IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, VOL. PAS-89, NO. 3, MARCH 1970

Switching Surges with Very Long Fronts(above 1500 ,us): Effect of Front Shape

on Discharge VoltageGIANGUIDO CARRARA, MEMBER, IEEE, LUIGI DELLERA, AND GIANLUIGI SARTORIO

Abstract-The results of dry positive-polarity switching impulsetests on a 4-meter rod-plane gap are reported. The tests were per-formed using two different shapes for the front of the impulse ap-plied: one shape consisted of the "smooth" front commonly used inswitching impulse tests; the second shape was obtained by super-imposing a short "bump" on a very long "smooth" front just beforethe crest was reached.The results show the effect of the relative amplitude of the bump

on the sparkover voltage and allow some conclusions on the correctlaboratory duplication of actual switching surges.

INTRODUCTIONTHE investigation being reported appeared useful in view

of the following considerations.1) Recent studies of ultra-high-voltage (UHV) networks have

shown that, with the help of suitable means, switching overvolt-ages may be limited to below 1.8 per unit, and eventually down to1.5-1.6 per unit.

2) Such limited switching surges will have very long times tocrest; their fronts, however, will not be completely smooth, butwill present "bumps" of short duration and of amplitude betweena few percent and a few tens of percent.

3) The switching impulse strength of air insulation increaseswhen the time to crest increases above the few hundred micro-seconds corresponding to the minimum strength. The strengthincrease may be of 20 percent or more if the front becomes suffi-ciently long. Such an increase, however, was found while testingwith switching impulses having "smooth" fronts.

4) If the so-called bumps are disregarded, the strength increasefor surges with very long fronts may be of great benefit in thedesign of air insulation.

5) If the bumps are disregarded, the air insulation strengthmay be assessed by testing with impulses having very long andsmooth fronts. Consequently, HV transformers may be a goodalternative to conventional impulse generators in producingswitching impulses.From all these considerations, some of which will be expanded,

it was considered interesting to assess the influence of the relativeamplitude of the bump on the switching impulse strength of airinsulation.The investigation was limited to a 4-meter rod-plane gap and

to dry switching impulses of positive polarity. A study of the prob-lem from the designer's point of view will reveal whether anextension of the investigation to other objects and to different

Paper 69 TP 697-PWR, recommended and approved by the Trans-mission and Distribution Committee of the IEEE Power Group forpresentation at the IEEE Summer Power Meeting, Dallas, Tex.,June 22-27, 1969. Manuscript submitted February 24, 1969; madeavailable for printing April 10, 1969.The authors are with the Centro Elettrotecnico Sperimentale

Italiano (CESI), Milan, Italy.

voltage ranges is justified. Of course in this case also, a study of thestatistical distribution of the switching surge front shapes that canoccur in the network becomes necessary.

ACTUAL SHAPES OF SWITCHING SURGE FRONTSFor networks in which the highest overvoltage factors may

reach values in the range of 2 to 4 per unit, the fronts of the switch-ing surges consist, in practice, of a high and steep rise of voltage("bump") superimposed on the power frequency.

Insulation strength stressed with this type of overvoltage isusually studied by means of a switching impulse shape repre-senting as closely as possible the characteristics of the bumponly, and the withstand voltages obtained with critical time tocrest are considered conservative values for insulation design.For UHV systems the present design trend is to reduce more

and more the switching overvoltages, and therefore the bumpstend to decrease to values of the order of some tens of percentof the peak value. As an example, Fig. 1 shows two switchingsurges recorded during a transient network analyzer (TNA)study on a 735-kV system. As one can see, the magnitude of thebumps becomes relatively low, and of the order of 20 percent ofthe peak value of the switching surges. In these cases, if the bumpis disregarded, the resulting time to crest of the surges is of theorder of several thousand microseconds.As a first approach, therefore, one may try to utilize, for design

purposes, the insulation strength as determinedwith impulses withsmooth fronts having very long time to crest, thus taking ad-vantage of the higher strength of the insulation and disregardingthe possible bump influence.

SMOOTH FRONTS: EFFECT OF TIME TO CRESTON DISCHARGE VOLTAGE

Up to now, to the authors' knowledge, laboratory tests havebeenperformedutilizing switching impulses consisting, inpractice,of a double exponential form with smooth front, such impulsesbeing considered as duplicating the actual switching surges onnetworks. As is well known, the discharge voltage of air insula-tion, when stressed with these switching impulses, depends on thetime to crest of the applied impulse [1 ]-[3].

Stresses with positive polarity in dry conditions only are exam-ined, since previous investigations have shown that this stressmay be generally considered the most representative for a studyof insulation behavior. The considerations to be made, however,may be generally extended to other test conditions.The dependence on the time to crest of air insulation strength

is so well known that no test would be necessary to prove it.However, since for studying the effect of the bump a close com-parison with smooth-front results was considered useful, thisdependence was once more investigated. Table I and Fig. 2 reportthe results of tests on a 4-meter rod-plane gap performed withimpulses having different times to crest. Fig. 2 shows that the 50-

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IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, MARCH 1970

(a)

(c) (d)Fig. 1. Examples of switching surge shapes obtained during TNA study of 735-kV network system for three-phase re-closing: closing resistance 550 ohms; insertion time 13 ins; dead time 0.3 ms; opening sequence phase 1-3-2. (a)Receiving end: line compensation 70 percent; overvoltage factor 1.5 per unit; AV/V = 23 percent. (b) Oscillo-gram of (a). (c) Receiving end: line compensation 50 percent; overvoltage factor 1.5 per unit; AV/V = 14 per-cent. (d) Oscillogram of (c).

IV)

1.C

1.050 10 5 0.l t)T( c

(b)

leoGc/ \A\ \AA AAA/X

(a)

DOOcI/a

(b)Fig. 3. Example of waveshapes obtained for impulsesgenerated with HV transformers [5].

percent discharge voltage reaches a minimum for time tocrest of the applied impulses in the range 100 to 300 /is ("crit-ical" time to crest). All air insulations present similar behaviorwith different critical times to crest, depending mainly on theelectrode shape and on the gap spacing [1]-[3].For UHV insulation testing, if the bumps are disregarded, only

impulses with very long smooth fronts are likely to be used.On this line, test circuits using step-up transformers suitably

energized from the low-voltage side were adopted by some lab-oratories in order to generate switching impulses with very longsmooth fronts [4], [5]. It should be noted in passing that theimpulse shapes obtained with transformer generation (Fig. 3),have a slightly different front shape. Actually, the time to crestfor an impulse generated with a conventional impulse generator isabout twice the time to 90 percent, whereas for transformer-generated impulses this figure is not higher than 1.3. These lastimpulse shapes also present some problems in the definition ofthe origin of the impulse. Fig. 3 shows the authors' determinationof the origin.Now the question arises about the limit of the relative value

of the bump for which insulation strength may be studied in thisway. This paper intends to study the problem, the tests performedbeing limited to a 4-meter rod-plane gap stressed with positive-polarity impulses.

Fig. 2. 50-percent discharge voltage of 4-meter rod-plane gapobtained in dry conditions with positive-polarity smooth-frontswitching impulse, as function of time to crest of applied impulses.

TABLE ITEST RESULTS OBTAINED ON 4-METER ROD-PLANE GAP: POSITIVE-

POLARITY SWITCHING IMPULSES, DRY CONDITIONS*

50-PercentDischargeVoltage Atmospheric Conditions

(ProspectiveTime Value) Pres- Tem- Absolute

to Crest TC V % sure P, perature t Humidity u(As) (kV) (mmHg) (IC) (g/m')

30 1320 751 16 6.0115 1215 761 17 12.7185 1180 761 18.5 13.0300 1215 759 18 13.4350 1200 758 22 14.0460 1280 759 19.5 13.7790 1420 760 19 14.01300 1470 756 18.5 14.21800 1480 758 11.5 5.0

* Values not corrected for atmospheric conditions.

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455CARRARA et al.: SWITCHING SURGES WITH LONG FRONTS

Fig. 4. Test circuit. A-impulse generator used for basic smoothfront. B-impulse generator used for superimposed imptulse. C-different connections for different capacitive coupliings betweengenierators: I-used for AV/V = 15-28 percent; 2 used forAV/V = 6-10 percent; 3 generator B short circuited when 11obump was sllperinmposed. D-capacitive voltage divider. F-capac-itor assembly for capacitive coupling between generators. I-testob;ject (4-mneter rod-plane gap). S-stray capacitanlce to grotimd.

(a) (b)

Fig. 5. Example of waveshape obtained with circuit of Fig. 4;maximum voltage V reached during tests was 1500 kV.

(C)

Fig. 6. Impuilse shapes obtained for tests.(e) (f)

(a) Long smooth front. (b)-(f) Bump amplitudes equal to 6, 10, 15, 21, and 28 percent,respectively, of total peak value.

GENERATION OF FRONTS WITH BuMPS

In order to obtain a high-voltage switching impulse representa-tive of the stress of actual switching surges as shown in Fig. 1, atest circuit consisting of two suitably coupled impulse generatorswas set ul). This circuit (Fig. 4) is suitable to generate a switchingimpulse with a smooth long front on which a shorter voltage im-pulse representative of a bump is superimposed. A short study ona Inodel was first performed in order to determine the bestparameters for the actual HV circuit, taking into account thetesting facilities available in the laboratory.The coupling between the two generators was obtained by

means of suitably connected capacitors, and the two impulse

generators were triggered with an appropriate tinme interval. Atypical impulse shape obtained with this circuit is showni in Fig.5.By varying the charging voltages of the generators aInd the

capacitive coupling between them, it is possible to obtain a cer-tain range of peak values V and of AV/V (ratio between thebump amplitude and the peak value).The times to crest of the impulses obtained Xwere 1800

As for the basic impulse with smooth front and 200 As for thesuperimposed bump. The bump was superimposedl on thebasic impulse with a time lag of about 1600 ,us in order to obtainthe same time to crest for the basic impulse and for the bumpedimpulse.

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, MARCH 1970

TABLE IITEST RESULTS OBTAINED ON 4-METER ROD-PLANE GAP: POSITIVE-POLARITY SWITCHING IMPULSES, DRY CONDITIONS,

BUMPS SUPERIMPOSED ON LONG SMOOTH-FRONT IMPULSE*

Bump Magnitude 50-Percent Atmospheric ConditionsReferred to Peak Discharge Standard

Applied Impulse Value AV/V Voltage V50% Deviation o Pressure P Temperature t Absolute Humidity u(Fig. 6) (percent) (kV) (percent) (mmHg) (OC) (g/m3)

(a) 1480 4.6 758 11.5 5.0(b) 6 1460 - 758 11.5 6.0

-> (C) 10 1380 3.0 758 8 6.8(d) 15 1330 - 758 12 4.8(e) 21 1280 4.0 758 13 5.3(f) 28 1220 6.0 758 12 4.8

* Values not corrected for atmospheric conditions.

V50%(MV)

1.4

12

1.0 I_

0.6

OA +

0.2 __

oU 2U AN t:U 9U 1UUO 20 40 60 80 10Q

A&V/V(%)

Fig. 7. 50-percent discharge voltage of test object as function ofrelative amplitude of bump. AV/V = 0 percent corresponds tosmooth front, 1800 ,us time to crest; AV/V = 100 percent corre-sponds to smooth front, 200 ,us time to crest.

TEST PROCEDURE AND RESULTS

The tests were performed by applying to the rod-plane gapten switching impulses for each of several different voltage levelsin order to determine the 50-percent discharge voltage. Firsta test with the smooth long-front impulse, and then the tests withthe superimposed bump were performed.The tests with a bump consisted of applying to the test object

a constant long-front wave, the peak value of which was in therange of 60 to 90 percent of the 50-percent discharge voltageobtained during the first test, and superimposing on it the bumphaving variable crest values. Five tests were performed, withaverage values of bumps in the range of 6 to 28 percent of totalpeak value of the applied switching impulses.

Finally, a test applying smooth short-front impulses havingtime to crest equal to that of the superimposed bump was per-formed.

Fig. 6 shows the various impulses obtained with different bumppercentages, and the results are reported in Table II.The standard deviation obtained during the tests with wave-

shapes (b) and (d) is not reported because its reliability is verylow, due to the few voltage levels applied.The 50-percent discharge voltage as a function of the average

ratio between the bump and the total peak value of the appliedimpulses is reported in Fig. 7. The point corresponding to AVI/V= 100 percent is derived from Fig. 2. As one can see in Fig. 7,the influence of the bump begins to be consistent for a AVI/V

ratio higher than about 1 percent, whereas for bumps of theorder of 30 percent and higher, the 50-percent discharge voltagehas already attained its lowest value. The decrease of the 50-percent discharge voltage for bumps between 5 and 30 percentis about 20 percent, which represents the total differencebetween the discharge voltages corresponding to impulses with1800- and 200-,us times to crest.

CONCLUSIONS

1) If a switching surge with a very long front presents on it ashorter bump, that is, a sudden faster voltage rise during which itattains its crest value, the sparkover voltage is affected by thepresence of that bump. In other words, it is not possible to saythat the insulation strength for an impulse with a bumped frontis the same as that determined applying to that insulation animpulse having a smooth front and the same time to crest andpeak value as the impulse with the bumped front.

2) The 50-percent discharge voltage of an impulse with abumped front decreases as a linear function of the relative ampli-tude of the bump from its maximum value, which corresponds tono bump present, to its minimum value, which corresponds to abump of relative amplitude equal to 30 percent, and is equal tothe value obtained with an impulse having a smooth front andthe same time to crest as the bump itself.

3) All tests were performed on a 4-meter rod-plane gap underdry conditions and using positive polarity; therefore one must becareful about making generalizations. It is felt, however, thatapproaches to the problem are enlightened enough to make possi-ble the decision on the usefulness of performing further tests ondifferent test objects, in different conditions and for differentvoltage ranges.

REFERENCES

[1] L. Paris, "Influence of air gap characteristics on line-to-groundswitching surge strength," IEEE Trans. Power Apparatus andSystems, vol. PAS-86, pp. 936-947, August 1967.

[2] Y. Watanabe, "Switching surge flashover characteristics of ex-tremely long air gaps," IEEE Trans. Power Apparatus andSystems, vol. PAS-86, pp. 933-936, August 1967.

[3] T. Udo, "Sparkover characteristics of large gap spaces and longinsulation strings," IEEE Trans. Power Apparatus and Systems,vol. 83, pp. 471-483, May 1964.

[4] G. N. Aleksandrov, V. L. Ivanov, and V. P. Redkov, "Electricalstrength of representative air spacing on EHV lines duringswitching surges," Elektrichestvo, no. 1, pp. 65-71, 1966.

[5] G. N. Aleksandrov and V. L. Ivanov, "How the electricalstrength of large air spacing varies with the frequency of oscilla-tory voltage," Elec. Tech. (USSR), vol. 2, pp. 297-309, June1964.

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