R) AND
SAFETY FACTOR (y) POR VARIOUSLIOHTNINO
SURGEDISTRIBUTIONS
Then it is necessary to consider Case I and Case II separately.
Examples of selection of the rated switching and lightning impulse
withstand voltage are given in Appendix D.
As regards apparatus of the first type ( Case I ), choice of the rated
switching and lightning impulse withstand voltages is usually made as
Choose the rated voltage of the surge arresters on the basis of the
temporary overvoltages (Block 6 ); thus the protective levels of
the arresters under switching and lightning impulses will also be
determined at least within certain limits.
Choose the rated switching and lightning impulse withstand
voltages of the apparatus on the basis of conventional safety
factors dictated by experience ( SCI5.4-l and 5.5.1).
in system design or suggest suitable
to reduce temporary
economic incentive exists to reduce insulation
levels ( start again
from Block 2 ).
voltage of the apparatus
disregards the severity of the
actual switching and lightning
by which the apparatus connected in parallel with the surge arresters may be stressed since it is based
on the protective level of the surge arresters only.
levels of the apparatus
of Case II are usually
a) Choose the rated switching-impulse
on the basis of an acceptable
risk which may be
estimated directly on the basis of the expected distributions
the overvoltages
and of the discharge
or by means of the correlations given in 9.2.3 between
the risk of falure
and the statistical safety factor ( simplified
Actually these correlations
apply to selfrestoring insulation
only, but norinally
acceptable for the entire equipment ( SM Note under 9.2.3).
NOTE -Protection
against switching surges afforded by surge arresters
installed close to equipment in Case I and gaps installed at the line-entrance
may often be discounted as regards Case II apparatus for the following
i) Most of the types of equipment belonging to Case II ( especially the lineentrance apparatus)
may at times be isolated from the surge arresters
installed in the station to protect Case I apparatus;
ii) With the present technology of surge arresters, the protective level against
switching impulses is often greater than, or equal to the highut awitching
overvoltage which may occur with correct bebaviour of system apparatus.
Insulation must therefore be designed to withstand these overvoltage; and
iii) Spark gaps may not provide a substantial degree of protection against
switching surges if undesired sparkovers are to be avoided.
In Fig. 12 the block between
Block 4 and Block 8 is therefore
b) Adopt suitable means in system design to reduce switching overvoltages, if this is possible and if an economic incentive
No economic incentive may exist in reducing insulation : for instance if the withstand
to normal operating voltages
overvoltages calls for higher insulation than
switching surges ( dotted line between Block 5 and Block 8 ).
Oparatlng voltage
Charactariaticr
of thr ryatom
ovrrvoltagrr
lntrodktion
to reduce trmporary or
awitchlng ovarvoltrgra
overvoltagaa
Salecttion
of surge
ir----L----
aa regarda operating
ovarvoltagsa
1.... .. I.(I..,..........,.......,*........*... ..:
........ ... ...... ...*.............
PoeaMe protscuvo
effect of ourgo
arroatora and spark gape
of ratad lightnlng
and awltching withstand
impulao voltage
of Case I apparatus
:a.........,......... *.... . ..
and switching withstand
Jmpular voltage
of case II apparatus
lntroductioc of means to
raduce llghtning ovorvoltagra
----_---------------l
of earth wires
spark gape
at the line-entrance
BL~~K-D~~~AY
4----Selection
df rated
lightning withstand
overvoltage0
OF INSULATIONCO-ORDINATION
AND DESION
Verify that the rated lightning impulse withstand voltage, corresponding ( in Table 3 of IS : 2165-1977* ) to the rated switching
impulse withstand voltage determined above [ Items (a) and (b) 1,
guarantees a satisfactory performance of the apparatus under
Thisshould be done on the basis of the
lightning overvoltages.
expected distribution of the lightning overvoltages and of the
discharge voltages or by means of the correlation given in 9.2.3
but for the sake of simplicity it is often done on a conventional
basis ( see Appendix D ).
Consider that only the highest value of rated lightning
impulse withstand voltage of each line should be used for
apparatus not effectively protected by a surge arrester ( see 6.4 of
IS : 2165-1977* ).
Provide means to reduce the amplitude of lightning surgest
(feedback from Block 12 to 10 ) or choose a rated lightning
impulse withstand voltage higher than the one determined on the
basis of Table 3 of IS : 2165-1977* if too high a risk of failure to
lightning overvoltages results from Item (3). In the latter case the
value of the rated lightning impulse voltage shall be selected from
the series ,in 6.1(b) of IS : 2165-1977*.
( Clauses 351.6,
7.2.2.1, 7.2.2.2 and 7.2.2.3)
A-l.1 Initial Capacitive
Voltage Spike -- During the initial period of
about one microsecond under the conditions of a lightning surge, the
transformer may be approximately represented as shown in Fig. 13A as
a capacitance voltage divider of ratio s where s < 1. If Ct is the sum of
the capacitances of the higher-voltage and lower-voltage arms of this divider, the initial transference may be simulated as shown in Fig. 13B, 13C
and 13D by a series circuit comprising a source U. = sU,, a capacitance
Ct and the capacitance or resistance of the external lower-voltage system.
Vi is the surge voltage on the higher-voltage side during the initial period.
The source voltage U, is the open-circuit transferred voltage.
lInsulationeo-ordination( second
revision) .
jConsider possible changer in line design such as tower footing and shielding wires,
install surge arresters other than those intended to protect Case I apparatus, make use of
protective spark gapr. The actual degree of protection provided by spark gaps is
dbewud in AppendixC.
frrnrformar
130 Equlvdont Circuit of
Cep8dtwue
Trw4emksion
8s a Gpecltivr
External System having
Gpacitrnce
Fm. 13
Ext;zzRrn
lmviy
INITUL CAPMXTNE VOLTAOE Srmx
If the external system may be represented by a capachnce
C. as in
Fig. 13C the equivalent circuit is a voltage divider having tbe ratio:
c; + c,
If during the initial period, the impedance of the external system is
the surge impedance of a cable or the resistance of the load, this system
may be represented by a resistance R asshown in Fig. 13D. Typical values
are from 10 ohms to a few hundred ohms. The transferred surge voltage is
then dependent on the steepness aswell as on the amplitude of the surge.
For high values of R theinitial voltage is approximately
& and for low
values of R it is given appr ximately by Us = s S R Ct where S is the
maximum steepness of the sz rge.
The above ex$essioru do not take into account the effect of superposition of the surge voltage on tbe power-fiqucncy
Isr32w-Em
may be made for the power-frequency
voltage by substituting ti Ur the actnal peak voltage Us and by introducing the fstor #.
For a star/delta or delta/star connected trarqformer the value of fi is typically about 1-15. Far a star/star or ddta/ddta
connected transformer the
of p b typicauy ablxit I-05. However, slightly higher values than
For switching surges the value of p may be
theW?maybeeWXmWWL
U, will be limited to the front-ofwave
sparkover value of the surge arrester or spark gap on the higher-voltage
rideofthetrausfbr~r[truIS:3070(PartI)-1974*].
side is given by:
gspike on the lower-voltage
u, = JWR
i?or a transfixmer without external connections to the lower-voltage
term&u&
the value of fxtor J may range from zero to at least 04,
The value of s may be measured
depending on the winding arrangement.
in a low-voltage impulse response test ( for example, with a recurrentsurge osoiliogrnph ). Values of Ct generally lie in the range lW* to lO_
Z$xra- The valua of 8 &
the mmn~turcr
Ct 8~ di&ult
to calculate bra new transformer MC
may only be expected to give a rough estimate without guarantee.
should be compared with the appropriate
test voltage of Table 1, List 1 or 2, of IS : 2165-1977t.
of the transferred surge may be reduced by:
a) Using a surge amstcr with a lower front-of-wave
voltage OIIthe higher-voltage side,
b) A&Gig capacitance
voltage side, and
between each phase and earth on the lower-
c) Alding a surge arrester on the lower-voltage
phase and earth.
For mm&cal
side between e&cl
example see Aa.
A-l f WeIy
W~E&WWI
V&age - The transference of emf by
inductive coupllug between windings in a 3-phase tramliumer or bank of
trar&rmers
may be evaluated fbr any winding connections by considerii
the surge voltage as .a single-phase alternating voltage.
The e&ct of delta whdimgs on the zero-phase-sequence
of the siuglqbam
voltage should be taken into account.
-tadC?Id8gerrmat
systems: Part I Non-lkar
l&kg-r--vmuiation~
(smadrmisirr).
IS-: 3716- 1978
Figure 14 shows the results for eight different connections
transformer, assuming the system voltage ratio is unity.
As in the analogous power-frequency considerations the transferred
voltages at the terminals are determined by the emfs and by voltage
division between the internal impedance of the transformer and the
external circuit impedance.
The efms may be assumed to have the same
waveshape as the surge on the higher-voltage system, if the effects of
internal oscillations in the windings are neglected.
lower-voltage system to these emfs is usually in the form of a voltage of
similar shape to the surge with a superposed oscillation.
The amplitude of the inductively-transferred
voltage depends also on
the voltage ratio and 3-phase connections of the transformer and on the
relative impedances of the lower-voltage system and the transformer.
The voltage on the lower-voltage side of the transformer is given by:
9r G/N
4 = is a response factor of the lower-voltage circuit to
transferred surge emf,
r = is a factor depending on the transformer connections
Fig. 14),
Ua - is the peak voltage to earth on the higher-voltage side,
X = is the system voltage ( phase-to-phase)
The value of q depends on the waveshape of the surge and on the
electrical parameters of the lower-voltage circuit.
For lightning surges on a transfermer having Category 1 equipment
without appreciable load connected to the lower-voltage aide, the val,ue of
q is generally not greater than about 1.3 although this value, may be
For switching surges on a similar system without appreciable
load, the value of q is not greater than about 1.8.
Generally lower values of q apply if an appreciable, load is connected
due to voltage division between the load impedance and the leakage
inductance of the transformer ( su#Note ).
For Category 2 equipment, voltage division takes place betweenthe
leakage inductance of the transformer and the subtransient inductance of
the generator, and if these are about equal, q has the value of about O-9
for lightning and switching surges.
Values oft for a surge on one phase only ( for example, a light&g
surge) and for equal surges of opposite polarity on two phases ( one .fYPe
of switching surge) are shown in Fig. 14 for eight different 3-phase
connections of the transformer.
I8 : 3716- 1978
The calculated value of Us is an estimate of the longer-duration
transferred voltage, which in practice includes longer-term
capacitive transference and transferred voltages corresponding to internal
oscillations within the windings.
Its amplitude will be limited by the
protective level of the surge arrester or protective spark gap. In the case
of the former this will be the higher of the standard lightning impulse
sparkovcr value and the residual voltage value for the lightning surges
[ JC~IS : 3070 ( Part I )-1974* 1. For switching surges, except when the
transformer is connected to a highly-inductive load, such as an induction
motor, over-voltages on the higher-voltage side may be assumed not to
exceed per-unit overvoltage of 3 (see Note below ).
The value of Us should be compared with the peak values of the
appropriate power-frequency test-voltage of Table 1 of IS : 2 165-1977t.
It may be found necessary to reduce the value of the lightning or switchAdding extra
ing surge on the higher-voltage side of the transformer.
capacitance to the lower-voltage side has little effect upon the amplitude
of the inductively-transferred voltage but it may be desirable to consider
the addition of surge arrester.
NOTE- When the circuit is switched off on the higher-voltage
side of a transformer
which may be loaded on the lower-voltage
side by reactors or other inductive load,
then dangerous overvoltages may be attained under the most unfavourable
of operation but, in general, Us does not exceed the peak value of the power-frequency
test voltage since Q is less than 1-O on account of voltage division between the trans.
former and the load inductance.
A-2. NUMERICAL
A-2.1 Example
145 kV star/l2 kV delta
145 kV side
12 kV side
= 60 kV (assuming List
1 of Table
:165-%37;;j
Surge arrester on higher-voltage
side of the transformer:
= 120 kV
*Lightning arresters for alternating
lightning arrestera (jrst revision ).
tInstrIation
=28kV
( secoad repision ).
Part I Non-linear
Front-of-wave sparkover vdtrgs
==463kV
=4OOkV
1*2/50 impulse sparkover @MI
a) Liyning
swgeon onr#hasb
Initial voltage spike for transfbrmer disconnected from loadz
U, - roUp - 04 x I.15 x 463 = 213 kV
Assuming a ratio of I.25 between the impulse withstand test voltage
and the service overvoltage,
cable connections f&n
that is, --&-
w 48 kV, would require the
the transfbrmer to have a capacitance
ct+c,
%i3-O23
hence Cm> 344 Ct
Having obtained from the transfixmer manufacturer a value of
farad, the cable capacitance per phase should be ;?t +st
942--x 1W farad.
Ct = IV
If the load is connected then this will reduce further the p&cVd$age
on the lower-voltage rid&.
Due to inductive transferarce :
u, = Pe &IN
1.15 x 1.3 x 0.577 x 400 c9 28_5 kV
W5 kV gives a ratio of 1*4 with the 39.6 kV peak test voltage
(28 4 p) so that the insulation to Tabk 1 List 1 of IS : 2165-1977* shall
be satirfhc~.
b) Wcking~~esofoppofitrjo&@yon&uo~hass
Amuming a PA.
l*O,q-
switching ovwal&ge
ori two phases of 2.5, and
1*8,andr= l-15, then:
1-O x.1.8 x 1.15 x 145 x ,/2- x 2-5
d/sx
=5@5kV
ls : 3714 - 1978
In this case the voltage peak exceeds the peak of the test voltage of
39.6 kV for Table 1 of IS : 2165-19778. To allow for this either insulqtion
for List 2 of the Table 1 of IS : 2165.1977* shall be used ()hat is, ha&g
test voltages of 75 kV lightning impulse withstand, and 28 kV rrns/39*6
peak) or switching surges between phases on the higher-voltage transmission system shall be limited, alternatively a surge arrester may be
selected on the higher-voltage side of the transformer havitig a lower
protective level or consideration may be given to fitting a surge arrester
on the lower-voltage side.
A-2.2 Example B-Category
A-2.2.1 Typical &amlpIe for 220 kV Transformtrs
Delta generator transformer
245 kV stat-124kV
Lightningimp&
Power-frequency test voltage
Imp&e test voltage
245 kV side = 1050 kV ( Ref.
Table 2 of IS : 2165-1977* )
24 kV side = 50 kV
24 kV side = 125 kV
( Ref. List 2 of Table 1 of
IS : 2165-1977* )
Surgs arrester on higher-voltags sid6 of
Front-of-wave sparkover
l-2/50 impulse sparkover and =
transfornur:
198 kV
746 kV
649 kV
a) Li&?rir& SUf~6SOn-Otl6~hU~6
spike for transformer
from 24 kV
G#==&J,
= O-22 x 115 x 746 = 189 kV ( maximum)
( the value for s having been obtained from the transformer manutacturer ).
This value would obviously be too great for insulation on the lowervoltage side.
&tuning a ratio of I.25 between the impulse test voltage and the
service overvoltage, that is a value. not exceeding 125/l-25 = 100 kV
would require the addition of external capacitance.
100 = O-53
hence C, > O-885 Ct
*Insulation co-ordination ( secondmkion ).
IS I 3716 - 1978
The value of Ct is obtained from the transformer manufacturer for a
wave steepness S = 1 200 kV/ys.
The dther methods listed in this Appendix for reducing the capacitive
peak ( use of surge arresters ) may ah be considered.
The overvoltage due to inductive transference is:.
U, = pqr wfl
1.15 x 1.0 x 0.577 x 649 = 41.5 kv
41 kV is less than the 70.7 kV ( 50 1/r ) peak tests voltage, and this gives
a ratio of 1.7, which can be considered adequate.
b) Switching surges of opposite polarities on two pfzases
Assuming a p.u. switching overvoltage on two phases of 2, then the
inductive transference is:
I.0 x 1.0 x 1.15 x 245 x dflx
2 = 43.5 kV
10.5 x VT
4 = l-0, and
r = = 1.15
43.5 kV is about equal to the inductive transference of lightning
(41.5 kV).
A-2.3 Example
Ty@*calExamplr&
300 k V Transformers
kV star/24 kV
kV side
= 1050 kV
Surge arrester on higher-voltage side of transformer:
Front-of-wave sparkover voltage
l-2/50 impulse sparkover and residual voltage
( smnd d&m
125 kV ( assuming List 2
ofTable 1 of
IS : 2165-.1977* )
= 240 kV
==9OOkV
= 785 kV
IS : 3716- 1Y-m
a) Lightning surge on one phase:
Initial voltage spike for
transformer disconnected from 24 kV
U,s = spup
= O-2 x l-15 x 900 = 207 kV maximum,
the value for s having been obtained from the transformer manufacturer.
This would obviously be too much for the insulation on the lower-voltage side.
Assuming a ratio of l-25 between the impulse-withstand test voltage
and the service overvoltage that is a value not exceeding 125/l-25 - 100
kV, would require the addition of external capacitance.
_!!! = O-48
c, + c, ,< 207
Hence C, )
steepness $ - 1 200 kV per microsecond [see IS : 3070 ( Part I )-1974* 1.
Alternatively the other methods listed in this appendix
Uue to inductive transference:
G = kf
1.0 1;.50*577 x 785,
414 kv
414.kV is less than 70.7 kV, ( 50 42)
test voltage and this gives a ratio
of 1.7 which may be considered adequate.
b) Switch&
surges of opposite polarity on two #iases:
u _ 1.0 x 1.0 x 1.15 x 300 x dT_x 2
12*51/5
-45kV
45 kV is about equal to the inductive
(41.4k.v).
lLightuing arresters for alternating
lightning arresters (first reviiion).
of lightning surge
rystetns : Part I Non-linear
(Cimssc 4-1.2)
B-l. GONFlDENGE LIMXTS
Tests may provide only more or less accurate estimates of the true
values of the withstand strength of equipment.
An increase in the accuracy may be obtained by an increase in the
extent of the test. The exttnt of tests must, however, be limited for
reasons of costs, the diminishing return of gain in the accuracy and possible
destructive efktx on the equipment. For these reasons IS:2165-1977*
prescribes three difkent test methods in 7.2,73 and 7.4 according to the
In 7.3 ( a ) dealiig with the 50 percent disruptive discharge test,
note states that There are a number of procedures available, and any
of these may be used provided that the accuracy of the determination is
within one half of the standard deviation with a confidence level of 95
percent . It can be said that there should be a 95 percent probability
that the 50 percent discharge voltage of the equipment at the time of
the test be within the boundaries given by the value estimated from the
test plus and minus one half of the standard deviation.
A test procedure fulfilling this requirement is the up and down test
with 30 shots.
The accuracy of the 15 impulse withstand test is considerably less.
The 95 percent confidence limits for the probability of discharge are in
Based on 95 percent confidence limits, it may be seen that irom a test
with 15 impulses only, it is not possible to conclude that the discharge
*Insulation co-ordination ( secondrmisim ) .
Bt37l6-1976
probability is lets than @lo, or that the probability of withstand is higher
~~~crrarfforodipehrse,~whenrhenumbgofdirchorgesexceadr
4,thctcstisaigdcantta
tut dthe hypothuia that the probability of
wiwzmdiagiskss
thano90.
B-Z. EXAMINATION
OF DIplpIsRBNT METHODS
Accept@ the above basic IimMions of tats with a small number of
impulses,the Mlowing exposesthe validity of differentmethodsof discharge
te&ng and tbc balance between the risksto the manufacturerand the purchaser bearing in mind the
tical necessityfor a limited number of
impulse8and, for .the mantlEzcuter, to design his products to have an
economicallyacceptabk risk of&hare on tests.
For the sake of simplicity it is assumed in this apptmdix that the
dischargep&ability Pt (U) df the diffcnnt piecesc&equipmentin a given
populati~ ( same type of equipment on which an impulse test U to be
canicdaut)fdkwsaGaussian
law with a constant standard deviation ut
cquaI for all the pieces
ofequipment. In this caseone parameter ( X given
in pa ofat, s88Fig. 15) is ru&knt to determine the dcviion of the inSulatio~strength of one piece of equipment 2 at the time t ( VW ) from
thcspec&dval~
K IN p.u.OF Qt
XI UlIo- URW
Fra. 15 DEFINITION
OF THE INSULATION STRENOTH
OF A PIECE08
AT THB TIME 1 AS FUNCTION
OF THB PARA-R
XS: 3716- 1978
The 90 percent withstand strength of a piece of equipment in a
population varies from one specimen to another.
Fig. 16 shows how this
may be described statistically in terms of K. The value of oP is very small
for those types of equipment ( for example, disconnectors ) which may be
considered essentially air insulating structures, since tolerances in dimensions are always very small.
To ensure the repeatability of tests, ambient and insulation conditions
should be kept as constant as possible during test ( or correction factors
should be used ) and standardized testing techniques should be adopted.
In principle, therefore, the discharge probability of a given piece of
insulation under test conditions should not be expected to change.
other words the Pi (U) ( ac Fig. 15 ) should be the same in different tests
However the value of the 90 percent withstand strength of insulation
may show variations from the average value derived from several tests by
the same methods carried out in the same laboratory at different times or
in different laboratories, due to differences in the ambient and insulation
conditions or in the test circuits. How the laboratory inaccuracy can be
described statistically considering the average value of the insulation
strength of one specimen as the true value is shown in (b) in Fig. 16.
Assuming that the distributions in (a) and (b) in Fig. 16 are Gaussian
with known standard deviations and that the design value of Uo has been
chosen by the manufacturer, the probability density of the deviation of the
measured 90 percent withstand strength of the population of a piece of
equipment may be calculated [ see (c) in Fig. 16 1.
An ideal test should be such as to prevent equipment having, at the
time of the test, either an insulation withstand lower than prescribed to pass
the test or an insulation withstand equal to, or higher than prescribed to fail.
Self-restoring insulation having, at the rated impulse withstand
voltage applied during the test, a probability of withstand equal to, or
higher than, the reference probability ( 90 percent ) should have a probability of passing the test equal to 1, while insulation having a probability
of withstand lower than the reference probability should have no chance
of passing the test.
The probability of passing an ideal test of an apparatus, the insulation
of which, during the test, differs by X from the prescribed value, is
represent& in Fig. 17 as a function of X by the solid line.
Actual test, however, depart from the ideal test and follow in the
K) plane curves similar to the dashed line.
Figure 17 shows the curves for the tests proposed in 7.2, 7.3 and 7.4
As far as the test of 7.3 is concerned an up and
of IS : 2165-1977*.
do- ) tat based on 30 shots was taken into consideration.
co-ordination ( srcondf&ion ).
IS : 3716- 1378
a) Distribution of the 90 percent withstand voltage of the apparatus of population p.
b) Distribution of the di!erence between the 90 percent withstand voltage of a given
apparatus measured m various laboratories and the actual one.
c) Distribution of the 90 percent withstand voltage of any apparatus of population p
FIO. 16 FREQUENCYDENSITYOF THE MEANRED 90 PERCENT
WITHSTANDSTRENGTHOF A POPULATIONOF APPARATUS
The probabjlity density of the deviation of the measured withstand
strength [ see (c) in Pig. 16 ] of a given population is represented in Fig; 18
by Curve 1. By multiplication of the values of this curve by the value of the
probability of passing a given test procedure as a function of K ( see Fig. 17 )
Curve 2 is obtained, This curve represents the density distribution of the
l3 t 37lS - 1978
-1 -04~[).6-O*&-0.2 0 0,2 0.4 0.6 O-8 1
K IN p.u, of pI
1 - Up and down test 30 impulses ( ?.!Z of IS : 216~1977)
2 = 1512 test ( 7.3 of IS: 2165-1977. )
3 - 3/O test (7.4 ofIS: 2165-1977)
*Insulation co-ordination ( sacoadfaGon ).
17 PROBABILITYOF AN EQUIPMENT TO PASS THE DIFFERENT TYPE
OF TEST AS A FUNCZIONOF ITS INSULATINQCHARACTZRISTICS
accepted deviation
R of the population of apparatus for the given test
If the area limited by Curve 1 and its abscissa is taken equal to 1,
Area A in Fig. 18 represents probability Rm of rejection of a good product
( manufacturers risk ) while Area B represents the probability of rejection
of a deficient product.
As a matter of fact the intended value of insulation
design W will be chosen by the manufacturer on the basis of the sum of
Areas A and B, that is, on the probability Pt of failing the test. Area C
represents the probability R. of acceptance of a deficient product ( customers risk ). By repeated calculation of Rc and PIfor different values of
3716h78
K IN p-u-
FICL 18 PROBABILITY Pt OF TEST FAILURE ( AREAA + B )
MANUFACTURERSRISK % (AREA A )
( AREA c )
the intended value of the insulation strength ( IV), curves can be constructed which show the relation between the risk of the user to accept a
deficient product and the manufacturers
Ps of failing the
Figure 19 shows such curves on the assumption
deviations up, 01 and at are those stated in the figures.
Figure 20 gives the correlation
Rm ( manufacturers
RC ( customers risk ) for the cases considered in Fig. 19.
risk )
It is to be noticed that if ea and ur are zero ( homogeneous population and no laboratory inaccuracy ) one of the two values Rm and R,, is
In other words there is onIy a risk either for the user or for the
This risk as well as the value of Pr may be obtained
directly from Fig. 17.
1!3:3716-1978
1 = 50percent
2 = 15/2 test
CU~OM~RS
3/O tat
RISK Rc AS A FWNCTION OF PROBABILITY P! m
1 P 50 percent
15/2iest
3/o test
R&K RR;~K~iF~~~~~~ QP MANUFACTURBR~
I6 t 3716- 1976
.APiENDIX
( ckuse~ 5.4.1, 5.5.3 ilnd 9.22 )
C-l. For a given waveshape of the applied impulse, let us cali:
the disruptive discharge probabilities of the
a) pi (W-and 2 (2
msulatmn an of e spark gap as a fundion of the crest value U
of the impulse*.
b) Ptp (v) the probability that the insulation may discharge before
the protective gap sparkovcr as a fbnction of the crest value U of
the nnpulse.
The discharge probability curves of insulation Pi (U) and of the
spark gap pD (u) connected in parallel are expressed byz
Pi(v>
Pi(u)-
PD(U)PPD(U).[l-Pi(U)l+PD(U)Pi
. ..(Sl)
fv)~i--ab~v)~-~*~~~
Nom -On
the asmm&~
spark gap Slow a Gauniam law, *I%im*
PI, (V) ir cxplv=d hp_
mzq MdlhF
2-,(u)
081(v)
~thc5opcrcalt-cnlaeoftbctimc-~koverofthe~gap,ur
fam&ndtbcnatvalueUofthc8pplialimpulse;
irtlE5opCrcentvahleoftbetim. argeoftheinmh&qa8a
fanctionoftheuutvahaeUofdreappliimp&q
is the standscd deviation of the time-torpartovcr of the rpark gap,
~8functioooftheaat~eUofthe8pplicdimp~
is the standard deviation of the tim~tmiischarge of the insuIation,
a8 a function of the crest v&m of the applied impulse.
If, for a particular combination ofthe insulation to be protected and
of the spark gap, there is a negligible probability that the tin&-to-&+
charge of equipment may be lower than the time-to-sparkover of the gap
in +he entire range 0 < U < Z&W, Pip. (U) becomes zero and fonnuh
(11 may be written:
&(V)ePi(U)
~l-pD(u)~
The formulae given below are gwxmlly valid for two piecu of insulation in paraW.
IA the case ofcombinationer dim&h
ki&weshallconsidertbe~oftbeprot~vegapasbeing
id4 .
and protective gaps of this
By making use offormula (7), the risk offSlure ofa protected piece
of insulationcan be evaluated by means of the lb&wing form&
P.(~~fo(u)~~+~
pi(u)*
(u)*fo
If the protective gap can be consideredideal , the second term in
formula (15) is equal to zero, and we obtain the f&ing
formuh:
GlL8ac
pi(u)*C1-pP,(u)]f,
. ..(16)
The ratio of the 50 percent lightning impulse sparkover voltage to
the 50 percent switching impulse sparkovervoltage of a spark gap fan be
chosen from a wide range ( 1 + I-5 ) by changing the electrode cxmfiguration. It is, therefore, possible toselect the discharge probability curve of-the gap Pp (U) to switching impulse3 quite irrespective of the discharge
probability curve to lightning impulses.
Design of a spark gap with a view to switching impulse will make the
average expected nu.mber ofa
-a. ofxhe gap per year, due to switchi~~~%jG$t%
a value .& such as not to make system performme
under switching surge signiicantly worse. Therefore, the probability of
sparkover of the gap due to switching overvoltages reaching the level
evaluated on the assumption that equipment operates as designed ( SW9.3 )
shall be very low.
Cousequently, even if the s ark gap behaves like an ideal protective
no protection against them and iusuldevice, the spark gap will provr&
ation shall be designed to withstand this type of switching overvoltages.
This is evident from formula ( 14 ) ( cpst of ideal spark gap ).
Pi -Pi(U).
[I--p(U)
J=Ppi(U)
. ..(17)
As regards switching ovenroltagesuceedin~ the values based on the
assumption of correct behaviour of equipmcn~ we can assume that the
overvoltage value is such that a gap sparkover will almost certainly be
caused and formula ( 11 ) then becomw
-Pi(U)
P*(U)
18:3716-En8
and formula (15)
u mSx
pip(
u)fo
. ..(19)
those of the gap. It
possible to zero over
effect is, in this case, due only to Pin ( U) that is,
between the time-to-discharge of the insulation and
is, therefore, necezsaty to make Pip ( U) as close as
the entire range of interest of U.
Design of a spark gap with lightning impulses in view will be such as
to limit the average expected number of flashovers per year of the gap to
lightning surges to an acceptable value Nr.
In this connection, it shall bcremembered that in many cases the
sparkover of the gap does not lead to any supply interruption.
Let us consider the example of a spark gap installed on the line side
of the breaker. If the lightning stroke causes the line to flashover, a
coincidental sparkover of the gap is of no significance. On the other
hand, if the overvoltage amplitude does not reach the sparkover level of
the line at the point struck, it is unlikely to cause a sparkover of the gap,
even if the gap withstand level islower than that of the line. This is due
to the reduction in surge amplitude at the point of installation of the
spark gaps, because of attenuation as well as the possible presence of other
lines and surge arresters at the station.
Thus, in contrast to the case of switching surges, it is possible to
accept in certain cases spark gaps with a 50 percent discharge voltage to
lightning impulses lower than that obtained, basing the design of the
apparatus on switching surg_es and using the combination of the rated
impulse withstand voltages glen in Table 3 of IS : 2165-l 977*.
Therefore, it may be concluded from formulae 11 and 14 that, spark
gaps my offer a limited degree of protection in the case of lightning overvoltages of the order of the rated lightning impulse withstand voltage of
Aa regards lightning overvoltages much higher than the rated light&g impulse withstand voltage of the apparatus gap sparkover will almost
certainly be caused. In this case formula 18 and the same considerations
as regards Pip ( U) previously made in respect of switching surges apply.
For air insulation of equipment the condition Pip ( U ) o 0 may b
fulfilled both for lightning and switching impulses by making use of gap
h;rvhg a high critical sparkover voltage to switching surges ( kV/cm ), that
l~dation cwxdimtion ( sawd r&rbn ) .
k gapsmuch shorter than air distances of the apparatus, for instance by
making use of spark-gaps having a conductor-rod
For non-self&storing
insulation of equipment the check that factor
Pip ( U) is reasonably small should be made by means of chopped wave
withstand test carried out at a voltage level based on the highest overvoltage which may be expected in the system and with a truncation time
to be chosen on the basis of the time-to-sparkover of the gap.
These tests are not laid down in IS: 2165-1977*
should be agreed upon between the user and the supplier.
( Clause 9.3 )
D-l. Table 3 illustrates the choice of the insulation level of a piece of equip
ment for Case I ( see 9.3 ) protected against both switching and lightning
overvoltages by surge arresters mounted at it3 terminals.
refers to 420 kV transformer and Example 2 to a 765 kV transformer.
D-2. The insulation levels of the transformers depend on the protective
levels of arresters against both switching and lightning impulses ( Block 7 of
Fig. 12 ). The protection level of a particular surge arrester depends, in its
turn, on its characteristics and rating. The rating of surge arresters in both
Examples 1 and 2 is chosen as the available rating immediately above the
temporary overvoltagcs anticipated on the system ( Block 6 ). Temporary
overvoltages include voltage rise during faults, overvoltagcs due to inrush
currents, sudden load rejection and other causes. The overvoltage to be
expected is influenced by the earth fault factor, system configuration, the
characteristics of system equipment, and operating practices see 3.3 ).
D-3. Table 4 shows the choice of the insulation level of a piece of equipment for Case II with no surge arrester protection or with remote surge
arrester protection. Example 1 refers to the line-to-earth insulation of a
420 kV disconnecting switch on the line-side of the breaker, no surge
arresters being installed at the line entrance.
Example 2 refers to a 765
kV disconnecting switch at the same location.
D-4. The rated switching iinpulse withstand voltage is selected first from
Table 3 of IS: 2165-1977* on the basis of the statistical switching overvoltage level at the equipment location and on the basis of an acceptable
risk of failure ( Blocks 4 and 8 of Fig. 12 ).
*Insulationco-ordination( scrond WV&I).
1) Bo~&Dah
Highat voltage for equipment u.
carapoadine line-to-ground voltage
rmsvaluc
Peak value U.*T
Detamkdog tempomy overvoltage (derived
fram system rtudics ):
safetyfiwtol.
Forswitchingovcrvoltagcs.
For lightning ovcrvobgcs
Rated voltage ( ms)
Afar switching impulse sparkover voltage
MUXliitnin6 impubc sparkover voltage
Mar front-of-wave sparkover vobge
MUXresidual voltage at rated di&arge current
3) R&ction Lad:
To switching impuka
To lightning impulses
Ul4lT
I ExAuPU2
kV (p.u.)
343(1~00)
kV ( p.u.)
452 ( 1.32 )
855 ( I.37 )
kV ( p.u. ) 765 (2.23)
kV ( p.u. ) 878*(256)
IntetJur
4) rnndatioa Li?d ( Phase-to-ffirlh ) stiw
Min&uin&u~o~eentionalswitching impulse with- kV @.u. )
Rated switching impulse voltage
Ratio of the rated r+c$i~
&pulse withstand
;olge
to the swltclumg Impulse pmtectb
kV (p.u. )
conventional lightning impulse withstand v&age
Rated light&g impulse withstand voltage
Ratio of the rated 1 htuing impulse withstand
to the ligmiing impulse protective
kV (P.u.
625(1-00)
1230 (1%)
1440+(2+0)
880 (2%)
1415 (2.26)
950 (2.77)
1425 (2.28)
100 (%20) 1800 (276)
1 175 (942)
NOrr - State Rlectricity Boards with 420 kV power system are reqututed to provide
the d&aib for Rxample 1 in accordance with the practices they arc follow@ m their
*Maximum front-of-Gave sparkover divided by 1.15 ( SW5.2.1).
voltagefor qdpm~t
Ur ( mu )
Comspondiq line-ground
rma value
Peak vaIue u.~rl~skV ( p.u. )
S4S(l*OO)
625(L.t36j
Statistical overvoltage at the equipment location LV ( p.u. )
910 (265) 1255 (23)
( value exceeded in 2 percent of casa only ) due
to rcclaing operationa
Maximumacccptcdrisk~6ashoverta~ndfor
-10-t
recloaingopc!ratioa
statxcal
x&ty factor
corrupondingtot8e
nurim*-taf
I& of flashover ( derived
i&n Fig. 7)
Minimum rtatistical awitching imp&c withstand
Ratal switchbg imp&c withstand voltage adectcd
4*10-
Risk of Hashoverawnrpanding to tht rated awitch10-b
ingimpul~~withstaadvoltageaelectui
impulse withstand kV ( p.u. ) 1425 ( 4.15 ) 2 460 (S-84)
impulse withstand voita6e
--For both ezamples, it is assumed that the only critical switching
overvoltages am these due to line re-energization ( that is, it is necessary
to check the risk offitihue due to reclosing operations only ).
By making use of the simplified statistical approach ( SN 9.2.5 ) and
choosing an appropriate standard deviation it is pcssible to determine
the statistical safhty factor y corresponding to the mazimum
risk of &shover;
from factor y it is possible to derive the minimum
statistical twitching impulse withstand voltage and then to select the rated
switching impulse withstand voltage immediiely above.
In the exam*
the correlation between risk of ihilure and: ztatistical
safktyfdorgiveniaFii.f(~
-8percent)is
I3-5. Oncethe rated swit&i+mpuJse
withstatxd voltage has been obtained
a corresponding rated lightning impulse withstand voltage is se&etz~I Eom
the same line of Table 8 of IS: 21651977.
considering that only the
highest value of rated lightning impulse withstand voltage should bc used
for apparatus not &ectivdy
protected by surge diverters
( SII 6.4 of
IS : 216%4977* ).
It is then neceszq
to verify that this value guarantees a satisfactory
under atmospheric overvoltages, that is, a risk of failure not
performan-/
%faUua
ceudilmtioa (mDndf48iSina).
U : 3716- 1978
higher than the permissible one. This can be done in a similar way as
previously for switching surges ( Blocks 9 to 12 in Fig. 12 ).
In the case under consideration an approximate distribution of the
lightning overvoltages, at least ior the case of disconnecting switches in the
open position, may be evaluated quite simply on the basis of the line
characteristics and of empiricalor semi-empirical laws for wave attenuation.
However, lightning overvoltage stresses vary from point to point in a
substation. In general, it is therefore extremely difficult and time consuming
to achieve the necessary knowledge of stresses by making use of the
statistical or simplified statistical approach.
Only the highest overvoltage stresses in the most common contingencies ( most common position of breakers and disconnectors ) are therefore evaluated.
It is then verified that the rated lightning impulse withstand voltage selected, as said above starting from the rated switching
impulse withstand voltage, exceeds the maximum credible atmospheric
overvoltages by a suitable margin ( e 10 percent ). If a rated lightning
mpulse voltage higher than the one determined on the basis of Table 3 of
IS: 2165-1977. is desirable, the value shall be selected from the series in
6.1 ( b) of IS : 2165-1977*.
D-6. In Table 5 there is an evaluation of the increase of the risk of failure
for the rated switching impulse withstand voltage lower .than the selected
value given in Table 3 of IS : 2165-1977*.
TABLE S RISK OF FAILURE OF INSULATION AS A FUNCTION OF THE
RATBD SWITCHING IMEWLS&~;~m~OLTAGBS
OF THE EQUIP-
Altamalior (a)
switching impulse withstandvoltage
( selected value )
Statirtical safety factor
Riskof failure corresponding
statistical rafety factor
Ahmativa (b)
EXAMPLB 1
kV ( p.u. )
1050 (506)
1550 (248)
Statistical safety factor
( sacond reoisiou
950 ( 2.76)
5 lo-
1 425 (2.28)
Af&mativ~ (c)
4.10-r
1300(2+8)
&lo-
tS : 3716 - 1978
( Clause 9.3 >
PARTS AND EARTHED
SPECIFIED IMPULSE WITHSTAND
E-l. In installations which, for various reasons, may not be impulse tested,
it is advisable to take steps to avoid flashover occurring below the impulse
withstand level which would have been prescribed in the case of a test.
E-2. The condition to be fulfilled is that the statistical switching and
lightning impulse withstand voltages in air between live parts and earth
should be equal to the rated switching and lightning impulse withstand
voltage as specified in IS: 2165-1977*.
This results in a minimum
clearance to be observed which depends on the configuration of the live
parts and the nearby structures ( electrode configuration ).
E3. No distance is indicated for an equipment which has an impulse
included in its specification since compulsory clearances might hamper
design of the equipment, increase its cost and impede progress.
impulse test, even when only a type test, is sufficient to prove that
impulse withstand condition is fulfilled.
E-4. Table 6A and 6B are suitable for general application, providing as a
first approximation a clearance to be specified in relation to the insulation
1eveL These tables have been compiled for easy use.
E4.1 In Table 6A ( Urn < 245 kV ) reference is made in the first column
to the rated lightning impulse withstand voltage and in the second column
to the air clearances for unfavourable configurations of live and.earthed
E-4.2 In Table 6B ( U, ) 245 kV ) reference is made in the cd (1) and
(2) to the values defining the insulation levels and in the co1 (3) and (4) to
air-clearances for electrode con8gurations of the conductor-structure
type and rod-structure type.
TM. The c rod-structure configuration is the worst electrode configuration
normally encountered in practice; the conductor-structure configuration
covers a large range of normally used configurations.
In Table 6B
reference is made to the electrode configuration because of its notabk
intluence for U, > 245 kV.
E-6. The values of air clewances given in Table 6 are the minimum values
selected by electrical considerations, and do not include any addition tbr
construction tolerances, effect of short-circuits, safety of personnel, etc.
These values are valid for altitudes not exceeding 1000 m.
Quuhtion
co-ordination ( saod midon ).
:5MJ
1050;1175
1I75;l%KJ;i425
1300;1425;1550
1425 ; 1.590; I &IO
1550;1t?lw;2lfJO
1800;~950;24@0
AWUymdmT
IS t 371Q- h7S
CO-ORDI[NATION
Revirion )
Altcrodon
8, &ace 1.2, fine 1 ) -
Delete the word only .
(P496 8, dam-e12) - Add the following new clause after the
the subsequent clause:
ezistingandWmmber
61.3 This guide also covers guidance on phase-to-phase insulation conciples and rules for which are enumerated in IS : 2165
(Part2)-I!383 r (~uAp$endixF).
Add the following Appendix aAer the
aistii:
( Czawc 1.3 )
APPLI%ATION
F-X. VotTAoE
FOR PHASETO-PHASE
STRESSES IN SERVICE
F-l.8 The dielectric stresseson phase-to-phase insulation may be class&d
a) Power-frequency voltage under normal operating conditions,
b) Teqorary
c) S~itdaing overvoltages, and
d) L&ght&govervoltages.
F-1.1 wqw
V&age
- Under normal operating conditions
the power-Gequenq voltage between phases will not be greater than the
bighesi v&age for equipment V,.
Therefore, LJ, has been taken as a
rei&znx value for insulation co-ordination purposes.
: Part 2 Phase-to-phase
co-ordiition,
F-l.2 Temporary
- The clauses of phase-to-phase temporary overvoltage are the same as those of the phase-to-earth temporary
overvoltges described in the Standard, except for earth faults which,
normally, do not create significant temporary phase-to-phase overvoltages
( less than 1.2 U, ).
The insulation performance under temporary overvoltages, which are
normally of short duration generally less than one second, is verified by
the power-frequency test at voltage levels indicated in Tables 1 and 2 of
IS : 2165 ( Part 2 )-1983* for the ranges A and B or specified in certain
cases by the relevant Apparatus Committees for range C.
F-1.3 Switching Overvoltages
- External phase-to-phase insulation is
determined by considering the actual stresses due to switching operations,
faults and other causes, the behaviour of the insulation when subjected to
these stresses and the accepted risk of failure.
F-l .3.1 Overvoltages Stresses - The overvoltage between two phases results
from two overvoltages to earth, generally having opposite polarity.
Experience shows that various combinations of these overvoltages can
occur. Nevertheless, the phase-to-phase overvoltage on a three-phase
system may be characterized by two situations:
Situation 1 : When the peak of the highest phase-to-phase overvoltage is
reached on any phase of the three-phase system;
Situation 2 : When the peak of the highest overvoltage between phases is
attained for any two phases.
From these two situations the two phase-to-phase components of the
phase-to-phase overvoltage are determined.
Whilst, by definition the
phase-to=phase earth voltage is always greater in situation 1, it should be
noted that the maximum value of the phase-to-phase overvoltage obtained
in situation 2 is greater than that obtained in situation 1.
The ratio usually observed between the statistical phase-to-phase
overvoltage ( situation 2 ) and the statistical phase-to-phase overvoltage
( highest component of situation 1 ) lies in the range of l-5 and 1.8, the
values being greater for the higher system voltages. These ratios are the
basis for the selection of rated switching-impulse withstand voltage phaseto-phase with respect to the phase-teearth values,
F-1.3.2 Insulation Behaviour - The behaviour of any insulation depends
on the geometric field distribution and on the type of the dielectric material.
In the case of external clearances and for a given total phase-tcr
phase test voltage, the division into components using the highest possible
llnaulation co-ordination:
Part 2 Phase-to-phase
positive component on the one electrode ( and the
on the other ) is the most severe condition.
compotieht
However, a different division may be used when testing the insulation
provided that the same test severity is achieved by increasing the total
The effect of the third phase voltage is
generally small and can be neglected in most cases.
For the internal insulation, for example, transformers, it seems that
the division of the components is of no importance.
insulation, for example, SFs iwulated, three phase enclosed installations
the influence of the components has not yet been sufficiently investigated.
F-1.3.3 Risk of Failure - Evaluations
of the risk of failure between
phases have to take into account the division of the phase-to-phase
overvoltage into two phase-to-earth components, the statistical distribution of
the maximum phaseto-phase
overvoltages and the flashover probability of
Such calculations have been made based on available
system data and typical insulation withstand characteristics.
a) the two situations mentioned
the same risk of failure; and
in F.1.3.1
result in approximately
b) the risk of failure between phases is smaller than ( or equal to )
that between phase and earth if the phase-to-earth
tested according to IS : 2165 (Part 1)-1977* and if the phase-tophase tested in the same way considering the test procedure given
in F-li3.4.
F-1.3.4
Test Procedure- To achieve the desired correlation
risks of failure between phases and phase-to-earth
the withstand voltage of
the insulation has to be equal to or greater than the rated switchingimpulse withstand voltages given in Table 3 of IS : 2165 ( Part 2 ) -1983t.
To fulfil this requirement one of the test procedures given in IS : 2165
( Part 1 )-1977* has to be applied.
The permitted number of disruptive
discharges includes all discharges between phases and to earth. Among the
different division of components which are possible, according to F-1.3.2
the one which is preferred is that giving the same amplitude
components ( Uner/UPm - 1 ).
a) during the phase-to-phase test the phage-to-phase component shall
not be higher than the rated switching-impulse withstand voltage
Taking into account the values given in Table 3 of
IS : 2165 ( Part 2 )-1983t
this implies a ratio Uneg/UDos already
higher than O-73. A further increase of this ratio is necessary
to avoid an excessive number of flashovers to earth because of the
influence of the negative component applied to the second phase;
*Insulation co-ordination: Part 1 Phase-to-earth
insulation co-ordination,
iPart 2 Phase-to-phase insulation co-ordination,
b) for asymmetric test arrangements
only two test series, applying the
positive component successively to the two phases are required.
If the ratio is different from 1, four test series are necessary to
cover all cases by permuting the polarities and the values; and
the influence of surrounding objects during the test is kept to a
The selection of the ratio U&U,
= 1 results in a maximum
acceptable risk of faihue between phases which is lower than the maximum
acceptable risk of failure to earth.
F-l.4 Lightning
- When a direct lightning stroke occurs
to a phase conductor or a backflashover takes place the lightning stress
between phases does not normally exceed the lightning stress to earth.
F-2. CLEARANCE IN AIR BETWEEN PHASES TO ASSURE
A SPECIFIED IMPULSE WITHSTAND VOLTAGE IN
F-2.1 In complete installations ( for example substation ) which cannot
be impulse-tested as a whole, it is necessary to ensure that the impulse
The statistical switching- and lightning-impulse withstand voltages for
phase-to-phase insulation in air should be equal to, or greater than, the
rated switching- and lightning-impulse withstand voltages as specified in
this standard: Following this principle minimum clearances have been
determined for different electrode configurations.
Tables 7 and 8 are suitable for general application, as they provide
a specified minimum clearance in relation to the insulation level. These
clearances may be lower if it has been proved by tests on actual or similar
configurations that the required rated impulse withstand voltages are f%lfilled, taking into account all relevant environmental conditions which can
create irregularities on the surface of the phase electrode, for example,
rain, pollution.
As regards range C, lower clearances may also be used if it has been
confirmed by operating experience that the switching overvoltages are
lower than the rated values indicated in Table 3 of IS : 2165 ( Part 2 )1983* for a given voltage U,.
No distance is indicated for an equipment which has a phase-tophase impulse test included in tke specificaation,since mandatory dearanca might hamper the design of the equipment, increase its cost and impede
In Table 7 ( U, < 309kV ), first column, the rated lightningimpulse withstand voltages are listed. The second column lists air clear*Insulationco-ordinntion:Part 2 Phase-to-phore insulation co-ordination, principles
utd ruler.
antes for u&voumbIe con6gurations of energized parts with a rehtively
smah curvature radius. These clearances have been derived by the
testing procedure desc&ed in 8 of IS : 2165 ( Part 2 )-1983*.
In the first cohmm of Table 8 ( U,,, > 300 kV ) rated switchingimpulse withstand voltages phase-*phase are listed. The second column
lists clearances for a conductor-conductor ( parallel ) configuration having
a symmetrical gap geometry. The same clearances may also be used for
other configurations with symmetrical gap geometry such as crossed conductors ofrod-rod gape.
The third cohrmn refers to a configuration such as rod-conductor
having an asymmetrical gap geometry.
For ring-ring gaps or configurations with large smooth electrodes
having a higher degree of f&d homogeneity, lower clearances than those
given in the second cohunn may be used, provided that the influence of
the environmental conditions ( AWabove ) is taken into account.
The cIearancesfor the phase-to-phase insulation ( SMTables 7 and 8 )
can be applied together with the clearances for the phase-to-earth
i%BI.R7
CO-%7ONSBRlWEEN~~NYAIUD
IarNamuPHAsR-To-PRASRAIR
-PORUm<=kV
(CkllfCCIWI)
&mUUX
hA86+tbP~ASB
hIpcmABuocIIB
RATBDLIQETHIX~IMPULS~
WITEBTARDVOLTAOL,
PHASE-T~-PEAEQS
(P-X)
lO!X-l
lInn1l8tioncowrdiiation: Put 2 Phase-to-phaskinsulation co-ordiiation principla
mdNle&
The values of air .clearances given in Table 7 and 8 are dictated by
dielectric considerations. Other factors such as construction tolerances,
the effect of short circuits, wind, safety of personnel, maintenance,
corona effects etc. are not included.
The indicated values are valid for altitudes not exceeding 1 000 m.
The effects of higher altitudes are under consideration.
TABLE 8 CORRELATION
BETWEEN INEULATION
LBVBLS AND
MINIMUM PHASE-TO-PHASE
AIR CLEABANCES
FOR UEJ > 300 kV
(Chn.r~ F-2.1 )
.?iWXTO~X?@IY~ULSBWITHSTA~
V~LTAGUCPHA(IE-TO-PHAEJB
(fi=)
(ETDC19)
M~~~J~PEA~~~o-PEAsEAIB
hlUE~CZlrOB~NIIOUBATIOI?S
Conduc;to$ductor
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