SCIENCE

Charge storage on insulation surfaces in air underunidirectional impulse conditions

M.A. Abdul-Hussain, BSc, MSc, PhDK.J. Cornick, MSc(Eng), CEng, MIEE

Indexing terms: Insulators and insulation, Overvoltages, Discharges {electric), Discharges (gas), Electric fields

Abstract: In many insulation systems, overvol-tages below the flashover level can give rise to dis-charges which produce free charge carriers. Thesecharges may be deposited on insulation surfaces,resulting in a high surface field and consequentdistortion of the normal field. This may result inincrease, or decrease, in flashover voltage. Investi-gations were undertaken to examine the factorsand mechanisms governing the production ofsurface charge and the resulting field distortion.The experimental equipment consisted of a simplepoint-insulation-plane electrode assembly,mounted in a glass test cell filled with air, to whichunidirectional impulse voltages were applied.Polycarbonate and polymethylmethacrylatesamples were tested. Measurements of the dis-charge currents, residual surface field and resultingdust figure patterns were carried out. Under bothpositive and negative impulse voltages, dischargecurrent pulses occurred resulting in high surfacefields on the insulation sample. On the wavetail ofthe applied impulse discharge, currents occurredin a reverse direction to those originally producedby the applied impulse. With repeated impulses,the stored surface charge produced by oneimpulse significantly affected the discharge activityof the following impulse. Under flashover condi-tions, a substantial surface field remained follow-ing the flashover.

1 Introduction

In many practical insulation systems, a gas/solid dielec-tric interface is situated in close proximity to a high-voltage conductor. Under transient overvoltageconditions, corona discharges may occur in such systemsgiving rise to free charge carriers. These charge carriersmay accumulate on solid insulation surface, producingelectric fields which seriously distort the normal field con-ditions. As a result, the flashover level of the insulationassembly may increase or decrease. The magnitude anddistribution of the stored surface charge will govern thechange in flashover voltage, while its decay rate will indi-cate the time period over which this effect will occur. Themechanisms and factors governing the magnitude anddistribution of surface charge and its influence on flash-over voltage are, therefore, of some concern. This is espe-

Paper 56O5A (S2, S3, S8), first received 14th April and in revised form22nd August 1986The authors are with the Power Systems High Voltage Laboratories,UMIST, PO Box 88, Manchester M60 1QD, United Kingdom

cially true with the increasing use of low-loss insulationmaterials and the need for better utilisation of thesematerials.

Within recent years, a number of studies have beenmade to examine the processes by which free charge canbe generated and accumulated on insulation surfaces.These studies have included the use of a sharp-pointedemitter [1], attached, conducting surface particles [2],surface conduction under direct voltage [3, 4], coronadischarges on the high-voltage electrode [5], and anexternal corona source [6], for surface charge pro-duction. Other studies, of a more practical nature,include work on an epoxy-insulated busbar assembly [7].In these and similar studies, surface charge detection andmeasurement is usually carried out by electrometer probemethods, although dust figure [8] and liquid crystal [9]methods have also been used, with good effect in deter-mining the finer detail of charge patterns. Other impor-tant information on the basic processes involved incharge production has been obtained by examining thedischarge current pulses producing the discharge [5, 10].

Although the previous work has provided much valu-able information on the subject of surface charge and itsinfluence on insulation flashover performance, there isstill a great deal to be understood. In particular, moreinformation is required on the basic processes involved inorder that the possibility of surface charge productionand accumulation may be predicted, and its effect assess-ed in the early stages of insulation design. It was for thispurpose that the present work was undertaken.

This paper describes investigations carried out with asmall-point/insulation/plane electrode assembly mountedin a glass test cell filled with air, and tested under uni-directional impulse conditions. The specific objectiveswere to examine the characteristics of the dischargecurrent pulses producing the stored surface charge; toassess the relationship between these pulses and thestored surface charge; to determine the distribution andmagnitude of the surface charge; to assess how thesurface charge distorts the normal field conditions; toexamine the development of surface charge under repeat-ed impulses, including flashover. The results of theseinvestigations are analysed and their relevance to engin-eering applications are discussed.

2 Equipment and experimental techniques

2.1 EquipmentThe experimental equipment consisted of a glass test cell,impulse generator, surface charge and discharge currentmeasuring circuits, and an electric-field data-acquisitionfacility, Fig. la. The glass test cell housed a point/insulation/plane electrode system and an electrometer

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987 731

probe. The point electrode had a tip radius of 0.1 mmand its distance from the surface of the insulation samplecould be adjusted with a micrometer. The insulation

glass test cell

field probe

fielddata

acquisition

impulsegenerator

dischargecurrent

mear.urementand

display

I VDU

plotter

disc file

mpulse

insulationsample

balancingcapacitor

oscilloscope

Fig. 1 Test equipment

a Overall equipment layout; b Balanced detection circuit

samples were in the form of discs 90 mm diameter and1.5 mm thick mounted in an offset aperture in theearthed plane electrode. The bottom side of the samplewas painted with conducting paint, to ensure goodcontact between the sample and the plane electrode. Ini-tially, the insulation sample would be centralised underthe point electrode. Immediately after an impulse hadbeen applied to the point electrode, the electrode wouldbe lifted and the earthed plane electrode would berotated so that the insulation sample passed under theelectrometer probe, and a scan along an arc of the insula-tion sample would be obtained. By continuously rotatingthe earthed plane and synchronously moving the elec-trometer probe transversely to the direction of rotation,the full surface of the insulation sample could be scanned.Before any test, the insulation sample would be scannedand checked to be free of residual surface charge.

Following the electrometer probe scan, the sample wasremoved from the glass test cell and dusted with powder

to reveal the surface charge figure. In general, Xeroxreproduction powder was used to obtain a fine imagethat could be easily photographed. This powder adheresto both positive and negative polarity charges. If,however, it was considered necessary to determine thepolarity of the charges, then a mixture of jeweller's rougeand talcum powder was used [11]. Jeweller's rouge andtalcum powder attach to negative and positive charge,respectively.

For discharge current measurement a balanced detec-tion circuit was used, Fig. lb. This type of system waschosen in order that very small discharge current couldbe detected in the presence of the large displacementcurrent generated under impulse voltage conditions. Dis-charge currents and impulse voltages were recorded withan oscillograph. In addition, the surface charge scanswere digitally recorded and fed to a minicomputer fordata processing.

The impulse generator was arranged to produce a 25/350 us double exponential impulse voltage wave. Thewavefront capacitor of this impulse generator was largeand the wavefront resistor small, to ensure that large dis-charge currents did not produce wave distortion.

The experiments were carried out on polycarbonate(PC) and polymethylmethacrylate (PMM), samples inroom air having a relative humidity of 50%.

Although the point/insulation/plane test system usedin the experiments is not truly representative of practicalconditions, it was considered that the various factorsinvolved in surface charge phenomena could best beresolved with a simple test arrangement having welldefined test conditions. Details of the test equipmenthave been given in References 12 and 13.

2.2 Surface charge measurementsSurface charge measurements on insulating materials areusually performed with a capacitance probe coupled toan electrometer amplifier. The sensitivity of this type ofsystem depends on the distance from the probe tip to thedielectric surface; the closer this distance the greater thesensitivity. The resolution depends on the probe diam-eter; the smaller this diameter the better the resolution. Ifthe capacitance probe tip is placed near to the dielectricsurface and has a narrow aperture, then, to a goodapproximation, the electrometer output voltage can berelated to the surface charge intensity by a calibrationfactor [14].

Under the conditions prevailing in the present investi-gations, the density of the deposited surface charge ishigh, resulting in a high surface field intensity. If thecapacitance probe tip is brought into close proximity tosuch a charged surface, then a discharge from the chargedsurface to the probe tip is likely. Under these circum-stances, it is necessary to ensure that the distance fromthe capacitance probe tip to the surface is large enoughto avoid such a discharge. This means that the electricfield at the probe tip is not solely due to those elementsof surface charge directly under the probe tip, but is theintegral effect of the component fields at the probe tipproduced by surface charge elements over a much greaterarea than the probe tip area. The output from the elec-trometer is therefore proportional to the charge densityover this small area, rather than proportional to thestored surface charge at the particular point directlyunder the probe tip. In view of the complex and highlyfilamentary surface charge deposited by surface streamersand the integrating effect of the capacitance probe, nosimple factor of proportionality relating electrometer

732 IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987

output voltage to surface charge density can be defined.The effective electric field at the tip of the capacitanceprobe can, however, be calculated, and, for this reason,all results presented will be scaled in effective field inten-sity at the capacitive probe tip rather than surface chargedensity. The effective electric field strength plotted is nottherefore directly related to the surface charge density,but will be a rather blurred replica of this charge. Calcu-lations are, at present, being carried out to solve thethree-dimensional field due to surface charge in terms ofelectrometer voltage, in order that the surface chargedensity can be evaluated from the electrometer measure-ments. The capacitance probe head used in the presentmeasurements consisted of a probe-tip wire of 0.2 mmdiameter, insulated and centrally located in a 0.3 mmdiameter hole drilled in a 3.13 mm diameter shield elec-trode.

In certain cases, it is possible to obtain greaterresolution at the perimeter of discharge figure, where thecharge density is not so high as at the centre, Fig. 2, by

probe gap = 0.2mm

probe gap = 0.8mm

0 10 20 30

distance,mm

Fig. 2 Sensitivity of field probe

Positive impulse = 16 kVp, point-insulation gap = 2.5 mm

reducing the probe distance to a smaller value thannormal. How close the probe tip could be placed abovethe insulation surface in the experiments was decided ona trial and error basis. Any discharge from the surface tothe probe, owing to the probe being too close to the

surface, was immediately evident by a major disturbancein the electrometer output voltage and dust figure.

Experimental results

3.1 Discharges and charge storage at inceptionvoltage

Initial studies were carried out to establish the nature ofthe surface charge deposited at the discharge voltageinception level. The point-to-insulation surface gap wasset at 4 mm and a PMM sample was used.

Under positive polarity impulse conditions (Fig. 3), asingle high-speed current pulse of 40 mA magnitude was

50mA10 us

200 p A200 ps

20 40 60distance, mm

b

80 100

Fig. 3 Discharge at inception voltagePositive impulse = 11 kVp, point-insulation gap = 4.0 mm, probe gap = 3.0 mm

detected (Fig. 3a). This occurred near to the crest of theimpulse voltage wave and some 14 /is after the impulsehad started. The resulting field-intensity profile detected(Fig. 3b) covered only a small area and was conical inshape. The dust figure obtained from this sample (Fig. 3c)exhibited the typical filamentary pattern [8, 15] expectedunder the prevailing test conditions. The capacitanceprobe tip was positioned 3 mm above the sample surfaceduring the field scans, to avoid discharges to the probe.Comparison of the field profile (Fig. 3b) and the dustfigure (Fig. 3c) clearly illustrate the point previouslymade, that the electrometer output is not an exact replicaof the surface charge but is rather a blurred image of thatcharge.

Under negative-polarity impulse conditions and at thedischarge inception level (Fig. 4), multiple high-speedcurrent pulses were generally observed (Fig. 4a). Inaccordance with breakdown conditions in a stronglydivergent field, the negative discharge inception voltageof 7 kVp was much lower than the positive value of11 kVp. The resulting field intensity figure (Fig. 4b),although similar in shape to the positive impulse figure,was much smaller in magnitude and diameter, reflectingthe lower discharge inception voltage. The dust figureproduced by the negative impulse has a well defined cir-cular figure (Fig. 4c). This reflects the diffuse negative dis-charge which might be expected [8, 16].

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987 733

In order that any reverse discharges (see Section 3.2.3)might be detected, the discharge currents in both thepositive and negative voltage tests were also displayed on

Id

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5 mA200ps

10mA5ps

can occur. Owing to the surface charges deposited by thefirst discharge, the surface streamers of this second dis-charge are forced to follow paths between the filaments of

100mA10ps

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200MA200ps

20 40 60 80distance, mm

b

100

Fig. 4 Discharge at inception voltageNegative impulse = 7 kVp, point-insulation gap = 40 mm, probe gap = 30 mm

much slower time sweeps (Fig. 3a and 4a) and a verymuch greater sensitivity for positive impulses. At thesensitivity level chosen, it was found that, for positiveimpulses, a certain amount of noise was detectedresulting in the diffuse traces shown. At the inceptionvoltage level of both positive and negative impulse tests,no reverse discharges were detected.

3.2 Discharges and charge storage above inceptionvoltage level

3.2.1 General: During the wavefront of both positiveand negative impulses, when the voltage was aboveinception level, discharge current pulses were detectedwhich were of the same polarity as the applied impulse;these will be termed 'direct' discharges Id. During thewavetail of both positive and negative impulses, dis-charge current pulses were obtained which were of theopposite polarity to the applied impulse; these will betermed 'reverse' discharges (/,.).

3.2.2 Direct discharges: With positive impulse voltages,at voltages above the discharge inception level, morethan one short duration direct discharge current pulseoccurs (Fig. 5a). Following the first direct dischargecurrent pule, at an instantaneous voltage close to theinception voltage level, (i) Fig. 5a, a second direct dis-charge current pulse occurs at a much higher voltage onthe impulse wave, (ii) Fig. 5a. The first direct dischargecurrent pulse will deposit charge on the insulationsurface, which results in a field profile similar to thatshown in Fig. 3b. This field will weaken the field at thepoint electrode, produced by the applied impulse voltage,such that a much higher instantaneous voltage is neededat the point electrode before the second direct discharge

734

20 40 60 80distance, mm

b

Fig. 5 Discharge above inception voltagePositive impulse = 22 kVp, point-insulation gap = 4.0 mm, probe gap = 3.0 mm

the first discharge. At the periphery of the first discharge,however, the insulation is free of charge and, at thispoint, the surface streamers of the second dischargebranch radially into a hand-shaped figure (Fig. 5c).

The foregoing process may be repeated a number oftimes, giving rise to several direct discharge currentpulses. The magnitude and timing of such pulses wasfound to be very variable, but strongly dependent on theinstantaneous voltage at which the first direct dischargeoccurred. The resulting surface discharge dust figurewould show several hand-shaped extensions to the dustfigure produced by the first discharge, and the overalleffect would be an increase in the area of depositedcharge.

When the impulse voltage level is increased to a valueclose to the flashover level, a further type of direct dis-charge current pulse is observed, (ii) Fig. 6a. This type ofpulse, which carried a much greater amount of chargethan the short duration pulses previously described, pro-duces much greater streamer activity on the insulationsurface and is termed a 'prebreakdown' direct dischargecurrent pulse. The extent of the surface discharge produc-ed by this pulse is much greater than that produced bythe short-duration direct discharge current pulses.3.2.3 Reverse discharges: Immediately following thelast of the direct discharge current pulses, the chargestored on the insulation surface will be large. Conse-quently, the field at the point electrode will be low, asdescribed previously. As the impulse voltage starts to fallon the wavetail, the electric field at the point electrodewill become increasingly governed by the stored surfacecharge. Eventually, the field at the point electrode will be

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987

produced predominantly by the surface charge, and willreach such an intense value that a reverse dischargeoccurs.

20 40 60 80distance,mm

b

100

Fig. 6 Discharge near toflashover voltagePositive impulse = 25 kVp, point-insulation gap = 4.0 mm, probe gap = 3.0 mm

Under positive impulse conditions, the reverse dis-charge current pulse pattern consists of a large number ofvery rapid pulses, Figs. 5a and 6a, which occur atapproximately 500 /is after the wave crest. These rapidpulses of current remove charge from the central area ofthe surface charge, until the field intensity at the pointelectrode is insufficient for further discharges to occur.The result of this charge removal on the field-intensityprofile is to remove the top of the conical figure shown inFig. 3b, and create a crater-shaped profile, Figs. 5b and6b. The resulting dust figures, Figs. 5c and 6c, indicatethat surface charge has been completely removed from alarge central area of the insulation surface.

Under similar test conditions but with a negativeimpulse voltage applied, the overall pattern of results issimilar to that obtained under positive impulse condi-tions. Direct and reverse discharge current pulses areobserved, Fig. 1b, and the dust figure indicates a largecentral area devoid of charge, Fig. 7c. However, distinctdifferences in the detail of Figs. 6 and 7 can be noted.

Under negative impulse voltages, the initial direct dis-charge current pulse is of short duration and is of largeamplitude, (i) Fig. la. Following this pulse is a seconddirect discharge current pulse which, although of muchlonger duration, is of much smaller amplitude, (ii) Fig. la.This pulse has characteristics of a glow discharge at thepoint electrode. These direct discharges will deposit alarge amount of negative charge on the insulationsurface. Again, similar to the positive impulse results, the

field produced by this charge will be of sufficient intensityfor reverse discharges to occur during the wavetail of theimpulse wave, Fig. la upper trace. The reverse dis-charges, unlike the positive impulse reverse discharges,

Ir

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I

(n)

10mA200u:

• M M • W

20mA5ps

60 80distance,mm

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Fig. 7 Discharge near to flashover voltageNegative impulse = 43 kVp, point-insulation gap = 4.0 mm, probe gap = 3.0 mm

are few in number, widely separated and of large magni-tude. As a result of these discharges, the electric fieldprofile is again of the crater shape seen under positiveimpulse conditions. However, the gradient of the field inthe radial direction, Fig. 1b, is much greater than thatobserved under positive impulse conditions, Fig. 6b. Thedust figure obtained, Fig. 7c, exhibits the diffuse circularpattern typical of negative discharges. The central area ofthe figure is again devoid of surface charge due to thecharge clearing action of the reverse discharges.

3.3 Effect of gap setting on discharges and chargestorage

To determine what effect gap distance would have onstored surface charge, tests were carried out in which animpulse voltage of 24 kVp was applied to gaps in therange 0.5 to 14 mm using PMM samples.

In very short gaps, under positive impulse voltage, thefield intensity at the point electrode is very high, and sothe first discharge takes place at a low voltage very earlyon the front of the impulse voltage wave, Fig. 8a. Thisfirst discharge will deposit charge on the insulationsurface, which would tend to inhibit further dischargesdue to the weakened field at the point electrode.However, in this very short gap, the discharge inceptionvoltage of the first discharge is so small, compared withthe crest value of the impulse voltage wave, that on thecrest of the impulse voltage wave three smaller dischargepulses occur. It is considered that these discharges addcharge to the outer periphery of the deposited charge,

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987 735

due to the high radial field existing. Further, it is con-sidered that these discharges take place, not from theelectrode point to the insulation surface below the point,

ri IIIf\

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Fig. 8 Variation of discharge with gap settingPositive impulse = 24 kVp, probe gap = 3.0 mm

but radially outwards and downwards, where the electricfield gradients are still at a critical level for discharge.Despite a large amount of reverse-discharge activity, thefield intensity is still high following the impulse. The dustfigure resulting from discharges has a central area thatcontains negative charge, surrounded by an area of nocharge which, in turn, is surrounded by a large area ofpositive charge.

The discharge activity described in this very short0.5 mm gap remains substantially the same when the gapwas opened out to 4 mm. However, the reverse-dischargepulses became steadily weaker, and the outer diameter ofstored surface charge slightly smaller. At a 7 mm gapsetting, Fig. 8c, only a single direct discharge currentpulse was detected, and the reduction in reverse-discharge activity and the resulting surface charge profileis quite evident.

In relatively large gaps, the reverse-discharge activityis clearly becoming weaker and in a 10 mm gap, Fig. $d,it is entirely absent. With the 10 mm gap, the residualfield profile due to the surface charge now attains thesimple conical shape described earlier, and is of greaterintensity than the previous gap setting.

For very large gaps, 14 mm, Fig. Se, the field betweenthe point electrode and insulation is such that the firstdischarge consists of a branched streamer. Only the tips

736

of this multiple streamer will reach the insulation surface,depositing small patches of charge. The field profile doesnot indicate these individual patches of charge, but doesindicate a profile much more ragged than previouslyseen. This results from the field integrating effect of theprobe previously mentioned. The magnitude of the fieldprofile is much smaller than that obtained with the pre-vious gap settings, Figs. Sa-Sd, and again no reverse-discharge current pulse was detected.

When a negative impulse was applied to a series ofgaps, the resulting field profiles were similar in form andmagnitude, but of opposite polarity, to those under posi-tive impulse conditions. In very small gaps, the field atthe point electrode is very high and, consequently, thefirst, and only, direct-discharge current pulse appearsearly on the wavefront of the impulse. This leads to alarge diameter charge profile which is diffuse, but has aclearly marked circular boundary. Following the directdischarge current pulse, there is the very much smallerand much longer type of current pulse as described inSection 3.2.3 and shown in Fig. la.

Again, in a similar manner to the positive impulseresults, intense reverse-discharge activity commencesearly on the wavetail. The reverse discharge is character-ised by a number of high-speed pulses, which removecharge from an area directly under the point electrode,giving rise to the characteristic crater-type field profile.This produces a filamentary figure, which is characteristicof a positive discharge and is superimposed on the diffusedust figure produced by the negative direct discharge.

With increasing gap setting, the diameter of the fieldprofile reduces steadily, and so also does the number ofreverse current pulses. Ultimately, in gaps greater than7 mm, the reverse current pulses cannot be detected andthe field profiles indicate this by reverting back to asimple conical profile. In a similar manner to the positiveimpulse conditions, this leads to a slight increase in theresidual field at a gap setting of 8 mm.

3.4 Effect of repeated impulses of constantmagnitude

The possibility of surface charge build up under repeatedimpulses is of great importance in practical systems. Tostudy this phenomenon, tests were carried out on a 5 mmgap and a PC insulation sample. Repeated positiveimpulse voltages of 14 kVp were applied. The test voltagechosen, although well above the discharge inceptionvoltage of the test arrangement, was well below the flash-over voltage level.

Prior to the test series, the PC sample was checked toensure it was free from surface charge. A sequence ofthree impulse voltage applications was then applied tothe system. The resulting discharge currents were record-ed, and, following each impulse voltage application, thesurface was scanned with the electrometer probe. Nomore than 10 min elapsed between each impulse voltageapplication.

During the first impulse test (Fig. 9a), a single andlarge direct-discharge current pulse Id occurred duringthe impulse voltage wavefront, at a voltage of only 60%of the impulse voltage peak value. This discharge depos-its sufficient charge on the insulation surface to build upthe surface field strength, to such an intensity that thefield at the point electrode is substantially reduced andfurther discharges inhibited.

During the second impulse test (Fig. 9b), the direct dis-charge occurs later on the impulse voltage wavefront,and is of smaller magnitude than that produced during

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987

the first impulse test. Charge is again deposited on theinsulation surface and this increases the insulationsurface field strength (Fig. 9b) above the level produced

2 0 T

20mA10 ps fi rst

impulse

2 0 0 M A2 0 0 MS

10us second1 1 1 impulse

200\JS

Id

Ir

1

1 I , t l l M

j

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20mA

200 p A2 0 0 M S

thirdimpulse

Fig. 9 Variation of discharge with repeated positive impulses ofHkVp

Point-insulation gap = 5.0 mm, probe gap = 2.0 mm

by the first impulse voltage. The result again is thatfurther direct discharges are inhibited. However, the elec-tric field strength at the insulation surface is now of sucha magnitude that, during the wavetail of the impulse, andwhen the impulse voltage has decreased to below 20% ofits peak level, multiple reverse discharges occur, Ir. Thesereduce the field strength at the insulation surface, asdescribed in the preceding text, and the field profile(Fig. 9b) exhibits a characteristic crater shape.

On the third impulse test, Fig. 9c, the direct dischargeoccurs during the period when the impulse voltage waveis near to its peak value. This direct discharge is smallerthan that produced during the previous test, and verymuch less than that produced during the first impulsevoltage test. Again, during the wavetail of this impulse, aseries of reverse discharges occur at a time almost thesame as that observed during the second impulse test,Fig. 9b. The field strength intensities of Figs. 9b and 9cexhibit only small differences which indicate that thesurface charge removed by the reverse discharges of oneimpulse are replaced by the surface charges depositedduring the direct discharge of the following impulse;charge balance is taking place.

3.5 Effect of repeated impulses of increasingmagnitude

A further phenomenon of practical importance is themanner in which the deposited surface charge varies withrepeated impulses of increasing magnitude. Furthermore,how does the surface charge stored under these condi-tions influence the final flashover voltage? To examinethis phenomenon, a series of impulse voltage tests werecarried out on a system similar to the one just describedbut with the gap increased to 10 mm. The results of thistest sequence, Fig. 10, indicated that, at the lower impulsevoltage levels, and with increasing impulse voltage ampli-tudes of 12.5, 17.5 and 27 kVp the surface field intensitysteadily increased. At a voltage level of 40 kVp (Fig. lOd),the field intensity has increased by only 10% above thelevel obtained at 27 kVp, Fig. 10c. It should be notedhere that 40 kVp is the single impulse flashover voltage.However, the shape of the surface field profile changedsubstantially; the simple conical formation, formed at thelower voltages, changed into the crater-like formation atthe higher voltages, owing to the large reverse dischargesat these voltage levels. If the reverse-discharge currentsare substantial, the resulting surface charge directly underthe point electrode is almost reduced to zero, and theelectrometer probe can only detect this zero as a substan-tial reduction in the surface field strength.

When the impulse voltage was increased to 45 kVp thepolycarbonate sample flashed over (Fig. lOe). The shapeof the surface field profile, although being considerablydistorted on the right hand side in the region of flashover,had otherwise remained substantially the same as in theprevious test at 40 kVp. Following flashover at 45 kVp afurther impulse at 40 kVp was carried out. The effectobserved (Fig. 10/) is that the charge stabilisation mecha-nism, similar to that seen in the previous test sequence, isin action, and the surface field intensity is starting toapproximate to the surface field intensity observed in theprevious 40 kVp impulse test (Fig. iOd).

3.6 Effect of a single impulse flashoverWhen a single impulse voltage, with a magnitude abovethe flashover level of the system, was applied to a freshcharge-free insulation sample, the resulting surface fieldprofile was very similar to that shown in Fig. lOe. Theresidual field and dust figures indicate a channel of nearlyzero charge, the flashover path, which is bounded onthree sides by a region of high surface charge.

4 Discussion

4.1 Positive impulse characteristicsThe results presented have shown that, in a point-insulation-plane system, the discharge current pattern is

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987 731

complex and consists of both direct and reverse currentpulses. However, the essential characteristics of the dis-charge can be explained in terms of relatively simple dia-grams, Figs. 11 and 12.

2 0 T

20 10040 60 80d istanc e, m m

Fig. 10voltages

Point-insulation gap = 10.0 mm, probe gap = 5.5 mm

Variation of discharge with repeated and increasing impulse

Under positive impulse conditions, the first indicationof discharge activity is seen when a single short-durationcurrent pulse occurs (Fig. lib). This initial current pulseis presumably due to a streamer, which propagates fromthe point electrode to the insulation surface, and therebranches into a number of surface streamers. Because thesurface of the insulation sample is free of charge prior tothe discharge, the branches of the surface streamer willhave equal opportunity to spread symmetrically away

738

from the point of impact. The surface streamer propa-gates along the insulation surface, until the field at thestreamer tips is no longer sufficient for further propaga-tion.

positive

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100m A200pA

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Fig. 11 Diagrammatic representation of discharge current pulsesunder positive impulse conditionsa 9 = applied impulseb 9 = inception voltagec 9 = above inception voltaged 9 = near to flashover voltage

The magnitude of the short-duration current pulsedepends primarily on the instantaneous value of theimpulse voltage wave at which it occurs. The greater thisvoltage the greater the area of surface charge. The wave-front duration of this current pulse was always very shortand the wavetail duration relatively long. A detailedexamination of this current pulse on an expanded timescale was not undertaken. However, in similar investiga-tions on a point-plane electrode system [17, 18] it wasshown that the pulse waveshape is complex and is theresult of several separate mechanisms.

If the voltage is raised to a slightly higher level thaninception, then, following the first short-duration currentpulse, there then follows a number of similar currentpulses, Fig. lie, whose number, magnitude and timespacing is very variable. The effect of these pulses is toextend the simple filamentary discharge pattern due tothe first short-duration current pulse, in a series ofbranched discharge patterns at the periphery of the firstdischarge pattern.

If the voltage is raised to a sufficient level, then, inaddition to the short-duration current pulses, thereappears a much larger current pulse (Fig. lid). The effectof this pulse is to greatly extend the radius of surfacedischarge in what might be a number of successive andoverlapping steps; the discharge is rapidly progressing toa flashover condition.

As the impulse voltage magnitude is increased, thedeposited charge on the insulation surface, due to direct

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987

discharges, will increase, resulting in an increased surfacefield. The effect of this on reverse discharges is that, as thesurface field increases, the point in time at which thereverse discharges commence will move closer to theimpulse wave crest (Figs, lie and lid). However, theinstantaneous value of the impulse voltage, at which thereverse discharges cease, will be constant; it is governedby the electrode geometry. The reverse discharges willoccur for a longer period and a larger amount of surfacecharge will be removed. The overall result of this actionis that following an impulse, the surface field intensity isalways governed by the reverse-discharge process, andnot the magnitude of the applied impulse. It must be keptin mind, however, that the area over which the surfacecharge is deposited will increase with the impulse voltagemagnitude.

4.2 Negative impulse characteristicsUnder negative impulse voltage conditions, the first indi-cation of discharge activity takes the form of a singleshort-duration current pulse which may be followed by aseries of very much smaller pulses (Fig. 12b). This occurs

negative

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10m A20

mA'd

Fig. 12 Diagrammatic representation of discharge current pulsesunder negative impulse conditionsa 9 = applied impulseb 9 = inception voltagec 9 = above inception voltaged 9 = near to flashover voltage

at a slightly lower voltage than under positive impulseconditions and is of much smaller amplitude, about 15%.The surface dust figure produced by this current pulse isin the form of a diffuse circular area of small diameter.

If the impulse voltage is raised to a slightly higherlevel, the limited short-duration current pulse is imme-diately followed by a current pulse having a muchsmaller amplitude, but much longer duration. This pulsehas characteristics indicating a glow discharge. Followingimmediately after this glow discharge pulse are a numberof extremely short current pulses (Fig. 12c). These pulseshave the characteristics of Trichel Pulses [19].

With further increase of impulse voltage the amplitudeof the initial short-duration current pulse increases, boththe amplitude and duration of the glow-type dischargepulse increase, and the Trichel pulses are delayed furtherdown the wavetail (Fig. I2d).

The amplitude of the Trichel pulses observed wassmall and their number increased with gap increase. Inlarge gaps, the effect of deposited surface charge has lesseffect on the field at the point electrode, and, in addition,large gaps are more capable of storing positive charge inthe vicinity of the point electrode; a condition essentialfor the production of Trichel pulses [19]. Accordingly, inthe smallest gap tested, no Trichel pulses were presentand only the long-duration current pulses were observed.The Trichel pulses vary in time spacing, depending on theinstantaneous voltage as might be expected from thistype of discharge activity.

Unlike the positive impulse condition when the dis-charge ceases at the crest of the impulse voltage, undernegative impulse conditions, the direct discharge con-tinues part way down the wavetail (Fig. I2d).

When the direct discharge activity has ceased, andwhen the voltage on the impulse wavetail has decreasedsubstantially, the net field at the point electrode becomesdominantly produced by the surface charge, and reversedischarges occur. At this point in time, reverse currentpulses are produced, Fig. 12c and d, which have wave-shapes similar to those produced by direct dischargesunder positive impulse conditions. That is they have alarge magnitude, almost equal to that of the direct dis-charge current amplitude and are of short duration. Inaddition, they occur widely separated in time and haverandom magnitudes (Fig. 12c). The resulting dust figureof the surface discharge shows that the reverse dischargesproduce a filamentary star-like area of zero charge,superimposed on the preceding diffuse circular figure,produced by the direct discharges.

The reverse discharges remove the central area ofdeposited charge in a similar manner to that seen underpositive-polarity conditions. The central area of theresidual field is also similar to that obtained underpositive-polarity impulse. However, the boundary of theresidual field is much more pronounced than that underpositive impulse polarity, and clearly indicates the welldefined edges of the dust figure. Again, under increasingvoltage conditions, although the boundary of the residualsurface field may increase due to the voltage level, itsamplitude will reach a maximum level at which it willremain, being governed by the reverse discharges.

5 Conclusions

Investigations into the mechanisms governing the pro-duction of charge in a point-plane system and its storageon an insulation surface have been described. It wasfound that, on the rising section of an applied impulsewave, of either polarity, discharges would occur, leadingto the production of large surface charge concentrationsand resulting high surface field strengths. The dischargecurrent pulses producing the surface charge were exam-ined and explained in terms of point-discharge pheno-mena, modified to take account of the modified fieldconditions caused by the surface charge.

Under both positive- and negative-polarity impulsevoltages, the surface field was sufficient to producereverse discharges when the applied impulse started todecay. This reverse-discharge mechanism had the effect of

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987 739

limiting the maximum surface field, following an impulseto a value governed only by the electrode geometry.

When repeated impulses of increasing magnitude wereapplied to the system, each impulse created dischargeswhich increased the surface charge stored and theresulting surface field intensity. The result of this actionwas seen to increase the flashover level to a value wellabove that which would have caused flashover of anuncharged system.

For the system and impulse conditions used in theinvestigations,' the effect of surface charge was alwaystowards increasing the flashover voltage; the surface fielddue to charge acts in opposition to the applied voltagefield. It will be appreciated, however, that under otherconditions, such as oscillatory impulse waves or impulsevoltage polarity reversal, this would not be the case, andthe surface field would aid the applied voltage field,leading to a reduction of flashover voltage.

6 Acknowledgments

The authors would like to thank Prof. C.B. Cooper of theElectrical Engineering and Electronics Department,UMIST, for his interest in this work, and are grateful forthe facilities provided in the UMIST Power SystemsLaboratories. They would also like to thank Mr. F.A.Camarena for his assistance in the computer presentationof the surface field results.

7 References

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740 IEE PROCEEDINGS, Vol. 134, Pt. A, No. 9, NOVEMBER 1987