Some practical aspects of overvottages on theCEGB transmission system

L. Csuros, Dipl.-lng., C.Eng., F.I.E.E., and K.F. Foreman, D.F.H., C.Eng., M.I.E.E.

Indexing terms: Power transmission, Lightning, Switching, Transients, Insulation

Abstract: The paper reviews the insulation co-ordination policy of the CEGB and the main practicalaspects of investigations into various types of overvoltages. Faults resulting from external and internalovervoltages are compared with those due to other causes. Overvoltage problems and remedial measuresare appraised in the light of the insulation co-ordination policy adopted by the CEGB.

1 Introduction

This paper reviews some problems which have been en-countered on the CEGB 275 kV and 400 kV systemswhere overvoltages arising from switching or lightninghave given rise to potential difficulty in operating thenetwork. The main purpose is to put these problems intoperspective from the operational standpoint rather thanto discuss the theoretical aspects. Relevant data are sum-marised from staged switching tests in the field, carriedout either to investigate particular incidents or as part ofa general programme of investigation of possible over-voltages. The frequency of occurrence of these incidentsis compared with that of other causes of circuit inter-ruption. The philosophy of overvoltage protection adoptedby the Board is considered in relation to these particularproblems.

All the incidents discussed in the following Sectionsoccurred in 'conventional' substations. Their relevanceto insulation co-ordination in gas-insulated substations isbriefly considered.

Only those overvoltages of sufficient magnitude tocause protective gap operations, flashovers, or damage toplant insulation come to the notice of operational staffand there will be exceptions even in some of these cases.On such a heavily interconnected network, transient over-voltages due to fault clearing and fault initiation are of toosmall a magnitude to cause a gap operation, as are mosttemporary overvoltages. Hence, while considerable interestcentres round these types of overvoltage abroad, wherelower insulation levels have been adopted and particularlywhere higher system voltages are contemplated, thesehave not caused difficulties to the CEGB hitherto.

2 Proportion of faults due to lightning and switchingovervoltages

Table 1 shows that during the five years of the surveythe number of incidents attributed to lightning was sig-nificantly less than those due to adverse weather and theincidents attributed to switching overvoltages were virtuallyinsignificant.

Figures listed in Table 1 were extracted from a com-puterised list of data relating to each fault reported. Uncer-

Paper 713C, first received 24th January and in revised form 26thMarch 1980. Subject review paperMr. Csuros was formerly with the Transmission &, Technical ServicesDivision of the CEGB and is now with Preece, Cardew & Rider,Paston House, 165-167 Preston Road, Brighton, Sussex BN1 6AF.Mr. Foreman is with the Transmission & Technical Services Divisionof the CEGB, Burymead House, Portsmouth Road, Guildford,Surrey GU2 5BN

IEEPROC, Vol. 127, Pt. C, No. 4, JULY 1980

tainties surround the cause and location of flashovers ina number of cases and it proved impracticable to eliminateall of these; hence these figures are included as a guideonly, showing the order of magnitude of faults due todifferent causes on the systems as they are now operated.

An earlier survey1 showed that the average number ofsurges per annum at each location monitored exceeding2-4 per unit was in the region of 008 at 400 kV and 0-54at 275 kV. This was based on monitors installed at theterminals of transformers and circuit breakers withinsubstations on three phases at 13 points on the 400 kVand 12 points on the 275 kV system.

3 Philosophy of insulation co-ordination andovervoltage protection

3.1 Form of overvoltage protection used

Overvoltage protection on the CEGB transmission systemsis provided almost entirely by co-ordinating gaps usuallycomprising horizontally arranged rod-gaps located at theprimary and secondary bushings of transmission trans-formers, the h.v. bushings of generator transformers andat the terminals of each e.h.v. reactor. Some overvoltageprotection is provided also by gaps across overhead-lineinsulators in the vicinity of line terminations and acrosscable-sealing ends. On overhead lines, gaps are providedacross each insulator set throughout the entire length ofthe line. A reduced setting is adopted on those insulatorswithin about 1-5 km of the end of the line, irrespective ofwhether the line terminates at a substation, or at a cable-sealing end such as at the approach to a substation or,alternatively, at a point along the length of a feeder wherea section of cable is interposed.

The gaps on the overhead-line insulators with the largersettings are intended to offer a measure of protectionfrom burning to the line fittings etc., by providing ananchorage to the arc, thus reducing the possibility ofpermanent damage. No gaps or arcing horns etc. areemployed on substation equipment other than transformersor reactors.

3.2 Gap settings employed

The nominal settings adopted with some indication of flash-over voltages are shown in Table 2.

Gaps on suspension insulators exhibit little dispersiondue to polarity effect. However, conventional co-ordinatinggaps unfortunately exhibit widely differing flashovervoltages depending on polarity. The setting chosen hastherefore to be a compromise between reducing the prob-ability of a gap operation during normal operational switch-

248

0143-7046/80/04248 + 14 $01-50/0

Table 1: Primary cause of major categories of incidents. Total during period April 1974—March 1979

Highest voltage of system (kV r.m.s.) 420 kV 300 kV

Linecircuits

Internalsubstationcircuits

Linecircuits

Internalsubstationcircuits

1 Total number of incidents attributedto lightning involving flashovers

2 Number of incidents in (1) listed underline circuits associated with one ormore flashovers on transformer h.v.connection in substation (see Table 4)

3 Number of incidents in (1) with flashoveron one phase to earth

4 Number of incidents in (1) with flashoveron two phases to earth

5 Number of double circuit faults involving

circuits of same voltage

involving circuits of differing voltages

6 Total number of incidents attributed toswitching surges involving flashovers

7T Number of incidents attributed to heavyrain involving flashovers

8 ' Number of incidents attributed primarilyto ice, snow, sleet, fog, freezing rain.Note high winds or pollution presentduring some of these faults

9^ Number of incidents attributed primarilyto wind

201

16

176

7

4

2

r

7

149

284

10

12

92

15

73

5

1

2

2*

3

210

166

5

3

13

Overhead-line circuit length (km) commissioned (excluding cables) at

Transformer feeder circuits listed under voltage of overhead line' Position of fault listed on basis of type of equipment and not type of circuit

Date31.3.7431.3.7531.3.7631.3.7731.3.78

420 kV78597988843191009132

300 kV40433945385938803925

ing, to an acceptable minimum, and at the same timeproviding as much protection as practicable to the trans-former and other substation plant.

3.3 Use o f surge arres ters

The Board uses surge arresters in a few locations wherethe disadvantages of co-ordinating gaps are consideredunacceptable.

On transformer feeders, the winding connected to theoverhead-line circuit is especially exposed to severe lightn-ing surges. Surge arresters have been used at 132 kV oncircuits with this configuration and would be used morewidely at 275 kV and 400 kV if type approved arresterswere available. At present, arresters are used only at thesehigher voltages in the few cases of transformer feederswhere the transformer is a generator transformer, andhence, not only is there a need to reduce the probabilityof a circuit interruption resulting from a gap operation toa minimum, but the risk and consequences of damage tothe transformer or generator are unacceptable. The pro-tection of transformers against lightning would normallyrequire the arrester to be installed on the line side of thetransformer. On at least one transformer-feeder circuitsurge arresters are installed on the opposite side of thetransformer to limit: energising overvoltages discussed inSection 5.2.2.

Arresters are used in place of co-ordinating gaps onsome 300 kV shunt reactor installations where frequent

gap operations might otherwise occur as a result of currentchopping.

Surge arresters are being used also at 275 kV and 400 kVto provide protection for generator transformers at locationswhere 'generator circuit breakers' are employed betweenthe generator and transformer, and arresters are needed tolimit overvoltages transferred to the l.v. delta windings(with inaccessible connections), which can be excessivewhen not loaded by the generator.

The number of existing surge-arrester installations on the275 kV and 400 kV systems including all the above casestotals approximately 20. A major factor in limiting thenumber of arresters installed at these voltages is thatdesigns installed at the Board's pollution-testing stationfailed to satisfy the requirements (i.e. no operation atnormal system voltage and no material degradation ofcomponents) for type approval unless greased at pollutedsites. Service experience of the limited sample, some ofwhich were greased, has been satisfactory.

3.4 Impulse-voltage-withstand tests on items of plant

The 50% sparkover values of gaps quoted in Table 2 havebeen established from a large number of tests carried outover the years on the electrode assembly in question.The Board specifies a rated withstand-voltage level forlightning and switching impulses for all plant items. Re-levant tests carried out on completion of manufacture ofplant are listed in Appendix 11.

IEE PROC, Vol. 127, Pt. C, No. 4, JUL Y 1980 249

Table 2: Gap settings used on CEGB transmission systems

Highest voltage of system (kV r.m.s.)Peak value of highest system (phase-to-earth) voltage (kV)

420(343)

300(245)

145(118)

Rated lightning impulse-withstand voltage (kV peak)(i) Transformers

(ii) Switchgear busbars etc.

Rated switching impulse-withstand voltage (kV peak)(i) Transformers

(ii) Switchgear busbars etc.Co-ordinating gap setting (m)Co-ordinating gap setting Uso 1 -2/50 jus positive (kV peak)

Co-ordinating gap setting U50 1 -2/50 us negative (kV peak)

Co-ordinating gap Uso switching surge positive (kV peak)

Co-ordinating gap U50 switching surge negative (kV peak)

Overhead-line gaps (last 1 -6 km) and cable sealing ends (m)Overhead-line gaps Uso 1 -2/50/us positive (kv peak)

Overhead-line gaps U50 1 -2/50 jus negative (kV peak)

Overhead-line gaps Uso switching surge positive (kV peak)

Overhead-line gaps U50 switching surge negative (kV peak)

Overhead-line gaps (mid line) (m)Overhead-line gaps Uso 1-2/50/us positive (kV peak)

Overhead-line gaps Uso 1 -2/50 us negative (kV peak)

1425(4-15)

1050(3-06)1-681030(3-00)-1140(3-32)850(2-48)-1200(3-502-541480(4-31)•1580(4-61)1280°(3-73)-1300(3-79)2-791630(4-75)-1760(5-13)

1050(4-29)

850(3-47)1-22745(3 04)-825(3-37)710(2-90)-825(3-37)1-901100(4-49)1270(5-18)1000*(408)-1050(4-29)2-131240(5 06)-1430(5-84)

550(4-65)650(5-51)

440*(3-73)

0-66410t(347)-505+(4-28)390t(3-31)-470+(3-98)0-99570(4-83)

- 6 0 0(5-08)560*(4-75)- 5 8 0 *(4-92)1-12660(5-59)- 6 9 0(5-85)

* Assumed level. Switching surge withstand level not specifiedt Values quoted for vertical rod-rod gap configuration* Estimated levelD Measured value 40/2000 wave

N.B. Figures in brackets are per unit values based on peak value of highest system voltage above (phase-to-earth)

3.5 Phase-to-phase clearances and insulation levels

Historically, the clearances between phases at 132 kVwere chosen to be in the region of \/3 times the phase-to-earth clearances. Subsequently, difficulties in impulsetesting with an increased lightning impulse level, led insome countries to the adoption of the same b.i.l. andclearances between phase and earth as between phases.The CEGB introduced the concept that phase-to-phaseinsulation should withstand a full lightning wave on onephase with the peak of the nominal system voltage ofopposite polarity applied to the other. Clearances shownin Table 3 are based on this concept but no tests arespecified.

4 Overvoltages not giving rise to difficulties in theoperation of the 275 kV and 400 kV systems

4.1 External overvoltages

From Table 1, the average fault rates over five years due tolightning are approximately 048 and 0-49 faults per100 circuit km each year on the 400 kV and the 275 kVsystems, respectively. While a number of co-ordinatinggap operations occurred on transformers (see Table 4),during the five years surveyed only one transformer failureis reported which could possibly be attributed to damageby lightning. Three cases are reported where lightningcaused permanent damage to 420 kV circuit breakers andone case at 300 kV.

Table 3: Phase-to-earth and phase-to-phase clearances

Highest voltage of system (kV r.m.s.) 420 300 145

Minimum clearance live metal to earth (m)SwitchgearTransformers*

Minimum clearance between live metal of different phases (m)SwitchgearTransformers*

3-002-896

3-603-556

2-102 083

2-402-515

1-201-067

1-401-27t

Metric equivalent of dimensions in existing standard quoted; these clearances will be revised in next issue"•" For 400/132 kV and 275/132 kV auto-transformers, this clearance is increased to 1 -37 m

250 IEEPROC, Vol. 127, Pt. C, No. 4, JULY1980

Table 4: Proportion of co-ordinating gap operations on transformer feeders compared with those at other locations in the period April 1974 —March 1979

Transformerfeeders

Transformerteed onto linecircuit midroute

Transformersteed at circuitends, e.g. at meshsubstation

Other e.h.v.transformerfaults, e.g. atdouble-bussubstations

Highest voltage of system 420 kV 300 kV 420 kV 300 kV 420 kV 300 kV 420 kV 300 kV

Estimated total circuits 31 17 16 26 26 97

Flashovers on both linecircuit and transformerconnections

Flashover on transformerconnections only

Circuits where flashoveroccurred on eitherline circuit ortransformer connectionsor on both

Total probable gapoperations

4

1

10

7

0

0

2

0

3

0

0

3

3

1

0

4

2

2

0

4

7

2

0

9

N.B. The incidents listed above are classified according to operating conditions at the time of the incident. The circuits are categorised under'estimated total circuits' on the basis of the configuration existing during 1976 and assuming all associated plant to be in service

The CEGB system is designed in such a way that trippingof one circuit (line, transformer or generator) does notjeopardise the continuity of supply. The simultaneoustripping of more than one circuit can be more serious.In Table 1 the ratio of double circuit faults to all line-circuit lightning faults is 3 0 and 3-3%, respectively, onthe 400 kV and 275 kV systems. It is to be anticipated thatthe system with the lower insulation level would incur agreater relative number of double-circuit faults. Surveysof earlier periods have indicated greater differences.

An estimate of the number of co-ordinating gap oper-ations on transformers at differing types of location withinthe network is given in Table 4. It is presumed that whererelay operations indicated a flashover on the transformerconnections, this occurred at the co-ordinating gap. In thecase of the majority of transformer feeder circuits involvedin an incident, the zones of protection were such that itwas impossible to establish whether a gap operation hadoccurred in addition to a flashover on the line. It appearsthat during the period of survey the number of gap oper-ations on transformer feeders was of a similar order tothose occurring on transformers teed along the length ofcircuits.

4.2 Internal overvoltages

4.2.1 Interruption of transformer magnetising current: Inthe early post-war days, occasional flashovers occurredon the co-ordinating gaps when de-energising old types of145 kV transformers. Staged tests2 indicated widely differ-ing values depending on the transformer, type of tapchanger and circuit breaker. When interrupting steady-state magnetising current with air-blast breakers, themaximum overvoltage recorded was 2-8 p.u., but in thelarge majority of cases less than 2 p.u. The interruption ofinrush magnetising current, i.e. a make-break operation ofthe air-blast circuit breaker, gave a maximum value of 3-7p.u., but in the majority of cases below 3 p.u. Typically,oil circuit breakers tended to give somewhat lower over-voltages than air-blast types. Occasionally, with the oilcircuit breaker, repeated chopping and re-ignition of the

arc caused asymmetry and substantial increase of themagnetising current such that conditions at final inter-ruption were similar to those existing when interruptingthe inrush current resulting from a make-break operation,with consequent increase in the overvoltages.

The majority of transformers on the 275 kV and 400 kVsystems have cold rolled-grain-oriented core material.Oil circuit breakers are installed only on the 275 kV systemand the relative proportion of these is dwindling. Sitetests on transformers of various ratings show that inter-ruption of the steady-state magnetising current by air-blast breakers is highly unlikely to produce a co-ordinatinggap operation. The highest value recorded during sitetests was 2 p.u., the other 55 operations producing over-voltages less than 1 -6 p.u.

The interruption of inrush current during make-breakswitching operations during site tests has caused co-ordinating gap operations. During one test, 3-2 p.u. wasrecorded on one occasion in about 275 operations, theremainder being less than 2-9 p.u., few exceeding 2-5 p.u.

As magnetising inrush current can be interrupted onlyfollowing a malfunction of equipment or human error,this duty will arise only very seldom and the occasionalco-ordinating gap operation from this cause is regarded asacceptable. No gap flashovers have been reported in servicewith air-blast or oil breakers, supporting the conclusionsfrom the tests that, with the generous insulation levelsused on the Board's system, switching operations involvingthe interruption of magnetising current of unloaded trans-formers do not produce excessive overvoltages.

4.2.2 Interruption of capacitive currents: Repeated re-strikes in circuit breakers interrupting capacitive currentswith an inductive source containing comparatively littlecapacitance can produce excessive overvoltages.3

System tests have confirmed also that the interruptionof line charging current by re-strike-free air-blast breakersdoes not produce noticeable overvoltages. Records obtainedwhen de-energising overhead lines by oil circuit breakersshowed overvoltages of less than 2p.u. on the 275kVsystem.

IEEPROC, Vol. 127, Pt. C, No. 4, JUL Y1980 251

When de-energising a 22-8 km long 145 kV cable with asmall oil volume breaker with cables on the source sidealso, overvoltages well below 2 p.u. were measured. Theseverity of the transient currents due to re-strikes, exacer-bated by the presence of cables immediately adjacent toboth sides of the breaker was, however, sufficient todamage the circuit breaker.

4.2.3 Energising uniform overhead lines or cables: In viewof the problems encountered on some overseas systemswhen energising long lines, extensive analytical, studiesand staged tests were carried out on the CEGB system.1'4>s

During site tests on a 121 km long 300 kV overhead linethe maximum overvoltage slightly exceeded 2 p.u. at thereceiving end in the absence of trapped charge. With atrapped charge ranging up to 045 p.u. simulating highspeed auto-reclosure, the maximum overvoltage recordedwas 2-7 p.u.5

The highest voltages measured during 12 energising shotswithout trapped charge on a 200 km 420 kV feeder was2-25 p.u. Tests on a 250 km long 420 kV line gave amaximum overvoltage of 2-8 p.u. at the receiving end withno trapped charge. With trapped charge in the region of0-5 p.u., the highest recorded overvoltage was 3 p.u. Inthis case, the 'dead' time was in the region of 350 ms, where-as under operational conditions on the Board's system thecorresponding interval is about 15 s with a consequentreduction in the magnitude of trapped charge and henceresulting overvoltage. The analytical studies agreed closelywith the results of the site tests.5 Service experiencesuggests that this type of overvoltage does not represent a.problem on 275 kV and 400 kV systems of the Board andthese investigations showed that even if high speed auto-reclosing was to be used, only occasional flashovers wouldarise with the gap settings employed on overhead lines.

No problems of overvoltages arose on uniform cablecircuits for the 275 kV system with route lengths of up toapproximately 20 km. On the 400 kV system, cablesinvariably form part of composite circuits consistingpartly of overhead lines (see Section 5.2.3).

4.2.4 Power-frequency overvoltages: Voltage rise can occuron a healthy phase as the result of an earth fault on anotherphase. The neutrals of all 300 kV and 420 kV transformersare directly earthed except for those at two substations,and the ratio of the voltage of the healthy phase and thevoltage before the fault, i.e. the earth fault factor, is wellbelow 1*4. Even when on some transformers the deltawinding will be omitted and some neutrals earthed throughlow-impedance reactors, the earth fault factor will be keptbelow 1-4.

Power-frequency voltage rise could be caused also byloss of load on very long lines or where capacitive circuitsare energised from a source with high inductance. Indicated50 Hz voltages on fault recorders of about 550kV r.m.s.have been reported on long 400 kV transformer feederscontaining lengths of cable when still energised from the66 kV system for the period (120 ms) between the clear-ance of the 420 kV breaker and the opening of the low-voltage oil circuit breaker. An overvoltage of this durationand type is termed a temporary overvoltage and as such isnot excessive; however, it could become sustained in theevent of a switching error or relay failure and would thenbe hazardous.

In this case no damage resulted. It is to be anticipatedthat the saturation of transformers would limit fundamental-frequency voltage rise to fairly insignificant values in mostcases. The plant and surge arresters are designed to with-stand these conditions and no problems have arisen on thisaccount. Transformer saturation can lead to temporaryovervoltages due to harmonic resonance as indicated inSection 4.2.5.

4.2.5 Temporary overvoltages due to transformer saturationand harmonic resonance: Temporary overvoltages mayresult from harmonics contained in the transformer magnet-ising inrush-current resonating with components of thesupply network. The resulting harmonic voltages super-imposed on the power-frequency voltage may cause sig-nificant overvoltages.6'7 These conditions can arise whenswitching in a transformer or when a close-up fault isdisconnected from the supply system. Loss of load andthe resulting voltage rise also can produce saturation.Severe harmonic resonance is usually confined to systemswith substantial capacitance, i.e. with long lines, cablesand particularly power-factor correction or filter capacitors.

Tests energising convertor transformers,6 with filters con-nected, produced temporary overvoltages of approximately1-5 p.u.

A somewhat similar phenomenon was recorded whenthe load was dropped from a generator transformer con-nected to a 35-4km, 420kV overhead line. On thisoccasion a temporary overvoltage of approximately 1-4p.u. was recorded when a load of 270 MW with a powerfactor of 0-85 was dropped. Although no problems haveso far arisen on the Board's system due to these temporaryovervoltages, they deserve attention particularly wheresurge arresters are installed to protect generator trans-formers, as they may be called upon to reseal under suchconditions.

5 Overvoltages which have caused some problems onthe CEGB transmission systems

5.1 External overvoltages

In one case (included in Table 1) a lightning fault occurredon the overhead line connected to a section of busbarwith the other circuits shown in Fig. 1. The line circuittripped due to an earth fault, but additionally two generatorcircuits tripped due to the apparently simultaneous oper-ation of co-ordinating gaps on the same phase of bothtransformers. Although the configuration of this networkfor fast-fronted surges is similar to that of an overhead-linecircuit with transformers teed at a point along its route, theprecise mechanism is open to speculation. At least twoincidents have occurred where two adjacent transformersteed to the same 420 kV line circuit have tripped simul-taneously during thunderstorms, thus resulting in theinterruption of the 400 kV supply (from both ends ofthe line circuit) to the substations in question.

Three cases have been reported (in Table 1) where it isbelieved that fault current was re-established in 'opening'circuit breakers which were damaged as a result, as anoverhead line tripped following a presumed lightningflash and earth fault on the line. The precise sequenceof events cannot be determined due to the absence ofany oscillographic records of suitable resolution. Accord-ing to a CIGRE survey, at least 10% of all flashes contain

252 IEEPROC, Vol. 127, Pt. C, No. 4, JULY 1980

more than four strokes and have an overall duration ofseveral hundred milliseconds.8 This information supportsthe view that it is possible for a line circuit to trip as theresult of a first stroke, and provided the earth-fault arcon the line is extinguished, the surge from a further strokecould arrive at the breaker terminals during or after theopening process.

From Table 4 it is worth noting that 7 out of 31 in-cidents involving the operation of a co-ordinating gapoccurred on transformers teed onto line circuits midroute.On the assumption that the co-ordinating gap operatesas the surge travels along the line past the T point, theseverity of the condition would be increased due to reflec-tion had the line been terminated at a transformer alone.It is interesting that a similar order of co-ordinating gapoperations appears to have occurred on transfomer ter-minated circuits (i.e. transformer feeders). While the sampleis small this suggests that had the teed circuits been trans-former feeders the winding insulation would have beenexposed to more onerous conditions. This supports theneed for improved surge protection on transformer feedercircuits. In fact the only transformer to fail, possiblyowing to lightning, and during the period of survey, formedpart of a transformer feeder circuit.

Incidents of the types described above have occurredso infrequently that they do not justify modification tothe present practices relating to overvoltage protection.This will be reviewed as further occurrences of such in-cidents arise.

5.2 In ternal o vervoltages

5.2.1 Shunt reactor and generator transformer switching:Numerous co-ordinating gap operations occurred in the pastwhen switching out 300 kV, lOOMVAr reactors beforeremedial steps were taken (four cases are reported in Table1). These gap flashovers occur immediately followingdisconnection of the plant from the system. Occasionallythe virtually instantaneous collapse of voltage on thereactor side of the breaker may cause the opening breakerto re-ignite, thus connecting an earth fault to the system,

earth fault on y - phoseacross 1 9 m gap opprox7km from generatingstation

double-circuit line

open

275 kV busbar

y - phase

Fig. 1 300kV line and generating-station busbar configuration whena line circuit tripped (attributed to lightning) and apparently simu-ltaneous operation occurred of co-ordinating gaps (l-22m) ongenerator transformers

typically until the next current zero, resulting in a systemvoltage disturbance. These gap operations have an oper-ational nuisance value due to the need for inspection ofplant before re-energisation but no cases of damage toplant have been reported. The overvoltage produced inpractical situations frequently approaches the switching-surge-withstand level of the reactor and there is usuallyalso a measure of uncertainty in assessing, on the basis ofworks tests, the maximum possible current which a givenbreaker can possibly chop when installed at site. Con-sequently, uncertainty frequently exists as to when surgearresters or other special measures are required. Wheresurge arresters have been used to protect reactors whichcan be banked with transformers protected by conventionalco-ordinating gaps, occasional co-ordinating gap operationshave arisen (two of the above cases) when switching out thebanked reactor and transformer.

numberof

test shots

numberof

test shots

2 2-1 22 23 24 2526 p .u . aovervoltage

numberof

test shots

2 2 23 24 25 26 27 28 29 P•"• ,

overvoltage

numberof

test shots

10-9-8765-4-3-2-1-

24 25 26 27 28 29 3 31 p.u.overvoltage

Fig. 2 Histogram of overvoltages obtained when switching a 300 kV100 MVAr shunt reactor by an air-blast breaker with resistors of27ka, 40 kQ. and 80k£l, respectivelya 16 shots R = 27kJ2b 14 shots # = 40kfte 6 shots R = 80 kftd 31 shots # = 80kJ2

2000 pF additional capacitance across one phase of reactor

IEE PROC, Vol. 127, Pt. C, No. 4, JUL Y1980 253

The histograms of Fig. 2a, b, c and d show the distri-bution of overvoltages recorded on a 300 kV 100 MVA star-connected shunt reactor due to current chopping by anair-blast breaker incorporating opening resistors of 27 kCl,40 kfi and 80k£2, respectively. The equivalent capacitanceof the shunt reactor and its connections to the circuitbreaker was approximately 4400 pF. The histogram ofFig. 2d shows the effect of increasing the capacitanceacross one phase of the reactor by 2000 pF. These histo-grams show that the unmodified circuit breaker (80kS7per phase) produces overvoltages well within the rangeof the sparkover voltage of the co-ordinating gap whenswitching the shunt reactor. As the shunt reactor is requiredto be switched daily, the resulting frequency of gap flash-overs is not acceptable. It should be noted that if thecircuit capacitance had been larger, the overvoltages wouldhave been further increased.

In some cases the shunt reactors are connected to the13 kV tertiary of transformers. Tests have been carried outon a 300/145 kV 240 MVA autotransformer with one and*two parallel 30 MVA shunt reactors connected to thetertiary delta winding. The switching was carried out bya 300 kV air blast circuit breaker. The number of testshots had to be limited as three out of six resulted ingap flashovers when one 30 MVA reactor was connected.The maximum voltage reached at the instant of gap break-down was 2-86 p.u. The overvoltages were somewhatlower (2-45 p.u.) with both shunt reactors connected.

The interruption of the first and second phase stillleaves a power-frequency voltage on the disconnectedphases due to the interphase coupling through the deltawinding. Any current-chop induced transient is super-imposed on these voltages. Transients produced on anyof the phases (e.g. due to re-ignition of the circuit breaker)will affect the other phases and can increase the current-chop induced voltage.

During contractual load rejection tests on a generatorit was found that, when a load of about 160 MW wasinterrupted on the 400 kV side of its associated 350 MVAtransformer, the co-ordinating gap flashed over due to anovervoltage of approximately 850 kV (2-5 p.u.). Thegenerator transformer combination represents an induct-ance comparable with that of a shunt reactor, the 50 Hzsource being on the same side of the circuit breaker asthe inductance. However, in this case the load currentis chopped and the oscillatory transient, depending on

closing open

A

-x- 1I

the circuit parameters (inductance, capacitance, losses,etc.), is superimposed on the power-frequency voltagesupplied by the generator. The installation of surge arresterswill eliminate such flashovers.

5.2.2 Energising overvoltages on transformer feeders: Manyof the flashovers reported during the last 15 years on theCEGB systems and attributed to switching surges occurredwhen energising transformer feeder circuits via the 'line',i.e. when closing breaker A with breaker B open (Fig. 3)(three cases in Table 1, see Section 6 also). Switchingrestrictions exist on at least 20 such circuits requiringenergisation by closing B before A, a sequence whichreduces any resulting energising overvoltages, sometimestermed 'best-end' switching.

On closing A, a voltage step is impressed on the line,the magnitude depending on the voltage across the breakerat the instant of closure and on the surge impedancepresented by the source in relation to that on the 'line'side of A. This step voltage will be nearly doubled onreflection owing to the much higher surge impedance ofthe transformer at C. Further overswing occurs on transferof the surge to D. The magnitude of the first major voltagepeak at D in relation to that at A will depend on theduration of the 'pulse' arriving at C, the natural frequencyof the transformer and the equivalent capacitance at D,and the energy required to charge this capacitance inrelation to that available at C. Subsequent voltage swingswill exceed the initial peak, and hence the amplificationwill be highest when the natural frequency of the com-bination of the transformer and equivalent capacitanceat D is close to the natural frequency of the line. Theseaspects are discussed in more detail elsewhere.9'10

An indication of the wide range of overvoltages encoun-tered on successive closing shots depending mainly on theinstant of closure of the individual phases from A, whenenergising a transformer feeder, is provided by the measuredvalues shown in Fig. 4. Comparison of these values (whichare more or less typical of many circuits), with the charac-teristics of conventional co-ordinating gaps, shows thatthe probability of a co-ordinating gap operation for over-voltages of this order even if of positive polarity is stilllow. Higher overvoltages can arise only if the applied stepis increased.

The initial voltage step may be increased if a residualvoltage of opposite polarity to the applied power-frequencyvoltage exists at the time of energising. This may occureither when a second or third phase is energised at a timewhen a voltage already exists as the result of couplingthrough the transformer from another phase previously

number ofshotsi >

Fig. 3 Transformer-feeder circuit

a Single line diagram of a transformer feederb Simplified equivalent circuit

1 1-2 U 1-6 1-8 2 22 2-4 2-6 28 3 32overvoltagep.u.

Fig. 4 Histogram of overvoltages recorded at the 275 kV terminalsof a 400/2 75 kV transformer feeder

13 test shots

254 IEE PROC, Vol. 12 7, Pt. C, No. 4, JUL Y 1980

energised, or the circuit is switched in at a time whenresidual voltages exist due to sustained resonant excitation.Tests on the CEGB system under normal operating con-ditions have confirmed that both effects can occur. Fieldmeasurements indicate that the residual voltage arisingfrom an earlier phase to close may have a peak valueexceeding, 1 p.u., although the likelihood of this valueexisting at the instant when the system voltage on therelevant phase is near its maximum value and of oppositepolarity requires the coincidence of several factors, anapparently remote contingency. Under these conditions inFig. 3 the maximum voltage step would be applied to thecircuit at A and the final overvoltage at D will be equal tothe total voltage excursion produced by the inherentamplification of this step within the circuit, less the residualvoltage at.D.

Resonant excitation can arise on a transformer feederwhen disconnected from the system where it comprisesa section of double-circuit construction and the parallelcircuit remains energised.9'10'11 On disconnecting thetransformer-feeder circuit from the system, any energytrapped in the line C2 (Fig. 5) will be discharged throughthe transformer winding usually in the form of a dampedoscillation. If the transformer saturates during this oscil-lation, the voltage waveform approaches a square wave.This oscillation can become a sustained 'ferro-resonant'condition if the losses incurred in each cycle are suppliedfrom an external source, in this case from the adjacent'live' circuit through capacitance Cx, the transformerbeing switched from an unsaturated to a partially saturatedstate every half cycle. The frequencies of excitation mostcommonly encountered are 16| Hz and 50 Hz, but it isbelieved 10 Hz has been encountered also. As the mech-anism does not require a unique value of inductance andcapacitance for ferro-resonance to occur at a particularfrequency, transformer-feeder circuits with apparentlywidely differing parameters can support ferro-resonanceof the same frequency. It is believed that all transformerfeeders in excess of 5—10 km length can support thiscondition irrespective of the length of the feeder andMVA of the transformer etc., although the probabilityof the condition becoming sustained cannot be predicted.

Usually during 50 Hz ferro-resonance, the three-phasevoltages are approximately 1 p.u., —\ p.u. —\ p.u., res-pectively. At 161 Hz, the corresponding values are in theregion of j p.u., — \ p.u., — g p.u., and similarly at 10 Hzit is to be expected that the peak voltage on the predomin-ant phase would be \ p.u. In practice, the voltages varydepending on the predominant phase and waveform.

Linear (as opposed to ferro-) resonance12 has occurredon two circuits when de-energised with their associated60 MVAr tertiary reactors remaining connected to the

X-'live' circuit

'live'circuit:'dead'circuit

Fig. 5 Transformer feeder coupling with adjacent circuit

a Simplified single-phase representation of a transformer feederb Simplified equivalent circuit of a

transformer-line combination. This condition dependson the virtually constant reactor inductance and linecapacitance resonating at 50 Hz. The essential differenceto ferro-resonance is that it arises on every occasion thereactor circuit combination is switched out with theparallel line circuit alive. Detuning the circuit quenches thecondition.

These residual voltages, if of opposite polarity to thesystem voltage at the instant of closure of the breaker,can increase the voltage step applied at A (Fig. 3) in thesame manner as would residual voltages resulting frominterphase coupling from the earlier phase to close. Wherea circuit is in ferro-resonance, the initial voltage stepimpressed on a later phase to close will be influenced bya combination of the decaying ferro-resonant voltage andinterphase coupling. There is insufficient informationavailable at present for a realistic quantitative assessment tobe made of these effects.

For a co-ordinating gap operation to arise on energis-ation of a transformer feeder, a number of factors musttherefore coincide, i.e. the instant of closure of the firstand possibly that of later phases in relation to the first.It may also be necessary for the circuit to be in a state offerro-resonance, an event which may typically only arisefollowing one in about 20 occasions on which the circuit isde-energised. On some circuits there have even been flash-overs across 300 kV cable sealing ends rather than theassociated co-ordinating gap, believed to be caused bynegative polarity energising overvoltages.

5.2.3 Overvoltages on other composite circuits: Whenoverhead lines or cables are terminated at one end by aseries reactor the equivalent circuit is identical to thatof the transformer feeder and, if energised from the 'line'end, similar overvoltages arise as on transformer feeders.Flashovers have occurred on the co-ordinating gap of a145 kV series reactor and investigations by Heaton andReid gave results very similar to those found on trans-former feeders.13

Flashovers have been encountered on two other typesof composite circuit: the first of these concern a cable/overhead-line/cable combination (see Fig. 6). A flashoveroccurred at C when energising from the pressurised headcircuit breaker at A, the length of the cross-bonded cableAB being greatly in excess of cable CD. Such a circuit com-prises the same elements as a transformer feeder, i.e. adistributed transmission line (cable AB) of comparativelylow surge impedance terminated in a series-tuned circuitcomprising the overhead line terminated in a short lengthof cable. The highest voltage measured at C during a fieldinvestigation with a limited number of tests was 3 16 p.u.;the records revealed that this voltage occurred on thesecond phase to close, and that significant coupling (up to1 p.u.) due to the cross-bonding could occur on a stillunenergised phase as the result of the earlier energisation

275kV 275kV

crossbondedcoble B overhead line C cable D

closing 10 km 285km 0-39 km open

Fig. 6 Composite circuit configuration producing high overvoltagesat C and D when energised from A

IEEPROC, Vol. 127, Pt. C, No. 4, JULY 1980 255

of another phase. The highest voltage occurred on thethird peak of the energising transient.

Some problems have arisen at two locations (one re-ported in Table 1) where restrikes in oil circuit breakerswhen switching 300 kV cables have created somewhatsimilar problems as energising a transformer feeder. Ineach case a flashover occurred on the 132 kV side of a275/132 kV transformer when the 145 kV breaker wasopen but with the 300 kV breaker closed. Tests showedthat voltage transients which were not excessive on the275 kV system gave rise to a 'highest measured voltage'of3-5p.u.at 132 kV.

6 Phase-to-phase switching overvoltages

There is now great interest overseas in phase-to-phaseswitching overvoltages as they become a greater problemon uJi.v. systems where lower insulation levels in relationto the operating voltage are used. The number of casesknown where flashovers have actually occurred betweenphases on the CEGB system is about four. Three of these(two reported in Table 1) occurred when energising 275/132 kV transformer feeder circuits from the 275 kV system.The flashovers occurred between 145 kV bushings (mini-mum clearance approximately l-27m). Clearances havebeen increased on transformers of more recent manufacture.The only other confirmed phase-to-phase fault resultedfrom switching out shunt reactors where overvoltageprotection was provided between phase and earth bysurge arresters.

0.5-0.4-0.3-0.2-o. i -

0.1-0.2-0.3-0.4-0.5"

1 2 • ••

a

1 i P"U phase-to-phase

U phase-to-earth2 3

max . componentb

p.u.

Fig. 7 Phase-to-phase energising overvoltages on a 400kV/275kVtransformer feeder at the 300 k V connections.

(Comprising 28 km of 420 kV line, 1000 MVA transformer, 25 nF300 kV cable)a a = smaller of the two phase-to-earth components of maximum

phase-to-phase voltage, expressed as ratio of phase-to-phasevoltage

Maximum possible value of a = + 0-5b M = ratio of maximum phase-to-phase voltage occurring during

any one 'transient' to highest phase-to-earth voltage occur-ring during any one 'transient' to highest phase-to-earthvoltage occurring during same disturbance

Maximum possible value of M= 1(After McFarlane and Wright)

256

Recent IEC proposals14 recommend increased phase-to-phase clearances determined from switching impulsephase-to-earth and phase-to-phase levels based on extensivestudies15 comparing phase-to-earth overvoltages withphase-to-phase overvoltages and withstand levels of typicalconfigurations. These studies suggest also that excessivestress between phases can result in a phase-to-earth flash-over.

Some results from site tests carried out specifically tomeasure phase-to-phase voltages occurring at the secondarywinding when energising a 400kV/275kV transformerfeeder of the CEGB are shown in Fig. 7. Derivation ofphase-to-phase voltages from oscillograms of phase-to-earthvoltages obtained during the tests indicated that voltagesof 4-12 p.u. (phase-to-earth components 2-34 p.u. and— 1 -78 p.u.) arose between phases when energising a 420 kVline with trapped charge in the region of 0-4 p.u. on bothphases. Energising a 420 kV line via a 500 MVA transformerproduced phase-to-phase values of 2-89 p.u. (phase-to-earthcomponents of 1 -23 p.u. and — 1 -65 p.u.).

7 Appraisal of overvoltage problems and remedialmeasures

The insulation levels adopted for the 400 kV and 275 kVsystems in relation to the system highest voltage are thehighest combination permitted under IEC Publication 71.16

This is necessary to accommodate the dispersion exhibitedby the gaps used for overvoltage protection. More extensiveuse of surge arresters would permit lower insulation levelsto be adopted for substation plant, and this would alsoeliminate those flashovers due to switching surges andthe rare simultaneous operation of two co-ordinating gapsin the same substation. The large number of incidents inTable 1 attributed primarily to weather or pollution wouldsuggest that little improvement would result in the overallnumber of incidents, even if all co-ordinating gap oper-ations were avoided by the use of surge arresters.

Most of the overvoltage problems discussed in Section 5arise from the operational difficulties and nuisance resultingfrom co-ordinating gap operations. With the conventionalswitchgear now in use, only in some reactor installations, orin the case of transformer feeder and other composite

T, /T;, =1;2/5Ogs Tcr/T2 = 45/2300 ps

1600-

1400-U

kV 1200-

1000-

800

T c r /L= 500/3000 ps

POSITIVE

10 100

-40

-35

-30

2-51000 JJS

p.u.

kV

1600

1400-U

1200

1000-

800.

\

\X

•

\ NEGATIVE

b " * "

10 100

40

35

-30

25lOOOus

p.u.

Fig. 8 420 kV gap: relation between mean time to chopping Tc

a = prospective peak voltageb = maximum voltage across gap

(After Hughes and Lightfoot)

IEE PROC, Vol. 127, Pt. C, No. 4, JUL Y1980

circuits in the event of the most onerous chance combi-nation of circumstances, do these internal overvoltagesapproach the withstand level of the plant.

The flashover voltage of conventional co-ordinating gapsexhibits a large dispersion depending on polarity andwaveshape. For negative-polarity surges these gaps hardlyprovide adequate protection, for positive-polarity-switchingsurges they can break down at an unnecessarily low voltage.The protective characteristics can be improved by modific-ation of the electrode configuration (see screened co-ordinating gap.)17 Laboratory tests show a significantimprovement in respect of polarity dependence of thesegaps and they are now being adopted on the Board'ssystem. This gap provides significantly improved protectioncompared with conventional gaps but will not permit areduction in insulation levels. Fig. 8 shows that as with anyairgap these gaps appear to give little protection againststeep-fronted surges of high amplitude. In addition, thesparkover voltage of these gaps rises with fronts exceedinga few hundred microseconds and hence the protectionefficiency diminishes for such surges. Although surgearresters provide much more effective protection againstovervoltages, to date they have only been accepted withreluctance, owing mainly to a number of failures of earlierdesigns. Hence they have found application only whereother solutions are not acceptable as the number of casesof damage to plant due to overvoltages is insignificant.In some cases limitations may be imposed by lack ofspace at existing stations.

In the case of four shunt reactors switched by air-blastbreakers, switching overvoltages have been reduced bychanging the resistors across the interrupters from 8Okf2to 27 k£2 per phase and now co-ordinating gap operationsseldom occur. Surge arresters have been installed at othersimilar installations where co-ordinating gap operationswould occur on an unacceptable proportion of de-energis-ing operations, or where it is considered possible that over-voltages may approach the insulation withstand level of thereactor. To date no failures in service of arresters of moderndesign have been reported.

Several methods have been used during recent years toreduce the probability of co-ordinating gap operationswhen energising transformer feeders, i.e. transformer ter-minated lines. These include 'best-end' switching (seeSection 5.2.2) and preventing reclosure when the circuitis in ferro-resonance. The normal practice adopted forquenching ferro-resonance is to open the isolator betweenthe transformer and line circuit (at C in Fig. 3). Relaysare being installed on most transformer feeder circuitsto warn of the need to quench ferro-resonant conditions,and in the majority of cases to lock out the delayed auto-reclose should sustained ferro-resonance arise betweende-energising and re-energisation. The circuit can thenbe energised from the remote breaker A with little riskof a gap operation. The risk is further reduced but noteliminated by the use of screened rod gaps at D in placeof conventional co-ordinating gaps. On some 12 circuitsof particular operational importance, co-ordinating gapsat D are being replaced by arresters, thus permitting energis-ation of the circuit irrespective of whether or not it is inferro-resonance. Also, screened co-ordinating gaps are beinginstalled at C on these circuits to provide some limitedprotection in the event of energisation from the trans-former end with residual voltages present although unfor-tunately these surges will have long fronts. Only service

experience will show whether surge arresters can be just-ified at the location on the line side of the transformerwhere they will not only protect against energising over-voltages, regardless of time to peak, but lightning surgesalso.

Where switch disconnectors exist between line andtransformer the preferred method is to first energise theline alone and then the transformer. During autoreclosesequences the 'interrupters' of the switch disconnectorare opened to quench ferro-resonance if present and theline is re-energised, after which the interrupters are closed.

To prevent linear resonance on reactor loaded trans-former feeder circuits, when disconnected from the systemthe reactor is disconnected from its transformer at the sametime that the line is tripped. Linear and ferro-resonanceconditions are not dependent on the point along the lengthof the associated line at which the transformer is locatedand hence are not confined to transformer terminated lines.

In the case of cable-line-cable circuits, 'best-end' switch-ing has been adopted, requiring the circuit to be energisedfrom the end with the shorter length of cable, i.e. D inFig. 6.

Reference was made in Section 5.2.3 to two sites where145 kV co-ordinating gap operations had occurred whenswitching a 300 kV cable. As an interim measure theswitching sequence has been changed, some reduction inthe final overvoltage being possible also by suitable choiceof tapping position. It is intended to install surge arresterson the 132 kV side of this transformer.

Problems arising from near coincidence of naturalfrequencies of a combination of items of plant are inevit-able on a continuously evolving complex network and arebest dealt with on an ad hoc basis.

For historical reasons the switching-impulse test levelspecified for 300 kV cables is less (see Appendix 11) thanfor other items of plant. No cable failures attributable toswitching have been reported to date, but an increase inthis level to 850 kV has recently been agreed.

Special problems arise in the case of SF6 -insulatedsubstations where the connections between the switchgearand transformer are usually totally enclosed, thus pre-cluding the use of available forms of co-ordinating gap toprotect the transformer. It is the Board's practice to pro-vide some form of overvoltage protective device acrossthe primary and secondary windings of all transmissiontransformers and across the e.h.v. windings of generatortransformers. In these situations SF6-encapsulated arrestersare the only practicable devices available. In the absenceof conclusive evidence that, regardless of the circuit con-figuration and condition to be switched, the switchingovervoltages will always be less than the stated withstandlevel of the transformer and recommended margin, SF6

arresters are installed between the circuit breaker and thetransformer. A triggered SF6 spark gap proposed else-where18 would be an alternative for this application if ofsubstantially lower cost than SF6 arresters, especially inapplications where it would seldom be required to operate.

8 Conclusions and recommendations

8.1 Insulation levels adopted

With the present philosophy of using either conventionalor screened co-ordinating gaps as the main means of over-voltage protection, the insulation levels and the gap settingsadopted are of the right order. Even a relatively small

IEEPROC, Vol. 127, Pt. C, No. 4, JULY 1980 257

reduction of the gap settings would result in a large increasein the number of gap operations on energising compositecircuits and transformer feeders. Flashovers on a largenumber of occasions when attempting to re-energise wouldbe operationally unacceptable.

As the overvoltage protection by these gaps is far frombeing perfect, a general reduction of insulation levelswould not be acceptable without changing the policy ofovervoltage protection. Even with the rated lightning-impulse withstand level and the rated switching-impulsewithstand levels now adopted, full operational flexibilityof the system and in some cases adequate protectionof the insulation of plant can be obtained only with theaid of arresters.

Though switching-impulse insulation levels were notspecified at the time of purchase of much existing plant,it is believed that most plant is capable of withstanding theimpulse tests now specified although no test is carriedout under artificial rain in the majority of cases. No dif-ficulties are expected in this respect as the creepage dis-tances used by the CEGB are more generous than oftenused abroad.

The use of arresters would permit the insulation level ofassociated plant to be lowered. However, the advantageswould be limited as:

(a) the general performance of the system is largelydetermined by existing installations and would not beaffected materially by new philosophies at a few instal-lations

(b) the plant would not be interchangeable withouttransferring the associated overvoltage protective equip-ment and this might not always be practicable

(c) material savings in insulation level would be possibleonly where significant temporary overvoltages do notarise. It would seem that greater savings might be possibleonce confidence is acquired in the performance claimedfor the most recent designs of arrestersThere appears to be some contradiction between serviceexperience on the Board's system relating to phase-to-phase insulation and clearances advocated in recentdocuments of the IEC. The stresses actually arising on thesystem between phases and the associated withstand levelsexisting require further study to ensure that no unnecessaryincrease is adopted for these clearances with the inevitableextra costs, unless justified on sound technical grounds.The Board intends to introduce phase-to-phase impulsetesting in the foreseeable future when the basis of suitabletests is agreed within IEC.

8.2 Applica tion and performance o f surge arresters

The use of surge arresters for protecting shunt reactorsand some generator transformers, especially where theyform part of a transformer feeder, will continue to beimplemented.

The use of surge arresters on the opposite side of thetransformer to the line, on transformer feeder circuits,permits the removal of operational restrictions. The per-formance of those circuits where these arresters are in-stalled must be closely monitored;

(i) to ensure that full advantage of the arresters isobtained and that system disturbances from in-rushcurrents and the response of the autoreclose control schemeto ferro-resonance do not prevent re-energisation of thecircuit in ferro-resonance

(ii) to establish whether service experience indicates

that arresters are needed also on the line side of the trans-former to limit energising overvoltages which occasionallymay prove excessive during ferro-resonant conditions. Theapparent potential severity of lightning-surge conditionson transformer feeders and limitations of gaps are suchthat the actual failure rate and consequences of any failurewill be kept under continuing review.Where banked transformers are teed on to a circuit at a'single-switch' substation and the tripping of both trans-formers owing to lightning cannot be tolerated, surgearresters have to be used in place of co-ordinating gapson the line side of these transformers. This policy is re-quired also at those generating stations where the samecircuit configuration arises under unusual operatingconditions.

Where plant protected by conventional arresters canbe connected in parallel with plant protected by con-ventional co-ordinating gaps, the occasional gap operationcannot be avoided during switching when the two itemsare banked due to the overlap in the dispersion of sparkovervoltage exhibited by the two devices. Surge arresters willbe required increasingly on the system, and hence thereis an urgent need for the full type approval of conventionaland new designs of surge arrester for application on the275 and 400 kV systems. In particular their pollutionperformance should be such that no greasing is required.Where surge arresters are to be installed, reliable predictionof the amplitude of temporary overvoltages and dischargeenergy duties involved on circuits with long cables willbe necessary.

8.3 SF6 installations

Service experience of existing installations is too short fora meaningful assessment of performance and a furtherreview will be carried out when site test results and moreservice experience are available.

8.3.1 SF6 circuit-breaker performance: As the circuitbreakers used by the Board do not incorporate closingresistors, energisation overvoltages are considered to besimilar to those produced by air-blast breakers. Withregard to current chopping, no measurements have beenmade at site but as there are no 'opening resistors' thereis some concern that the overvoltage can exceed those ofsome air-blast breakers. There is an urgent requirementto clarify the actual voltages produced when in situ onthe network as overvoltage limiting devices cannot readilybe added to existing SF6-insulated substations betweenthe breaker and transformer.

8.3.2 Insulation levels: Metalclad equipment at present inservice was ordered with similar insulation levels specifiedas for conventional substations but exhibits higher with-stand strengths. Wider use of suitable surge arresters mightpermit a reduction in insulation levels and the economicsof this will be investigated. This is required to establish,with the time to breakdown of particular SF6 in-sulation structures, whether surge arresters are essentialat the line entry as is practice abroad, and whether inaddition (depending on Section 8.3.1) switching over-voltages dictate that surge arresters are required alsobetween the breaker and transformer.

8.3.3 Protective devices: Where the transformer terminalsare accessible, co-ordinating gaps or conventional 'open-

258 IEEPROC, Vol. 127, Pt. C,No. 4, JULY 1980

type' surge arresters if needed can be used as for a con-ventional substation. If the connections are not accessible,metalclad surge arresters are used, though triggered SF6

gaps may be used if and when available. If the triggeredgap is developed, a prototype will be installed on thesystem for economic and performance evaluation.

9 Acknowledgments

This paper is published by permission of the CentralElectricity Generating Board and Preece, Cardew & Rider.The authors gratefully acknowledge the help received froma very large number of colleagues in CERL, various depart-ments and Regions of the CEGB and also in other organis-ations, in particular the Electrical Research Association andin the British Manufacturing Industry. The authors in theseorganisations of the many unpublished and internal reportscontaining the field test information quoted in this paperare particularly acknowledged.

10 References

1 DWEK, M.G., HALL, J.E., JACKSON, R.L., and JONES, B.:'Field tests and analysis to determine switching transients onthe British system.' CIGRE Conference Report 13-03, 1972

2 YOUNG, A.F.B.: 'Some researches on current chopping in high-voltage circuit-breakers', Proc. IEE., 1953, 100, Pt. II, pp. 337 —353

3 CSUROS, L.: 'Transient overvoltage protection' in 'Power systemprotection-Vol. 3' (Macdonald, London, 1969), pp. 1-38

4 BATTISSON, M.J., DAY, S.J., PARTON, K.C., MULLINEUX,N., and REED, J.R.: 'Calculation of switching phenomena inpower systems', Proc. IEE, 1967, 114, (4), pp. 478-486

5 BATTISSON, M.J., BICKFORD, J.P., CORCORAN, J.C.W.,JACKSON, R.L., SCOTT, M., and WARD, R.J.S.: 'Britishinvestigations on the switching of long EHV transmission lines'.

CIGRE Conference Report 13-02, 19706 CSUROS, L., EKSTROM, A., and POVH, D.: 'Some aspects of

HVDC insulation co-ordination', ibid., 33-10,19787 POVH, D., and SCHULTZ, W.: 'Analysis of overvoltages by

transformer magnetising inrush current'. Presented at the IEEESummer Meeting 1977, Mexico City, Paper F77 717-2

8 ANDERSON, R.B., and ERIKSSON, A.J.: 'Lightning parametersfor engineering application', CIGRE Conference Report, 33—06,1980

9 CSUROS, L., FOREMAN, K.F., and GLAVITSCH, H.: 'Energis-ing overvoltages on transformer feeders', Electra, 1971, 18, pp.83-105

10 CSUROS, L., and FOREMAN, K.F.: 'Energising overvoltageson transformer feeder circuits', Electr. Times, 1972, 162, 9,pp. 37-40

11 DOLAN, E.J., GILLES, D.A., and KIMBARK, E.W.: 'Ferro-resonance in a transformer switched with an EHV line', IEEETrans., 1972, PAS-91, 3, pp. 1273-1280

12 PICKETT, U.J., MANNING, H.L., and Van GEEM, H.N.: 'Nearresonant coupling on EHV circuits. Pt. 1 Field investigations',ibid., 1968, PAS-87, pp. 322-325

13 HEATON, A.G., and REID, I.A.: Transient overvoltages andpower-line terminations', Proc. IEE, 1966, 113, 3, pp. 461-470

14 IEC TC 28: 'Insulation co-ordination, Part 3. Principles andrules for phase-to-phase insulation co-ordination'. Draft IEC28(CO)-51,1979

15 SCHNEIDER, K.H., CIGRE Working Groups 33-02, 33-03,33-06, Task Force 33-0303: 'Phase-to-phase insulation co-ordination', Electra, 1979, 64, pp. 137-236

16 'Insulation co-ordination1. IEC Publication 71-1 , 197617 HUGHES, P.C., and LIGHTFOOT, H.A.: 'Shielded co-ordinating

gaps for overvoltage protection of CEGB 132 kV, 275 kV and400 kV transformers' in 'Development in design and perform-ance of EHV switching equipment'. IEE Conf. Publ. 182, 1979,pp. 92-96

18 HAMPTON, B.F., JINMAN, W.J.T., MEATS, R.J., FELLER-MAN, J.J., and FOREMAN, K.F.: 'A triggered co-ordinatinggap for metalclad substations'. CIGRE Conference Report33-16,1978

11 Appendix

Voltage withstand and other relevant tests on plant

Waveshape kV (peak) except where indicated

Highest voltage of system (kV r.m.s.) 420 300 145

1 Transformer and reactor bushings routine tests

1.1 Two full-wave applications—negative1.2 Power-frequency withstand (60s) — dry

(kV r.m.s.)Type tests (including relevant routine tests)

1.3 Five full-wave applications — negative(one external flashover permitted providedfurther five withstood)

1.4 Power-frequency withstand (60s) — dry(kV r.m.s.)

1.5 Power-frequency withstand (60s) — wet(kV r.m.s.)

1.6 Five full-wave applications*(one external flashover permittedprovided further five withstood)

1.7 Repeat (1.6) above with oppositepolarity*

2 Transformers and reactors'1"routine tests*

2.1 Minimum induced overvoltage withstandto earth (kV r.m.s.)

IEE PROC, Vol. 127, Pt. C, No. 4, JULY 1980

1 -2/50

sine

1 -2/50

sine

175/2500

175/2500

1640

680

1640

680

6301050

1210

510

1210

510

460850

650

275

650

275

275

same levels as for test 1.6

sine 630 460 230

259

2.2 One reduced full-wave application (for ref.)2.3 Two full-wave applications2.4 Repeat (2.2) above (for ref.)

Type tests (including relevant routine tests)(Routine test for generator transformers)

2.5 Minimum induced overvoltage withstandto earth (kV r.m.s.)

2.6 One reduced full-wave application* (for ref.)

2.7 Two/three full-wave applications*2.8 One reduced full-wave application (for ref.)2.9 One full-wave application2.10 Two chopped-wave applications2.11 Two full-wave applications2.12 Repeat (2.8) above (for ref.)

1-2/501-2/501 -2/50

sinei switching

surge see1 notes

1 -2/501 -2/501 -2/501 -2/501 -2/50

50%-75% of level for test 2.31425 1050same level as 2.2

63050%-

1050

460-75%oflevelfortest2.7

85050%-75% of level for test 2.9142516391425same:

105012081050

levels as for test 2.8

550

230

550633550

3 Switchgear type tests

3.1 IEC 15 impulse-withstand test — dry3.2 IEC 15 impulse-dry opposite polarity to (3.1)3.3 IEC 15 impulse-withstand test — wet3.4 IEC 15 impulse-wet opposite polarity to (3.3)3.5 IEC 15 impulse-positive polarity — dry3.6 IEC 15 impulse test (both polarities) — dry

applied to one pole, sine voltage of oppositepolarity applied to other pole (kV r.m.s.)

3.7 IEC 15 impulse test (both polarities) — dryapplied to one pole, sine voltage of oppositepolarity applied to other pole (kV r.m.s.)

3.8 Rated 1 minute power-frequency withstand(kV r.m.s.) applied-dry, with breaker closed

3.9 As (3.8) but across breaker open3.10 Rated 1 minute power-frequency-wet,breaker

open and closed (kV r.m.s.)

1-2/50 11-2/50 j175/2500)175/2500 J175/2500

I 1 -2/50

^ 50 Hzrl75/2500

I 50 Hz

50 Hz50 Hz

1425

1050

10501425

170900

245

520610

1050

850

8501050

120700

175

380435

650

—

-

275275

50 Hz 275

4 Cables (excluding extruded solid cable) routinetest

4.1 15 min withstand, test on each drum length4.2 Dielectric loss test at increasing

voltages (kV r.m.s.)4.3 D.C. test on completed installation — 15

min withstandType tests

4.4 Dielectric loss tests at increasingvoltages up to (specimen lengths only)

4.5 Ten full-wave applications negative0

4.6 Repeat (4.5) with positive polarity0

4.7 Ten full-wave applications negative0

4.8 Ten full-wave applications with positivepolarity

4.9 Thermal stability test for 12 hours (6hfor 145 kV) following 20 thermal cycles(kV r.m.s.)°

4.10 Dielectric security test for 24h (kV r.m.s.)A

50 Hz

50 Hz

d.c.

sine50 Hz175/2500 \175/2500 J

O-5-5/5O|

O-5-5/5oJ

50 Hz50 Hz

395

385

710

385

1050

1425

307

275

265

525

265

750x

1050

211

162

152

305

152

380

640

114190

260 IEEPROC, Vol. 127, Pt. C, No. 4, JULY 1980

5 Overhead-line insulator setsType tests

5.1 Five full-wave applications — one externalflashover permitted provided an additional10 applications are withstood. Fittingsas midline

5.2 Repeat (5.1) but with opposite polarity5.3 Wet one minute power-frequency withstand

voltage test (kV r.m.s.)

1-2/501 -2/50sine50 Hz

1425 1050same level as 5.1

630 460

650

275

* Test carried out when specifically called for — waveshape to be agreed+ With bushings installed and co-ordinating gaps removed* Not applicable to generator transformers° All with dielectric at working temperatureA Test 4.12 is an alternative to tests 4.5-^.7x 850 kV for future type tests

Notes on voltage-withstand tests on plant as tabulated

11.1 Transformer and reactor bushings

While a wet power-frequency withstand voltage is appliedto transformer bushings all tests on the completed trans-former are carried out dry. Prior to application of voltagesshown in wet tests, bushing is sprayed for 2 minutes, withapproximately half this voltage applied.

11.2 Transformers and reac tors

Frequently, the tests 2.5—2.12 are specified in place of2.1-2.4. Switching impulse tests 2.6—2.7 have been in-troduced only comparatively recently and are not alwaysrequested. The waveshape used has a virtual front timeof at least 20 jus, and for the tail, the duration above 90%of the test level is at least 200 JUS, the total time to firstzero being at least 500/is. During tests 2.1 and 2.5, thevoltage across any part of a winding must be at leastequal to twice the normal working voltage for any tappingposition.

11.3 Switchgear

Type tests on the conventional switchgear and associatedequipment, for both lightning and switching surge with-stand, are based on the IEC '15 impulse method'. Two

flashovers within any test is claimed (IEC Publication71 Parts 1 and 2) to indicate the statistical withstand(10% probability of flashover) when subjected to thetest waveform. In the case of some existing SF6 switch-gear 25 impulses were applied with no flashovers permitted.

The figures quoted for 'switchgear' under tests 3.1—3.5apply to voltage and current transformers and bushings,isolators as well as circuit breakers. In the latter case theseare applied to an outer and centre phase in turn while thebreaker is closed, and with all parts of the phases to whichthe impulse is not applied, earthed. They are also appliedacross the circuit breaker while open with one side earthed.Reference should be made to BS 5311, 1976, Parts 4 and7 for details of test conditions and levels in circuit breakers.

11.4 Cables

Cables, cable joints and sealing ends are impulse-tested atthe levels shown while at working temperature, as a singlecomposite assembly at the type test stage.

11.5 Overhead-line insulator sets

The voltage levels quoted for overhead-line insulatorsets relate to the minimum level to be obtained when fittedwith the gap electrodes used midline. More relevant datais listed in Table 2.

JEEPROC, Vol. 127, Pt. C, No. 4, JULY 1980 261