SCIENCE

Anomalous breakdown in synthetic insulatingmaterials immersed in transformer oil and

subjected to switching surge voltagesTaher D. Eish, M.Sc.(Eng.), C. Venkataseshaiah, M.E., Ph.D., and C.N. Reddy, B.E., M.Sc.(Eng.), Ph.D.

Indexing term: Dielectrics

Abstract: The occurrence of anomalous breakdown in synthetic insulating materials has been reported inrecent years under alternating voltages, direct voltages, and direct voltages with ripple. In the paper, theauthors report the results of their investigations on anomalous breakdown phenomena under positive andnegative switching surge voltages, and discuss the mechanisms of breakdown.

1 Introduction

In modern synthetic solid insulating materials such as perspex(Polymethylmethacrylate, PMMA), when tested underinhomogeneous field conditions, breakdown occurs at voltagesmuch lower than those reported under homogeneous fieldconditions, and takes a breakdown path whose length is manytimes greater than the shortest distance between the electrodes.This type of breakdown has been called 'anomalous break-down', and many authors [1—6], in recent years, havereported on the occurrence of this phenomenon in syntheticsolid insulating materials, when subjected to alternating, directand impulse voltages.

Electrical equipment in service is invariably subjected toswitching surges. In extra-high-voltage systems, the switchingsurges, rather than lightning surges, determine the insulationdesign. Hence, a study of the behaviour of the insulation undersuch conditions is of great importance. In this paper, theauthors report the results of their investigation of theanomalous breakdown phenomenon in synthetic insulationsubjected to switching surge voltages.

2 Experimental setup and procedure

A rod-plate electrode system (Fig. 1), mounted inside a

HV electrode (steel)20

specimen

LV electrode withguard ring

coaxial leadtoCROmeasuring andr

protective "^circuit I , r

Fig. 1 System of electrode arrangement showing measuring andprotective circuits

a Direct breakdownb Anomalous breakdown/ Surface flashover

C,R = detector impedance (1000 pF and 15 Kfl)Dlt D2 = diodesZj , Z2 = Zener diodes

Paper 1698A, first received 30th March and in revised form 23rdSeptember 1981The authors are with the Electrical Engineering Department, IndianInstitute of Technology, Madras 600 036, India. Mr. Eish is a researchscholar from Mansoura University, Mansoura, Egypt

perspex chamber filled with transformer oil, has been used forthe investigation. The perspex (PMMA) specimens used weresquares with 85 mm side and 6 mm average thickness. Beforeuse, the specimens were cleaned with tap water, and dried witha clean cloth and chamois leather cloth to make them freefrom grease or fibres. The high-voltage electrode was insertedinto a 2 mm diameter hole of 2 mm depth, at the centre of thespecimen. The transformer oil was replaced after every five orsix breakdowns. The discharge pulses were measured using adetector impedance of 15k£2 in parallel with lOOOpFconnected in the earth lead of the specimen (Fig. 1), and anoscilloscope. Standard 250/2500 ̂ is switching surge voltageswere produced using a two-stage Marx-generator with amaximum energy content of 0.45 kJ. The surge voltage wasmeasured by means of a capacitance divider, which also servedas the load capacitance of the impulse generator, and a peakvoltage measuring instrument. The waveforms were recordedusing a double beam surge voltage oscilloscope, which has aresponse time of less than 5 ns.

3 Experimental results

Preliminary investigations showed that surface discharges, andhence anomalous breakdowns, do occur in perspex specimenswhen subjected to either positive or negative switching surgevoltages. Systematic investigations were therefore conductedto determine the surface discharge inception voltage Ui, andalso the anomalous breakdown voltage Uab. When a dischargeoccurred during the application of a surge, bright channelswere observed on the surface of the specimen, radiating out-wards from the high-voltage electrode. The discharge inceptionwas also indicated on the oscilloscope, in the form of dischargecurrent pulses superimposed over the original switching surgecurrent (Fig. 2).

To determine the exact value of the inception and break-down voltages, the magnitude of the applied voltage wasincreased in steps of 2 kV, starting at a very low value. At eachvoltage level ten surges were applied to the specimen, and, ifno discharge was noticed, the magnitude of the appliedswitching surge was further increased. With positive voltagesthe discharge inception voltage was found to be around 72 kV,whereas anomalous breakdown occurred in the range of 82 kV.With negative switching surge voltages, these values were 92and 105 kV, respectively.

3.1 Investigation of probability of anomalous breakdownWhen a switching surge voltage of sufficient magnitude isapplied to a dielectric specimen, the probability of theoccurrence of anomalous breakdown depends on theoccurrence of surface discharge (a) of sufficient energy, and(b) at a location where the material is most susceptible tobreakdown. It is thus a statistical quantity, and could be

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Fig. 2 Switching surge current pulses

a Prior to discharge inception (t/ = 50 kV)b At inception (£/,- - 72 kV)

different for different voltates. This has been determined overa range of both positive and negative switching surge voltages.

3.1.1 Tests with positive switching surge voltages: These testswere started at about the inception voltage, i.e. 72 kV peak. Ateach voltage level, 20 specimens were tested, and the percent-ages of anomalous, direct and no-breakdowns were determined.Each specimen was subjected to 50 surges. If the specimen didnot break down during these applications it was taken as no-breakdown, as some preliminary experiments have shown thatspecimens which withstood 50 impulses withstood even 150without breakdown. It should, however, be pointed out that,as the voltage magnitude was increased, the number ofapplications required for a breakdown decreased, and, at82 kV, breakdown sometimes occurred in a single application.

The percentages of no-breakdowns and breakdowns (bothanomalous and direct) obtained during these investigations areplotted in Fig. 3. At each voltage level, the individual ordinatespx, pi and p 3 represent the percentages of no-breakdown,direct breakdown and anomalous breakdown, respectively.This has divided the graph into three regions, as marked on the

region of direct breakdown

region of no breakdown

70 8085 90 100 A 110 120 130 K0 150 160 170 180U.kV

Fig. 3 Probability regions of breakdowns for positive switching surgevoltages

Figure. It can be seen that the percentage of no-breakdownshas a maximum value, i.e. about 100% at 70 kV, and decreasesprogressively to zero at about 85 kV. The percentage of directbreakdowns increases slowly up to 15% at 80 kV, decreases toa minimum at 85 kV, and increases thereafter to nearly 80%above 150kV. The percentage of anomalous breakdowns, onthe other hand, increases from 10% at 75 kV, reaches amaximum of 95% at 87.5 kV, and decreases to nearly 20% atabout 150kV.

3.1.2 Tests with negative switching surge voltages: The pro-cedure followed for determining the percentages of anomalous,direct and no-breakdowns was the same as given in thepreceding Section, and the results are plotted in Fig. 4. As isclear from the Figure, the same general pattern as in Fig. 3 isobtained for the three regions. The voltage limits are, however,shifted to higher ranges.

ioo no 120 160 170 180 190130 W) 150U.kV

Fig. 4 Probability regions of breakdowns for negative switching sirgevoltages

3.2 Effect of applied voltage on distance of breakdownDuring the above investigations, it was observed that thelocation of anomalous breakdown, as measured from the high-voltage electrode, was further away with negative switchingsurge voltages than with positive voltages. This distance X wasgenerally of the order of 4 -9 mm for positive voltage, and9—36 mm for negative voltage (Fig. 5). The figure shows thescatter in the values of X at each voltage for either polarity,and the arithemetical mean values are shown connected by acontinuous line. It can be seen that, during tests with negativeswitching surge voltages, the distance of breakdown is manytimes greater than that with positive voltages. It can also benoticed that, with negative switching surge voltage, the lengthsincrease at first with increasing voltage, reach a maximum atabout 120kV, and indicate a falling tendency thereafter.

The investigations in Section 3.1 also showed that the typeof breakdown (anomalous/direct) had some bearing on thetime t required for the collapse of the applied voltage acrossthe specimen. This has been determined by oscillographic

120. 130OkV

140 150 160

Fig. 5 Distance X of breakdown as a function of switching surgevoltage U

IEEPROC, Vol. 129, Pt. A, No. 1, JANUARY 1982 63

Fig. 6 Oscillograms showing collapse of applied voltage duringanomalous and direct breakdown

a No-breakdownb Anomalous breakdownc Direct breakdown

recording of the applied voltage, and typical oscillograms areshown in Fig. 6. It can be observed that, during an anomalousbreakdown, the voltage collapses after the peak value has beenattained (Fig. 6b), whereas during a direct breakdown itcollapses on the front itself (Fig. 6c).

4 Discussion

The results presented in this paper show that anomalous break-down occurs in synthetic insulating materials under switchingsurge voltages of either polarity, and that it is statistical innature. Figs. 3 and 4 show that there are three probabilityregions: no-breakdown, direct breakdown and anomalousbreakdown. Fig. 5 shows that the polarity of switching surgehas a profound effect on the distance of breakdown from the

high voltage electrode. These results can be explained using thetheory of anomalous breakdown summarised below.

There are two hypotheses to explain anomalous breakdownphenomena in solid dielectrics. Both presume that anomalousbreakdown is initiated by surface discharges. According to one.[2,3] , the breakdown is initiated by the formation of anegative space-charge canal inside the material, just below thedischarge channel, during the occurrence of surface dischargesin the negative halfcycle, and, on discharge during the positivehalfcycle, this leads to the formation of prebreakdownchannels. During subsequent discharges, these prebreakdownchannels extend in the direction of the opposite electrode,leading to the ultimate failure of the material. This, however,is applicable only under the application of alternating voltages.

According to the other hypothesis [1 ,4 ,5 ] , the hightemperature of the plasma canal of a surface discharge causeslocal heating of the material, decreasing the intrinsic break-down strength. The high electric stress at the tip of the plasmacanal [1,4] causes local intrinsic breakdown at the surface ofthe material. Further progress of this breakdown canal in thematerial, during subsequent surface discharges, will extend ittowards the plate electrode in a manner analogous to thetreeing mechanism, and result in the complete breakdown ofthe material. The breakdown of the entire material may becaused during a surface discharge, if the energy content of thesame is adequate, or during subsequent discharges by acumulative process. This is stated to be applicable to all typesof voltages. .

Referring to Figs. 3 and 4, it can be noticed that theprobability for anomalous breakdowns under surge voltageconditions increases at first with increasing voltage, reaches amaximum at a particular voltage Uab max., and decreases there-after. The probability for direct breakdowns, however,increases hereafter. This behaviour can be explained as follows.At voltages where the percentage of anomalous breakdownsreaches its maximum value, namely 87.5 kV for positive and117.5kV for negative surge voltage, the maximum stress at thehigh-voltage electrode itself could attain values correspondingto the breakdown stress of the material, as is the case withdirect voltages [4]. If the voltage is now increased beyond thisvalue, it could well be visualised that the probability for theoccurrence of a breakdown also increases. Whether it is goingto be an anomalous breakdown or a direct breakdown is amatter of availability of time for the development of a dis-charge channel. Since an anomalous breakdown is preceded bya surface discharge, longer times are to be expected; as againstdirect breakdown, in which no surface discharges are involved(Fig. 6).

From Fig. 5, it can also be noticed that, at any particularvoltage, the location of breakdown with negative polarity ismany times farther away from the high-voltage electrode thanthat with positive. At first sight, this appears to be in contrastto the existing knowledge on the length of surface discharges,obtained by lichtenberg figure techniques, according to whichthe maximum length of positive discharges is always largerthan that of negative discharges at the same voltage [7]. How-ever, similar anomalies during impulse gliding discharges in airhave been reported in Reference 8, where the lengths of negativegliding discharges are shown to be more than those of positivegliding discharges. A further point of interest is differingbehaviour of the distance of breakdown X with the polarity ofthe applied voltage. Similar behaviour under lightning impulsestresses has been explained in Reference 5, which considers thecharge distribution on the surface of the dielectric during theapplication of positive and negative impulse voltages. This canalso be extended to explain the behaviour under the conditionsof switching surge stresses.

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5 Conclusions

Anomalous breakdowns occur in synthetic insulating materials(e.g. polymethylmethacrylate) when subjected to positive andnegative switching surge voltages. The times for anomalousbreakdowns are longer than the times for direct breakdowns.

The anomalous breakdown is statistical in nature. As themagnitude of the applied voltage is increased, the probabilityfor anomalous breakdowns increases at first and falls there-after. At very high voltages, with positive polarity, theprobability for direct breakdowns increases considerably,whereas with negative polarity the probabiltiy for direct break-down is lower than that with positive polarity, and does notvary appreciably.

The distance of breakdown location is farther away fromthe high-voltage electrode for negative switching surge voltagesthan for positive voltages.

6 References

1 MASON, J.H.: 'The resistance of sheet insulation to surface dis-charges', Proc. IEE, 1960,107A, pp. 551-568

2 DRONSEK, G.: 'Nebendurchschlage in festen Isolierstoffen beiWechselspannungen', Dissertation, T.U. Braunschweig, 1967

3 KIND, D.: 'Hochspannungstechnik an Beginn des kunstoffs-zeitalteis', ETZ Arch., 1970, 91, pp. 134-139

4 NARAYANA RAO, Y., and REDDY, C.N.: 'Anomalous break-down mechanism in synthetic insulating materials subjected todirect voltages'. Proceedings of international symposium on highvoltage technology, 1972, Munchen, pp. 443-449

5 REDDY, C.N., and NARAYANA RAO, Y.: 'Anomalous breakdownin synthetic insulating materials subjected to impulse voltages',ETZArch., 1973, 94, pp. 289-293

6 PAUL, P., and REDDY, C.N.: 'Surface-discharge-initiated break-down in solid dielectrics immersed in high-pressure gaseousmixtures', Proc. IEE, 1978,125, (4), pp. 361-363

7 THOMAS, A.M.: 'Heat developed and powder Lichtenberg figuresand the ionization of dielectric surface produced by electricalimpulses', Br. J. Appl. Phys., 1951, 2, pp. 98-109

8 KLAY, H.: 'Anomalie bei Stossentladungen', ETZ Arch., 1964,85,pp. 284-293

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