SPLN 7D_1978

8/13/2019 SPLN 7D_1978

1/21

c 0 l t M t s s t 0 i l r r c T R 0 T E c H i l t 0 u E1 { T E R l { A T t 0 } t A r E(affili6el'0rganisationnternationaleeNormalisationlS0)

B E C t l M M A I I l l AT I ( l I Il E t A C E I

I I { T E R t { A T I ( l i l A LT E C T R t l T E C H I { I C A L( } M M I S S I ( | N(affiliatedo the nternational rganizationor Standardization lS0)

I E C R E C l | M M E l { l l AT I l l } I

Publication9-lPremidredi t ion First dit ion

1 9 6 5

Gomplfmentla Publication9-l l95SlRecommandationsour

Premi0reartie: arafoudresles araloudresi rdsistanceariable

Supplement0 Publication9-l l95SlRecommendalionsor ightningrresters

Part : l,lon-linearesistor-typerresters

Droi ts e eproductiondserv6s Copyrighta l l rights eserved

Bureau entrale a Commissionlectrotechniouenternationale1 , uede Varemb6

Gendve,uisse

8/13/2019 SPLN 7D_1978

2/21

c 0 t f i t | l t s s t 0 i tL E c T R 0 T E G H i i l 0 u E1 { T E R i l AT t 0 i l A L E(affili6el'0rganisationnternationaleeNormalisationlS0)

RECll I I IMATIl lATI( l I IE tA CE

I T { T E R l { A T Il I I A LEL E G T B ( l T EH l { I C A L ( l M M IS I t l T I(affiliatedo the nternational rganizationor Standardization lS0)

I E C R E C { l M M E i l D AT I l l I I

Publication9-lAPremidredition First dit ion

1 9 6 5

Complfmenta Publication9-l l95SlRecommandationsoures arafoudres

Premiireartie: araloudresr6sistanceariable

Supplemcnt0Publication9-l 95S1Recommendationsor ightningrrestersPart : l{on-linearesistor-typerresters

P E R P L l S T { K A , { NPUSAT PE$VELl0 l l i l | l lAg t t lH l tLiST?r t ( r {

t- ' \ - . , - j1) *

I n" . {g rade :. i ( i . i i i

I

Ira r r sga l , ZS APR ? ' l ' l

t - -

Droits e reproductiondservds Copyrightall rights eserved

Bureau entrale a Commissionlectrotechniquenternationale1, rue de Varembd

6endve. uisse

8/13/2019 SPLN 7D_1978

3/21

FonEwoRo

PnErecE

Clause

l . Ob jec t .

- 3 -

CONTENTS

A S transformers, eactors, urrent

Page

5

5

a aJ J

J J

3 5

3 7

3 7

37

3 7

2. General procedure n applyin g ightning arresters

i . Step-by-step rocedure or protection of power transformers nd statiort equipment

3.1 Determine the maximum phase-to-earth ower-frequency voltage at the arrester ocation

3.2 Estimate he magnitude and wave-shape f arrester discharge current 11

3.3 Determine he withstand strength of the insulation o be protected 13

3.4 Tentatively select arresters 15

3.5 Determine he impulse protective evel or the tentatively selected rrester 19

3.6 Co-ordinate he arrester protective evel with the irnpulse withstand strength of the insula-tion to be protected 19

31. Protection of other equipment

4.I Protection of series windings of equipment suchtransformers and so forth

4.2 Protection of dry-type insulated equipment

3 1

3 l

5. Switching surges iable to cause operation of the arrester

5.1 Classification y cause of switching surges

5.2 Classification by type of discharge hrough arrester

6. New definitions.

6.I Impulse protective evel of an

6.2 Rated impulse protective evel

6.3 Protective atio .

arrester

of an arrester

8/13/2019 SPLN 7D_1978

4/21

- 5 -

INTERNATIONAL ELECTROTECHNICAL COMMISSION

SUPPLEMENT TO PUBLICATION 99-1 (1958)

RBCOMMENDATIONS FOR LIGHTNING ARRESTERSPart L: Non-Iinear esistor-type rresters

Application guide of nonJinear resistor-type lightning arresters for alternatingcurrent systems

FOREWORD

I ) The formal decisions or agreements of the I E C on technical matters, prepared by Technical Committees on which all the

National Committees having a special interest therein are represented, express, as nearly as possible, an internationalconsensus of opinion on the subjects dealt with.

2) They have the form of recommendations for international use and they are accepted by the National Committees in thatsense.

3) In order to promote this international unification, the I E C expresses he wish that all National Committees having asyet no national rules, when preparing such rules, should use the I E C recommendations as the fundamental basis for theserules in so far as national conditions will permit.

4) The desirability is recognized of extending international agreement on these matters through an endeavour to harmonizenational standardization rules with these recommendations in so far as national conditions will permit. The NationalCommittees pledge their influence towards that end.

PREFACE

This Recommendation was prepared by I E C Technical Committee No. 3T,Lightning Arresters.

It forms the first supplement o I E C Publication 99-1, Recommendations or Lightning Arresters;Part 1, Non-linear Resistor-type Arresters.

It gives he text of Appendix C, Application guide of non-linear resistor-type ightning arresters oralternating current systems.

A draft was prepared by a Committee of Experts and was discussed t the meetings held n Interlakenin 1961 and in Bucharest n1962. As a resutt of this latter meeting, a draft was submitted o the NationalCommittees or approval under the Six Months' Rule in December 1962.

The following countries voted explicitly n favour of publication of this Supplement:

Australia NetherlandsBelgium NorwayCanada SwedenCzechoslovakia SwitzerlandDenmark TurkeyFinland Union of SovietFrance Socialist RepublicsGermany United KingdomHungary United States f AmericaJapan

I

8/13/2019 SPLN 7D_1978

5/21

- 7

SUPPLEMENT TO PUBLTCATTON 99-1 (1958)

RECOMMENDATIONS FOR I,IGHTNING ARRESTERS

Part 1: Non-linear resistor-type arresters

Application guide of nonJinear resistor-type ightningcurrent systems

arresters for alternating

l. Object

This guide covers he application of non-linear resistor-type ightning arresters o safeguard apparatusagainst the hazards of abnormal voltages of various kinds. Such overvoltages may cause lashovers andserious damage o equipment and thereby eopardize he supply of power to users. t is essential o prevent

this by the proper co-ordination of protective devices with insulation strength.

The subject s a broad one with many ramifications and it would require a volume of considerablebulk to explain all possible ases n detail . t is not proposed o do this in this guide, but th e more basiccases re discussed nd step-by-step irection oward proper and economical solutions are provided. Insome cases, xperience r predetermined practices give sulficient nformation for some of these steps o beperformed without further consideration. The more complex cases f some nstallations, o which a numberof lines or cables re connected, o doubt merit special tudy by experienced ngineers.

In the present state of the art, complete protection against damage rom overvoltage may prove to beextremely costly and it may not be good engineering practice o provide it. Procedures are recommendedwhich are expected o provide conservative olutions hat can be ustified economically. These proceduresare based on theoretical studies, he results of tests, and experience.

For supplementary information see IEC Publication 7lA, Recommendations or InsulationCoordination Application Guide.

2. General procedure n applying lightning arresters

This guide presumes hat lightning arrester earth terminals are interconnected with earthed parts ofequipment, also hat both line and earth connections of the arrester are as short as s practicable.

The procedure or the selection and location of arresters n relation to the insulation to be protectedcan be reduced o a series f steps, which are elaborated n subsequent aragraphs:

a) determine he maximum phase-to-earth ower-frequency oltage at the arrester ocation;

b) estimate he magnitude and wave-shape f the most severe rrester discharge urrent;

c) determine the impulse withstand strength of the insulation to be protected, bearing n mind thatair insulation decreases ith increasins altitude;

8/13/2019 SPLN 7D_1978

6/21

- 9 -

d) tentatively select he arrester voltage rating and class;e) determine he impulse protective evel for the tentatively selected rresters;

f) locate the arrester as close as is practical to the apparatus o be protected, bearing n mind thatthe proximity of earthed objects and the height of the arrester above earth may adversely affectthe arrester sparkover characteristics, articularly at very high voltages;

determine he voltage at the insulation to be protected as imited by the arrester, aking into consi-deration h e separation istances nd other factors applicable o the point of application;in the event hat steps c) and g) indicate hat the arrester selected s inadequate, t may be necessa-ry to choose another arrester atin g or cla ss, or in the case when an entirely new station s beingdesigned, o increase he nsulation evel of the equipment o b e protected.

3. Step-by-step procedure or protection of power transformers and station equipment

(For special reatment of dry-type power ransformers ee Sub-clause .2.1).

3.1 Determine he maximum phase-to-earth ov'er-frequency oltage at the qrrester ocation

3.1.1 By multiplying the highest system voltage by the coefficient of earthing at the point ofinstallation of the arrester Clause 7 in Publication 99-1). The value of the coefficient ofearthing can be estimated rom the curves n Figure 1. Experience shows that for arresterapplication the coefficient of earthing can be classified nto a limited number of groups.

a) Cofficient of earthing does not exceed 80% (effectively-earthed ystem)A value not exceeding 0% is obtained at the arrester ocation when for all sy stem condi-tions the ratio of zero sequence eactance o positive sequence eactance (XolXi isbetween 0 and f 3, and the ratio of zero sequence esistance o positive sequence eactance(RslX) is between 0 and + l. At this point the system s considered o be effectivelyearthed.

Notes'*:n'ffiilHi.:.',?;J#fJ;'"iiln$H:"'i,il?",T"Til:?:il:'"::H:?Jrffmay be ess han 80%.

' - 3ftHlJ,*-3::"J1ffi;t'#:t';?l;Ji",ilil:il:,"ji,?'Ji,:i;[1'J:e#J";5%-1. - The possibility of increases n the coefficient of earthing due to system sectionalizing should

be recognized as a factor to be considered.

b) Cofficient of earthing exceeds 80% (non-effectively earthed neutral, resonant earthed orisolated neutral systems)

This may be the case n systems which are earthed hrough resistors or reactors, ncludingground fault neutralizer arc snppression oils), or which have some or all neutrals solatedfrom earth. In such systems he coefficient of earthing may be 100% or higher at thepoint of arrester nstallation f the ratio XolXt is negative.

Note.-lf XolXt lies between 0 and -20, resonance conditions may occur. For systems with isolatedneutrals, the ratio XolXr is, however, usually lower than -20 so that resonance conditions arenot likely.

o l6,/

h)

i***;;;,,**;i*6ll';i

8/13/2019 SPLN 7D_1978

7/21

3 . r. 2

3 . 1 . 3

- - l l -

By checking he influence on the maximum phase-to-earth oltage of :

a) sudden oss of load

b) the effect of machine overspeeds.

By considering overvoltages aused by resonance effects, nduction from parallel circuitsand similar factors known to be significant; otherwise they generally are neglected. Seethe discussion f abnormal system oltages n Sub-clause .4.2).

3.2 Estimate the magnitude and wave-shape f arrester discharge current

Estimate the magnitude and wave shape of the discharge current, largely by the degree ofshielding against direct lightning strokes o lines, stations, substations and distribution transfor-mer nstallations. uch nstallations may be divided nto two classes:

effectively hielded, or

non-effectively hielded.

3.2.1 Magnitude qnd wave-shape f discharge current or effectively-sltielded nstallations or overheadline-cable unctions

Effectively-shielded nstallations have shielding against direct strokes provided for thestation and for all connected ines. The li nes may be shielded either on the whole length or ona few spans rom the station (line end protection). Shielding s regarded as effective f the pro-bability of shielding ailures or back flashes rom shield wires or earthed supporting structuresto the conductors or other live parts is so small that the risk is considered acceptable or thespecific application.

Practical experience f some countries shows hat the shielding s not very effective or mediumvoltage lines (below 100 kV), and these are treated as non-effectively shielded nstallations.For higher voltages, effective hielding practice n some countries prescribes hat the protectiveangle of the earth wires should not exceed 30" and that the earthing resistance f each owershould not ex ceed 0 ohms. The maximum lightning arrester discharge urrents n such a casevary usually rom about 4 000 A at 110 kV up to about l0 000 A at 400 kV substations. ineswith bundled conductors may attain a discharge current between 5 000 and 10 000 A. (Thesedischarge urrents are for the 8120microsecond wave used or lightning arrester esting).

Shielding or line end protection on medium voltage lines located on woodeu poles withan earth wire which is earthed at each pole, but with no e arthing of crossarms outside theprotected zone, may not prevent back flashes back flashovers) f the earth resistance at thepoles is high. However, the shielding will tap off the lightning stroke current, and also willlimit overvoltages entering from beyond the s hielding. Such overvoltages may be very highbecause of the wood insulation. This shielding, while not fulfilling the requirements or aneffectively-shielded tation, will limit the magnitude of the discharge current below thatdiscussed n Sub-clause .2.2 or non-effectively-shielded tations, he amount depending on theparticular conditions at the installation.

8/13/2019 SPLN 7D_1978

8/21

_ 1 3

The arrester discharge currents n effectively shielded nstallations depend on many factors.The most important of them are listed below:

a) the size of the installation and system voltage;b) the impulse insulation withstand strength of the incoming lines; in steel and concrete

tower lines the withstand strength of insulators (or gaps, if used) should be taken into

account ;In wood pole lines the additional impulse withstand strength of the wood must be takeninto account ;

c) the number of connected ines. Owing to reflection of travelling waves, he discharge urrentof arresters s affected by the parallel-connected urge mpedance of lines and cables;

d) the length of the shielded portions of the incoming lines. (Minimum values of lengthsinvolved are under consideration.)

The rate-of-rise of discharge urrent s dependent n the rate-of-rise of voltage. The maximumrate-of-rise of voltage when it enters he station from an effectively shielded ine is estimatedto be 500 kV per microsecond.

3.2.2 Magnitude and wave-shape f discharge current or installations or oyerhead ine-cqble unctionsnon effectively shielded

With installations or overhead ine-cable unctions not effectively shielded, both the insula-tion and the arrester can be subjected o nearby direct strokes producing extremely high voltageshaving very steep rates-of-rise, of the order of 1000 kV per microsecond, as well as highcurrents. Severe onditions may result also from a back flashover close o the arrester. Expe-rience indicates that for the expected ncident voltage a satisfactory degree of protection isobtained by co-ordinating he arrester esidual voltages t a discharge urrent of 5 000 o 20000 A(8120 microsecond value) depending on :

a) the importance of the installation;b) the probability of the occurrence of the higher currents;c) the size and voltage of the installation;d) the line insulation where ully insulated wood pole lines are used; hus an arrester connected

to a fully insulated ine of lower voltage s likely to be subjected o much higher surge currentthan one connected o a higher voltage line with earthed crossarms, unless he stroke occursso close o the arrester hat the impedance and insulation of the line cannot influence hesurge.

Note. This problem s related o the number of days n the year n which hunderstorms ccur n thelocality, heir severity nd he design f the shielding f the ine and erminal quipment nd he

nature f the errain. Where onditions resevere, t is advisable o calculate r to otherwise eter-mine he ate-of-rise f the voltage nd he magnitude f the discharge urrent.

3.3 Determine the withstand strength of the insulation to be protected

Determine he insulation mpulse withstand strength and in some cases he insulation switching-surge withstand strength of the equi pment being nstalled.

3.3.1 Impulse withstand strengthfor equipment nsulation other than air

The impulse withstand strength for equipment is defined by its full wave impulse test voltage(see able of Standard nsulation evels n IEC Publication 71, Recommendations or InsulationCo-ordination).

8/13/2019 SPLN 7D_1978

9/21

8/13/2019 SPLN 7D_1978

10/21

- 1 7 -

b) the need or the best protection. As a general ule, l0 000 ampere arresters referred o insome countries as station arresters) re applied to high-voltage systems 100 kV and above)and to important stations of lower voltage systems which are considered mportant enoughto require the best protection; 10 000 ampere or 5 000 ampere Series A arresters are usedon medium voltage ransmiss ion ystems, 000 ampere Series B arresters r 2 500 amperearresters n distribution systems or the protection of small ransformers;

c) special equirements which indicate that a higher class of arrester s advisable because:

1) the lightning severity s unusually high;

2) the switching surge conditions indicate the use of arresters with increased current dis-charge capacity or discharging ong lines, cables and capacitor banks n case he circuitbreakers estrike or other switching urges ause parkover of the arrester Clause 5);

3) of installations with a single ncoming line which are considered mportant enough torequire the best protection and particularly those which are not effectively shielded.A study of expected ischarge urrents s advisable n such cases. See Sub-clauses .2and 3 .6 ) .

3.4.2 Choose he tentative arrester voltage ating

Choose the tentative voltage rating based on the highest phase-to-earth power-frequencyvoltage determined as indicated n Sub-clause .1. This voltage rating should be chosen atleast equal to the highest phase-to-earth oltage in order to assure extinction of the powerfollow current under any circumstances, hat is, proper arrester operation. As the highest phase-to-earth voltage for a system often is known only very approximately, t is recommended ha tarrester voltage ratings equal to or very closely above this highest phase-to-earth oltage beused only when his is necessary n order to get a suitable protective atio between he equipmentinsulation withstand level and the arrester mpulse protective evel. Use of arresters with thevoltage ratings too low may result n an excessive ailure rate of the arresters n service. Whenthis risk is deliberately aken in order to protect weak insulation, consideration must be givento the use of arresters arranged so as not to endanger personnel and neighbouring equipmentshould the arrester ail.

Special conditions which should be considered n choosing he arrester oltage a ting are:

a) Abnormal system voltages

The selection of the arrester voltage ratings corresponding to the highest systemvoltages U-) multiplied by the coefficient of earthing, s based on the assumption hat inservice he highest system voltage (t/-) is only exceeded nder abnormal operating condi-tions, and that the probability of an arrester operation coinciding with a voltage exceedingthe highest system voltage is very small. If abnormal system voltages are likely to be afrequent occurrence, hereby ncreasing he probability of arrester operations during suchconditions, t may be necessary o use an arrester with a voltage rating higher than recom-mended above, depending upon the particular circumstances.

I

8/13/2019 SPLN 7D_1978

11/21

- 1 9 -

b) Abnormal system requency

Alternating current of frequency ess han 48 or more than 62 Hz(c/s)mayrequirespecialconsideration n the manufacture or application of lightning arresters nd should be subject

to discussion between he user and the manufacturer.

As noted in Sub-clause .6.3, his tentative choice of arrester class and voltage rating may

require modification.

3.5 Determine he impulse rotective evel or the tentotively selected rrester

Note. - The proximity of earthed apparatus and the height of the arrester above earth may adversely affect the

arrester sparkover characteristics, particularly at high voltages.

3.5.1 For effectively-shielded ines and installations (see Sub-clause .2.1)

Determine he residual voltage at a discharge urrent estimated n accordance with the consi-

derations n Sub-clause 3.2.1 for the tentatively selected arrester rom information furnished

by the manufacturer. When l0 000 ampere arresters re used 8120microsecond waves), o-ordi-

nation at the nominal discharge urrent normally provides a factor of safety.

3.5.2 For lines andinstallations which are not effectively hielded see Sub-clause .2.2)

Determine the residual voltage for a discharge current estimated n accordance with theconsiderations n Sub-clause 3.2.2. This may exceed he nominal discharge current of the

arrester.

Notes ,1. - The front-of-wave impulse sparkover voltage for an arrester (at rates-of-rise specified in Publica-tion 99-1) divided by 1.15 may be lower than the residual voltage at the standard nominal dischargecurrent of the arrester. If so, they need not be further considered. Arresters having both internaland external gaps may be an exception.

2. - The procedures in Sub-clauses 3.5.1 and 3.5.2 are sufficient for arresters located close to the insula-tion to be protected, and with the overhead lines brought directly into the station. The influenceof location of the arrester with respect to this insulation (separation distance) will be discussed in

3.6.2.3. The introduction of a cable between the overhead lines and the installation will be discussedin Sub-clause 3.6.4.

-?. - Consideration should be given to the need for obtaining the switching surge protective level of the

arrester from the manufacturer. as covered in Sub-clauses 3.3.3 and 3.6.2.3.3.

3.6 Co-ordinate the arrester protective level with the impulse withstand strength of the insulation to

be protected

3.6.1 General

The recommended reatment is different for different categories of installations dependingon whether hey are effectively hietded or not, on the number of lines normally connected, ndon the physical extension of the installation.

The following general principle is applied:

There shall be a certain protective ratio provided between he impulse withstand strength

of the equipment nstallation to be protected (a guaranteed minimum value) and the impulseprotective evel which is achieved at the piece of equipment n question under the assumptionsgiven in Sub-clauses .3 and 3.5. This protective ratio is intended to cover exceptional cir-

cumstances uch as abnormally higher magnitude or steepness f infrequent occurrence.

8/13/2019 SPLN 7D_1978

12/21

_ 2 1 _

The recommended minimum protective ratioimpulse protective evel s l .2.I t is pre-supposedare directly connected ogether.

between insulation withstand strength andthat the earths of all arresters nd equipment

The reduction in the electric strength of air insulation with increasing altitude should beconsidered, using the recommendations or the specific apparatus.

3.6.2 Co-ordination based on the type of instollations

3.6.2.1 For non-effectively shielded nstallations tt,itha single incoming overhead ine

A typical nstallation of this type consists f a single ransformer with or without simpleswitching equipment, generally without measuring equipment, which is connected o a singleincoming line without shielding by earth wires.

Install the arrester ight at the transformer.

Compare the full-wave impulse withstand strength of the transformer (determined inSub-clause .3) with the impulse protective

evel of the arrestersdetermined

n Sub-clause3.5.2) and check hat there s a satisfactory protective atio.

3.6.2.2 For non-effectively hielded nstallotions v'ith several ncoming ines

Installations f this ype differ rom Sub-claus 3.6.2.1 ince n normal operation more ha none line is connected. Typically, this is a medium-voltage station, and even f there is morethan one transformer the extension of the station area s rather small. Some switchine andmeasuring quipment s usually nstalled.

Install a set of arresters at or close to the transformer or transformers and check theprotective ratio as in the previous case of Sub-clause .6.2.1, aking into account hat theincomirrg overvoltage waves are reduced by sharing of energy when several ines meetin the station. However, consideration should be given to the case when one or more of thelines are disconnected by switching.

When, in such a station, one or more circuit-breakers or disconnecting witches are open,the corresponding ine entrances r certain parts of the station may be eft without protectionfrom the arresters at the transformers. If such cases are recognized o require additionalprotection, non-linear esistor arresters, aps, or transmission-class xpulsion arresters reinstalled at the respective ine entrances.

3.6.2.3 For effectively shielded nstallations

The incoming overvoltages re imited in amplitude and steepness, hich generally permitsa certain separation between he arresters and the insulation to be protected see Sub-clause3.5.1), as will be discussed ater. Depending on whether t is a small or large nstallation,proceed as follows:

3.6.2.3.1 For small installations with one ncomins overhead ine

I Install one set of arresters at a point which provides protection to all equipment bu tgives preference o the transformer. Separation between he arrester and the insulationto be protected s permissible n effectively-shielded nstallations.

8/13/2019 SPLN 7D_1978

13/21

- 2 3 -

ii) Determine he maximum permissible separation distances etween he arrester and theprotected equipment which will not permit excessive oltages at this equipment nstal-lation. This can be calculated because he characteristics f the surge entering he instal-lation are reasonably well known. A precise determination can be made using methodsdetailed n the trade literature. A graphical method is being considered which may beadded as Appendix D at a later date.

3.6.2.3.2 For large installations with several incoming overhead ines, tronsformers. switchgear andmeasuring equipment

Determine as well as possible he most strategic arrester ocations and the number ofarresters which will give the wanted degree of protection to different pieces of equipment.This can be rather difficult.

A graphical method is being considered which may be added as Appendix D at a laterdate. Examples of cases reated by model studies and digital computer programmingmethods are given in the trade literature. Consideration must be given to the possibility

that the station may become sectionalized or that lines are disconnected during service,with or without disturbances. t must be possible o maintain the protection of at leastthe transformers under all circumstances. Sometimes his involves ncreasins he numberof arresters.

3.6.2.3.3 For installations with transformers having reduced insulation morestandord nsulation evels (see Tables n Publication 7 )

Install the arrester at the transformers and proceed as directed nor 3.6.2.3.2. n such installations, n many cases he arrester mustvoltages as well as lightning overvoltages.

than one step below

Sub-clauses .6 .2 .3 .1limit switching ver-

For switching overvoltages, he insulation withstand level as well as the s witching surgeimpulse protective level of the arresters have values different from those for lightningovervoltages and, therefore, the insulation coordination for switching surges must bestudied separately see Sub clause 3.3.3). The results of such a study may lead to modifica-tion of the arrester arrangement. As no standard values are established or the insulationco-ordination at switching overvoltages, t may sometimes e necessary o obtain guidancefrom equipment manufacturers.

3.6.2.3.4 For cable-connected nstallations of Sub-clause .6.2

For cable-connected nstallations he arrester or arresters should be applied as covered

in Sub-clause .6.4.

3.6.3 Other considerations

3.6.3.1 When co-ordination s not achieved

If co-ordination is not achieved by the procedure of Sub-clauses .6.1 and 3.6.2 with thearrester entatively selected n Sub-clause 3.4, it becomes necessary o consider alternativemeasures uch as :

a) selecting an arrester of better class or lower rating to obtain a lower impulse protectivelevel.Selecting an arrester with a lower voltage rating than that indicated by consideration of

8/13/2019 SPLN 7D_1978

14/21

- 2 5 -

Sub-clause3.4.2will nvolve some risk of arrester ailure resulting from the inability ofthe arrester gaps o reseal against a voltage exceeding ts rating;

b) changing the location of the arrester to reduce he separation distance and/or the arresterlead length;

c) increasing he insulation level of the equipment o be protected;d) improving the shielding.

3.6.3.2 Special conditions equiring reconsideration of the choice of the arrester

Special conditions affecting he need or reconsideration of the arrester entativelv selectedin Sub-clause .4 mav be :

3.6.3.2.1 High earth resistance or excessive eparation

(The consideration of this is dependent on the completion of Appendix D. )

The earths of all arresters and equipment (if possible) should be connected ogetherelectrically; however, f a direct connection between he arrester and the protected appa-ratus has not been made and the arrester earth resistance s high or the connections be-tween the arrester and the protected apparatus are of excessive ength, the impulse voltageswhich appear at the protected apparatus may be substantially higher than those across heterminals of the arrester. n order to obtain the desired degree of protection for the appa-ratus, t may be necessary ither to improve these conditions, or to select a class of arresterwith lower protective characteristics. he use of an arrester with a lower voltage ratingthan that indicated by consideration of Sub-clause .3.4.2 will result in a risk of failureof the arrester f it is required to operate when the power-frequency voltage across tsterminals exceeds he voltage rating.

3.6.3.2.2 Apporatus of lowinsulation strength

The protection of apparatus having low insulation strength may require special conside-ration in the selection or design of the arrester and should be referred o the manufacturer.

Equipments which can be included in this classification are old equipment built priorto the time impulse tests were made, and equipment not built for exposed ocations. Theproblem of protecting such equipment s similar to the problem discussed n Sub-clause .2 .

3.6.4 Cable-connected quipment for single-line nstallations

Cable-connected quipment nvolves a station, substation or individual apparatus connectedto a cable (with earthed metallic sheath) which in turn is connected o an overhead exposed inethat may or may not be effectively hielded at the line-cable unction.

3.6.4.1 Location of arresters

3 .6 .4 .1 .1 General

Install arresters at the equipment, or at the overhead ine-cable unction or at both theequipment and the unction, if necessary. rrester installations must be made at the over-head line-cable unction if it is impossible o apply the arresters at the equipment. Limi-

8/13/2019 SPLN 7D_1978

15/21

- 2 7 -

tations of space or the arresters t the equipment may also make application at the unctiondesirable.

In the case of unshielded ines, t may be advantageous o mount additional protectivedevices a few spans before the overhead ine-cable unction.

The arresters nstalled at the equipmentshould be connected o the station earth withthe shortest possible ead. Arresters nstalled at the cable unction should be earthed an d

interconnected at the unction with the cable sheath f a metallic sheath s used. For theprevention of circulating currents n the cable sheaths t may not be desirable o earth theseat the equipment end also. f the cable has a non-metallic sheath, he arrester at the unctionshould be earthed at the unction and interconnected with the station earth by a conductorinstalled adiacent o the cable.

3.6.4.1.2 Arrester ocated only at equipment

If the arresters are located only at the equipment, consideration should be given as towhether he insulation at the overhead ine-cable unction will be protected.

3.6.4.1.3 Arrester ocated only at overhead ine-cable unction

lf arresters are installed only at the overhead ine-cable unction, consideration shouldbe given as to whether the insulation of the equipment will be protected. The protectiondepends on factors such as he impulse withstand strength of the insulation o be protected,the class and voltage rating of the arrester at the junction, the length of the cable andwhether or not the overhead ine is shielded. For precise determination of the maximumpermissible cable ength up to which protection will be provided to the equipment nsula-tion, refer to the existing iterature and/or have a study made by those well versed n theart. Good results an be achieved y analogue etwork studies.

A graphical method for calculation of permissible able engths s being considered whichmay be added as Appendix D at a later date.

3.6.5 Protection of trantsformer nearthed eutral(s)

3.6.5. General

This applies o star (Y)-connected ransformer banks, he neutral(s) of which s (are) solatedor earthed hrough a high impedance. Surge voltage may appear on the neutral as a resul tof overvoltage at the line terminals propagating through the transformer windings. Withwood pole lines of unearthed construction this voltage can be very high. All neutral points

brought out through a bushing should be protected by lightning arresters. n the case oftransformers with insulation graded toward the neutral point, it is even more important toprovide protection.

Although the currents hrough the arrester due to lightning and switching overvoltages resmall up to I 000 A), account must be taken of their lonser duration.

3.6.5.2 Select and install the arresters

Install the arrester between the neutral terminal and earth (i.e. the transformer earthterminal) selecting t as directed n Sub-clause .4. The arrester ating should be at least 0. 7

8/13/2019 SPLN 7D_1978

16/21

- 2 9 -

times the highest phase-to-phase oltage of the system 0.7 (In), provided the transformer sfully insulated.

For transformers with graded insulation, refer to Tables C I and C Il for classificationof these ransformers. nformation on protection should be obtained rom the manufacturer.

Tnslp C I

Categories o.fgraded insulatiott

Category Recognized conditions of earthing

Neutral end of winding, solidly connected to earth through a connectionwhere no impedance has been added intentionally.Note. - The connection to earth via a current transformer is deemed to meet this

cond i t ion .

Neutral end of winding connected to a regulating transformer whoseneutral is

oris not

connected to earth and isprovided

with an appro-priate voltage-limiting device.

Neutral point of winding not connected, or connected to earth via animpedance or a resistance, with an appropriate voltage-limiting deviceconnected between the neutral point of the winding and earth.

Neutral point of the winding connected to earth via an arc-suppressioncoil with a suitable voltage-limiting device between the neutral pointof a winding and earth.

Tenrs C I l

Insulotion evels or the neutral ends of h,indings f transformers aving raded nsulation

(For system highest voltages o.f 72.5 kV and above)

In su la t io nto earth

Category 1

Category 2

Separate-source power-frequency voltagek V r.m . s .

ENR +(ELT-ENR)

x

3 8

additional voltage due toregulating transformer

rated voltage of the transformer windingwith a minimum of 38 kV

ENR test voltage of neutral point of regulating transformerELT test voltage of the line end of the transformer windins

Category 3 | 35 to 65 I of the test voltage of the line end, determined according to th echaracteristics of the apparatus and the system

Category 4 | 58 to 65 )( of the test voltage of the line en d

Notes 1. - When choosing the category of neutral insulation, the possibility that the neutral earthing maybe altered at a later stage or that transformers may be interchanged, should be considered.

?. - Obtain the equivalent impulse strength for these power-frequency voltages from the manufacturer.

8/13/2019 SPLN 7D_1978

17/21

- 3 1 -

3.6.5.3 Compare he impulse rotective evel with insulation w,ithstand trength

Compare he impulse protective evel of the arrester, namely he impulse sparkover voltagein this case, with the equipment nsulation withstand strength. The impulse protective evelshould be not more than 0.833 imes the full-wave impulse withstand strength of the insula-tion at the neutral.

4, Protection of other equipment

4.1 Protection of series windings of equipment such as booster transfornters, reactors, current trons-formers, and so orth

Sometimes t is expedient o provide surge protection across he series windings of equipment.

4.1.1 Selection of arrester (See Sub-clause .4.1 or choice of arresters class.)

Select he arrester having a standard voltage rating which is equal to or greater than themaximum power-frequency voltage that will appear across the series winding under faultconditions.

4.1.2 Location of arrester

Install the arrester lose o the terminals of the equipment.

4.2 Protection of dry-type insulated equipment

The dry-type insulated equipment covered by this paragraph ncludes such apparatus as dry-type transformers and rotating machines which have full-wave impulse withstand insulationstrengths ower than those of liquid-immersed equipments. Generally the impulse withstandstrengths with waves of short duration are considered

o be the same, or nearly the same, as thefull-wave impulse withstand strength.

4.2.1 Protection of dry-type transformers

4.2.1.1 Apply the procedure of Clause 3 including selection of the arrester as directed n Sub-clause .4 .

4.2.1.2 Compare the front-of-wave impulse sparkover protective level of the arrester with thefull-wave mpulse withstand nsulation strength of the transformer, or the impulse withstandstrength for any shorter durations for which higher values are given by the manufacturer.The minimum protective ratio between nsulation withstand strength and protective levelrecommended n 3.6 .1 s 1 .2 .

Note. - No tests or the impulse insulation strength of dry-type transformers have been standardized by I E C.

4.2.2 Protection of rotating machines when required)

4.2.2.1 For machines connected o overhead ines either directly or through a short length ofcable:

a) install, at the machine terminals between ine and earth, both capacitors to slope of fthe wave-front to approximately 10 microseconds or more and arresters o provideadditional protection. Also connect arresters on the overhead ines ahead of the machinelocation or at an overhead ine-cable unction point;

8/13/2019 SPLN 7D_1978

18/21

- 3 3 -

b) select he voltage rating and class of arrester as directed n Sub-clause .4;

c) compare he full-wave mpulse nsulation strength of the insulation or the value as recom-mended by the manufacturer, with the impulse protective level of the arrester for asuitable protective ratio. (See Sub-clause .6.1.)

4.2.2.2 For machines connected o overhead ines through transformers, n some cases nstal-lations may not require protection, or the capacitors an be omitted at the machine erminals.In some cases, or instance, with star (Y)-delta (D) transformers, better protection can beobtained by a second set of arresters connected between phases. t is suggested hat theliterature on this subject can be consulted or an investigation can be made with recurrent-surge oscillograph.

When arresters are installed at the machine erminals , ollow the procedure n Sub-clause4 . 2 . 2 . 1 .

Note. The mpulse nsulation trength f rotatingmachines as not been tandardized nd no standardiza-tion s contemplated n the mmediate uture. For reatment f the co-ordination roblem efer o themanufacturer f the machine. t is usual o take he peak alue f the a.c. est voltage s he mpulse

insulation trength f the otating machine, n the absence f more precise nformation.

5. Switching surges iable to cause operation of the arrester

5.1 Classffication of cause of switching surges

Overvoltages which are a problem in insulation co-ordination (see Sub-clause 3.6.2.3.3) andmay overstress he arrester when it sparks over, can be produced by switching n several ways.They may be caused primarily by a behaviour nherent n the switch or by the conditions nherentin the system and circuit during and after switching.

5.1.1 Switching surges rom inherent switch action

a) chopping of inductive currents by circuit-breakers;the interruption of an inductive current such as the interruption of magnetizing currentof the transformer can cause overvoltages when the current is forced to zero by the actionwithin the switch before normal current zero:

b) restriking by the circuit-breaker during the interruption of capacitive currents when discon-necting ong lines, cables or capacitor banks;

c) restriking by disconnect switches when disconnecting busbars;d) prestriking on energizingcapacitive ircuits by disconnect witches.

5.1.2 Switching surges and voltages ront circuit and system conditions

a) energization of line or unit-connected ine and transformer (no initial charge on the line);

b) reclosing on line with trapped charge;c) energization, de-energization or load rejection of unit-connected ine and transformer

(harmonic overvoltages)d) voltage magnification upon closing or restriking. Higher transient voltages are produced

at a point remote from the point of switching;e) 3-phase clearing of faulted lines at one end only, or lack of simultaneous opening at both

ends, particularly with line-to-earth faults ;

8/13/2019 SPLN 7D_1978

19/21

_ { \ _

f) clearing, following loss of load with regulation and over-speeding f alternating currentmachines;

g) closing when out of phase;h) linear resonant effects nvolving an interaction of li near inductive and capacitive compo-

nents of the system as a consequence f energization or de-energization f the part of thesys tem;

i) ferro-resonant non-linear oscillations, caused by energization or de-energization nteractionbetween system capacitance and non-linear magnetizing mpedance of transformers whichmay result from single phase switching, open conductors, neutral instability, or over-excitation of certain 3-phase ircuit configurations .

5.2 Classification by type of discharge hrough arrester

5.2.1 Switching surges v,ith high surge energy

These urges re covered y Sub-clauses .1 .1 ) ,5 .1 .2 b) and 5.1 .2 ) .

When the arrester sparks over with this type of switching surge, he charge on the line, cableor capacitor bank discharges hrough the arrester mpedance. f the arrester s connected on theline side of the circuit-breaker, he arrester s subjected o the full discharge current from theline. This is a relatively ow-current ong-duration discharge. or long lines he magnitude ofdischarge current is dependent on the charge on the line and the surge mpedance of the lineand the arrester. he duration s dependent n the ength of the ine.

Two solutions have been employed o meet his problem:

a) select ircuit-breakers r switches which do not generate witching urges bovesparkover potential;

b) select arresters with sufficient discharge capacity to withstand the dischargewhich the arresters will be subjected. nformation on the discharge apacityshould be obtained from the manufacturer.

the arrester

currents toof arresters

5.2.2 Switching surges with medium surge energ))

T h e s e u rg e s r e c o v e r e d y S u b - c l a u s es . 1 . a ) , 5 . 1 . 2 a ) , 5 . 1 . 2 ) , 5 . 1 . 2 ) , 5 . L 2 . f ) a n d5.1.2 g).They can be affected y the length of the line or the amount of capacitance nvolved.

They may be a problem only with arresters aving ow discharge apacity, such as 5 000 A,Series A arresters.

When the arrester sparks over with this type of surge, he discharge urrent and/or durationcauses nly mediurn duty on the arresters. his can overstress he ower surge apacity arresters.

The choice of so lu t ions s the same as n Sub-clause .2 .1 .

5.2.3 Switching surges witlt low surge energy

These urges re covered by Sub-clauses .1.1 a).

When the arrester sparks over with this type of surge, experience ndicates hat the dischargecurrents are limited and handled without difficulty by lightning arresters applied as directedin this guide.

8/13/2019 SPLN 7D_1978

20/21

- 3 7 -

5.2.4 Switching urges with prolongedfollow urrent

These urges re covered y Sub-clauses .1 .1 ) ,5 .1 .1 ) ,5 .1 .2 ) ,5 .1 .2 f) ,5 .1 .2 ) ands . r.2 ) .

With this type of surge the arrester is subjected to prolonged overvoltage which preventsresealing of the gaps. This can cause he arrester o fail.

One of two solutions has been employed o meet this problem:

a) make changes n the system o eliminate such surges, r;b) choose an arrester either with sufficient discharge capability for the indicated duration, or

with sufficiently high sparkover to prevent its operation, taking account of the possibledecrease n protection provided by the arrester.

6. New definitions

6.1 Impulse protective level of an arrester

The highest peak value of impulse voltage that may occur across the terminals of an arresterunder the prescribed conditions.

The i mpulse protective evel s given numerically by the maximum of the following quantities:

- Front-of-wave mpulse sparkover oltage, divided by l. l5t;- 1.2150 parkover voltage 2)- Residual (discharge) oltage at a given discharge urrent.

6.2 Rated impulse protective level of an arrester

The impulse protective evel with the residual voltage eferred o the nominal discharge urrent.

6.3 Protective atio

The ratio of the insulation withstand characteristics of the protected equipment to the arresterprotective evel, expressed s a multiple of the latter figure.

1) See 3.2 Note A) of Appendix C.z; IEC Publication 0 gives his as 1.2150 nstead f 1/50 as n IEC Publication 9-1.

8/13/2019 SPLN 7D_1978

21/21

- 3 9 -

{O

t 7

{o

\ 7

a) Yoltage conditions neglecting positive an dnegative-sequence resistance Rr : Rz : 0

1 2 3 4 5 6XaiX

b) Yoltage conditions or Rr : Rz : 0.1 Xr

1 2 3 4 5 6XolXt

c/ Voltage conditions or Rr : Rz : 0.2 Xt

Note. - Numbers on curves indicate maximum line-to-earthvoltage of any phase or any type of fault in percent of the line-to-line voltage for area bounded by curve and axes ofthe curves .All impedance values must be on the same base n ohms on samevoltage base .

Fo r a l l cu rv e s :

Ro - zero-sequence esistanceRr - positive-sequence esistanceRz : negative-sequence resistanceXo : zero-sequence inductive reactanceXr : positive-sequence ubtransient reactanceXz : negative-sequence reactanceX r : X z

The effect of fault resistance was taken into account. The resist-ance which gives the maximum voltage to earth was the valueused. The discontinuity of the curves is caused by the effect offault resistance.

Flc. l. - Maximum line-to-earth voltage at fault for grounded-neutral systemunder any fault conditions.