SCIENCE

Behaviour of pressboard-oil insulation underalternating voltages of industrial frequency in a

nonuniform electric fieldProf. P.N. Nikolopoulos, Dr. Ing., and D.N. Diamantopoulos, Ph.D.

Indexing terms: Insulators and insulation, Breakdown and gas discharges

Abstract: The behaviour of pressboard plates of 1 mm and 2 mm thicknesses immersed in mineral oil andstressed in the highly inhomogeneous electric field of a rod-plane gap under alternating voltages of theindustrial frequency of 50 Hz is described. First, the partial discharges inception and breakdown voltagesof the plates, measured under continuously increasing applied voltages, are given. Then the characteristicphases of the discharge development under applied voltages of each time-constant effective value aredescribed in detail and conclusions are drawn as to the position in the test object, and the kind of partialdischarges which appear with time. Also, the measured values of the discharge quantities are given. Finally,the measured relationships between life duration of the insulating plates and the relevant applied voltagesof constant value are presented and some conclusions are drawn on the overall behaviour of the plates.

1 Introduction

Pressboard plates are made of compressed paper impregnatedwith an appropriate resin, for example phenol, and then driedout. This material is, therefore, inhomogeneous; and also con-tains a relatively small proportion of gas bubbles. Its propertiesmust be a combination of the properties of the materials ofwhich it conGists.

Among other applications, pressboard is used inthe solid insulation of large power transformers andgenerators, thus representing a material of great importancefrom a practical point of view. For such an inhomogeneousmaterial, on the other hand, information is lacking on itsbehaviour under various types of test voltage. For this reasona research programme has been in progress for several years,aiming at the investigation of its behaviour; the materialused was in the form of plates. Up to now, results have beengained from investigations under the standardised impulsevoltage of 1.2/50 [1, 2] as well as under the alternatingvoltage of the industrial frequency of 50 Hz [3]. This paperrefers to the latter case.

2 Test objects, test arrangements and test procedure

2.1 Test objectsThe pressboard investigated can be combined with trans-former oil. Plates of this material have been investigated withthicknesses of 1 mm and 2 mm. The test object consisted ofa rod/plane gap; the two electrodes of which were in contactwith the insulating (pressboard) plates on either side of thelatter (Fig. I). Comparative measurements have also beencarried out on test objects with plane/plane gaps. The testobject was put into mineral oil of excellent quality; it hadbeen distilled to be used in power transformers, and wasfree of humidity.

2.2 Test arrangementsTwo test arrangements have been used in the investigationsas follows:

2.2.1 Measurement of discharge quantites (TA1) Fig. 2 showsthe test arrangement 1 (TA1) for the measurement of thevarious discharge quantities. As the range of frequencies of

Paper 2439 A, received 26th March 1982Prof. Nikolopoulos is, and Dr. Diamantopoulos was formerly, withthe Laboratory on High-Voltage Engineering and Measurement Tech-niques of the National Technical University, Athens 147, Greece

the latter falls into the range of frequencies of stray signals,the test object had to be shielded. Moreover, it was necessarythat the test arrangement itself did not produce similar straysignals. To meet these requirements, the test object was putinto a special screening room with an attenuation of 80 to100 dB, up to a frequency of 1000 MHz. Also, to get ridof stray signals coming from the network and cut theharmonics of the mains voltage (so that the integratingbridge could be balanced [4, 5]), two proper filters were used.Further measures consisted in the use of a discharge-freetest transformer and high-voltage capacitors, the establishmentof the high-voltage connections by means of tubes and doubletoroids, the use of double-screened measuring cables (in a care-fully studied layout to avoid induced voltages) etc.

The following instruments were chosen for the detectionof partial discharges:

(a) a device measuring the mean rectified current Imp andthe maximal peak value Up of the voltage impulses of thepartial discharges, with a wide-band circuit, the upper band-width frequency of which could be limited, by means ofadequate filters, to 3 MHz or 400 kHz

(b) an integrating bridge, measuring the partial-dischargeinception voltage Uh the (total) electric charge Qm per half-cycle and the energy W per cycle of the partial discharges[3,4,5]

(c) a discharge detector, measuring the peak values of theelectric charge q of the partial discharges at a narrow frequencyband of 10 kHz adjusted at 500 kHz [3, 6, 7]

(d) a radio-interference meter (quasipeak meter), measuringthe radio-interference voltage Uc at a narrow frequency bandof 9 kHz; the mean frequency of which was adjustable be-tween 0.15 and 1.9MHz [3,8,9, 10]

(e) single- and double-beam oscilloscopes of a high upperbandwidth frequency.

2.2.2 Measurement of life duration (TA2) In test arrangement2 (TA2), the life duration tb of the test objects was measuredas a function of the applied voltage Ua. As tb is a quantityof a statistical nature, ten test objects were connected in paralleland fed by the same test transformer, the voltage of whichwas kept constant throughout a test, i.e. until the last of thetest objects had broken down. The breakdown of any one ofthe test objects caused its corresponding high-voltage switchto open automatically, thus disconnecting it from the testcircuit, while a clock showed the relevant time to breakdown(life duration) tb.

IEEPROC, Vol. 130, Pt. A, No. 3, MAY 1983 145

2.3 Test procedureFrom various possible test procedures to be applied withTA1, it was decided to apply the same test procedure withTA2, i.e. by constant test voltages. A preselected voltage wasrapidly applied to the test object and kept constant untilthe breakdown of the latter, which, for the voltages selected,could take place after some minutes or hours. Meanwhile,

( i )

(ii)

2 dia

2 dia

(i)

1or 2

(iii) rn

50dia

70 dia

Fig. 1 Test-object arrangements

a For the measurement of the discharge quantitiesb For the measurement of the life duration(i) Rod electrode, (ii) pressboard plate, (iii) plane electrode, (iv) guard-ring electrode, (v) insulation between (iii) and (iv)Lengths in millimetres.

Table 1: Partial-discharge inception and breakdown voltages ofthe insulating plates

Thickness ofthe plates, mm

12

Voltage (kV,

Inception,

121 (0.30)13.51 (0.17)

effective value)

Breakdown,

282 (0.70)433 (0.54)

Coefficientkb

2.343.19

1 Rate of rise of the applied voltage: appr. 0.1 kV/s.2 Rate of rise of the applied voltage: 1 kV/s.3 Rate of rise of the applied voltage: 1.5 kV/s.

Fig. 2 Test arrangement for the measurement of the dischargequantities characterising the discharges (TA 1)

1 Mains supply, 2 voltage stabiliser, 3 circuit breaker, 4 regulatingtransformer, 5 and 6 filters, 7 high-voltage test transformer, 8 testobject with guard-ring electrode, 9 and 10 high-voltage standardcapacitors, 11 screening room, 12 voltmeter, 13 discharge detector,14 integrating bridge, 15 measurement of the mean rectified current,16 radio interference meter, 17 shunt (for 16), 18 oscilloscope

the readings of one or more of the instruments were takenperiodically, at very short consecutive time intervals of only10s, thus enabling the variation of the corresponding dischargequantities with time to be plotted.

3 Partial-discharge inception and breakdown voltages

Table 1 gives the partial-discharge inception voltage Ut andbreakdown voltage Ub of the insulating plates for boththicknesses of 1 mm and 2 mm. The coefficient k is defined as

k = TT (U)

Ua being the applied test voltage each time. Consequently

for the ratio of the breakdown to partial-discharge inceptionvoltage.

The inception voltage Ut was determined by increasingthe applied voltage very slowly from zero and noticing itsvalue, by which a reading of the most sensitive of the measuringinstruments first appeared; the procedure was repeated manytimes (each time with a new plate) and a mean value wasfinally taken. A similar procedure was followed for thedetermination of Ub.

It can be seen that, for the two thicknesses of the plates,the Uj values do not differ very much; their ratio being equalto only 1.13. This, as reported in the following, is due tothe fact that the predischarges start in the oil (which hasa lower electric strength than the plates), where the maximumfield strength is almost the same by both thicknesses, owingto the very sharp edges of the rod tip. The values of thebreakdown voltage Ub, on the other hand, have a ratio of

146 IEE PROC, Vol. 130, Pt. A, No. 3, MA Y 1983

43 :28 = 1.54; closer to the value 2 of the ratio of the twothicknesses. Furthermore, the coefficient kb is relativelyhigh this being due to the strong inhomogeneity of theelectric field combined with the difference of electric strengthsof the oil and the plates. Each of the figures given in Table1 in parentheses represents the ratio of the voltage value ofthe same position in the Table to the voltages of 40 kV or80 kV, which, according to the data given by the manufacturerof the plates, is their minimum breakdown voltage perpendicularto their layers,t assuming that this voltage increases pro-portionally to the thickness (by a constant breakdown fieldstrength of 40 kV/cm), which of course is only an approxi-mation. It can be seen that the breakdown voltages measuredhere are substantially lower than the corresponding ones ina uniform field, the difference being higher for plates of 2 mm.

4 Measurement of the discharge quantities

4.1 Discharge-development by constant test voltageNumerous series of measurements have been carried outusing selected values of the coefficient k, i.e. also of theapplied voltage Ua, as follows:

1 mm plates:k= 1.2, 1.3, 1.4,

Ua(kV)= 14.40, 15.60, 16.80,2 mm plates:

k= 1.3, 1.4, 1.5,Ua(kV)= 17.55, 18.90, 20.25,

It should, first of all, be pointed out that the behaviour of anumber of plates of the same thickness under one and thesame voltage of constant effective value is not always the sameand this results, from plate to plate, in different variationswith time of the corresponding discharge quantities. Thisis due not only to the statistical nature of the dischargemechanism, but also to existing differences on the surfaceor inside the volume of the plates, created during their manu-facture. Nevertheless, the high number of measurementsexecuted indicates that the discharge development consists,in principle, of several characteristic phases. In a certain case

1.5,18.00,

1.6,21.60,

1.6,19.20,

1.7,22.95,

2.024.00

2.027.00

B2

-2b2 lb3

t . min

Fig. 3 Typical variations with time of mean rectified current ofpartial discharges under applied alternating voltages of each timeconstant effective value

1, 2, 3 Curves referring to three different applied voltages [/„, > Uai

1, . . . . IV Typical phases of the development of the partial dischargestb\»• • • > fb3 Times to breakdown

tmeasured in a test-gap of 3 mm with a uniform electric field andsurrounded by mineral oil

either some or all of these phases may be present. We shalldescribe them briefly by means of Fig. 3; showing typicalvariations with the time of one selected discharge quantity,namely the rectified mean current Imp. In this Figure weactually have three curves, numbered 1, 2 and 3, correspondingto three effective values Ual, Ua2 and Ua3 of the appliedvoltage, where Ual > Ua2 > Ua3. The parts l a and l b , and2O and 2b, represent possible variations of the relevant partsof curves 1 and 2, respectively.

Phase I: Some 20-30s after the applied voltage has reachedits end value strong and irregular discharges appear in theoil in the vicinity of the rod electrode, causing high readingsof the instruments. The impulses corresponding to thesedischarges fall in the regions of the crest value of the appliedvoltage. Also of interest is the figure appearing on the screenof the integrating bridge (Fig. 4a), which cannot be balanced.*One can see the strong oil impulses on the crest values ofthe applied voltage, extending off the surface of the screen.

Fig. 4 Typical patterns of figures appearing on the screen of theintegrating bridge and corresponding to various characteristic phasesof discharge development

a Partial discharges in the surrounding oil (Phase I)b Partial discharges on the surface of the insulating plates (Phase II)c, d Volume partial discharges within the plates (Phase III)e Strong inner partial discharges (formation of discharge channels,

end of phase III)/ Mixed partial discharges (Phase IV)

Phase II: Here we have oil discharges on the surface ofthe insulating plates (on the side of the rod electrode); thecorresponding impulses falling not only on the crest but alsoat intermediate values of the test voltage. During this phase(in which the readings of the instruments vary continuouslybetween some maximum and minimum values, and the imageappearing on the screen of the integrating bridge is as in Fig. 4b)a relatively strong audible noise is present, accompanied bythe production of gas, evidently coming from the phenollayer on the surface of the plates, near the rod tip which hasdeteriorated. This deterioration most probably constitutesa necessary condition for the predischarges to be establishedon the surface of the plates. For voltages corresponding tok > 1.4 and k > 1.6 for plates of 1 mm and 2 mm thicknesses,respectively, this phase remains by continuously increasing

As it is well known [4], the integrating bridge, when balanced underan applied voltage Ua, lower than £/,-, permits the calculation of a lossfactor corresponding to the 'basic' losses of the test object, i.e. thepolarisation and hysterisis losses;at the same time, a horizontal straightline appears on the screen of the bridge, the length of which cor-responds to 2 \J2Ua. If then the applied voltage is further increased,from Ua to Uai > U{, and inner partial discharges are generated in thesolid insulation, near the zero values of the applied voltage, the previousstraight line on the screen is now widened to a parallelogram, as inFig. 4c or Fig. 4d, the height of which is proportional to the electriccharge, fed by the voltage source to the test object per halfcycle,and the area proportional to the energy dissipated by the partialdischarges per cycle.

IEEPROC, Vol. 130, Pt. A, No. 3, MA Y 1983 147

Table 2: Measured values of discharge quantities (maximum and minimum values) in the insulating plates — phase I has been excluded

Plate Appliednumber voltage Ua

kV

'mp. V- A

max.

Plates with a thickness of 1 mm1568

10141517

20212226

31334041

14.4

I C C

16.8

18.0

1.60

1.05

0.456.00

34

Plates with a thickness of 2 mm42434445

46474849

50515253

54555657

i o q1 O.I7

on oczU.zb

o 1 aZl.O

OO QR£.£.MW>

287

4620

601.2

4045

45335560

2.965

1.1541

min.

0.60

0.28

0.301.50

13

4.50.5

191.2

100.730.851.4

174

1619

1.3160.354

IfnpQ-

max.

0.250.940.72

1.821.590.93

3.351.85

24.82.17

8.341.752.28

8.650.73

10.043.9

13.61.28

10.74108

12.512.42532.25

8.1816.53.33

14.13

MA

min.

0.090.360.35

1.560.250.15

2.731.09

10.180.62

6.200.810.61

0.890.093.850.82

3.00.660.520.37

3.071.661.783.05

3.373.41.192.97

Qm,nC

max.

2.509.377.20

18.215.879.30

33.518.5

248.021.7

83.417.522.8

86.57.3

100.439

13612.8

107.4109

125124250325.5

81.816533.3

141.3

min.

0.893.643.47

15.622.501.55

27.310.9

101.86.2

628.16.1

8.90.88

38.58.2

306.645.23.68

30.716.617.830.5

33.73411.929.7

W, juJ

max.

39.5133.192.0

182270158

570.5315

3169360.6

852297.4355

1350155

1700750

2813227

21351730

2480238046005080

1393386011403009

min.

14.047.449.3

15643.115.8

465200

1302105.6

616137.3138

19017

765140

65011892.678

675318340606

598838237568

Op.mV**

max.

40

6800

12

26

123100

17000

30000400

3200500

64002500

980031

54004000

6000900055003700

3507600

332200

min.

1

800

4.8

12

5500

5000

360070

27009

80022

40002115

400

3600280020002000

554800

7.5130

min.

38.50847754

27795162.50

17.501416.5030.67

1116.5023.2715.77

112.17207131.50142

28081.3387

100.50

85.5070.5074.5076.33

60704866.33

Measuring resistance = 60 n

Besides the mean current lmp being measured by means of a separate device, another mean current lmpQ can also be seen in this Table, obtainedby dividing Qm by the duration of a halfcycle.

readings of the instruments until breakdown (curve l a , PhaseII t ) . It is, however, also possible (probably through theformation of some appropriate space charge) that thepredischarges weaken gradually, and finally any dischargeactivity stops completely (curve l b ) .

Phase III: This follows phase II, when fc < 1.4 or 1.6for plates of 1 mm or 2 mm thicknesses, respectively, the tran-sition to phase III from the previous phase being accomplishedeither in a shorter (curve 2a) or longer time (curve 2b). Inthis phase the readings of the instruments remain, forrelatively long time intervals, almost constant and of a ratherlow value; the relevant discharge impulses falling near thezeros of the applied voltage and the screen of the integratingbridge showing a picture as in Fig. Ac or Ad. It is obvious thatwe are dealing with volume partial discharges, taking placein gas bubbles in the solid dielectric [11, 12]. It is importantto note that, when phase III has been introduced and the testvoltage continues to be applied on the test object, a breakdownof the latter will certainly follow. In the course of time, theinner partial discharges become stronger and less numerous,as can be seen clearly on the screen of the integrating bridge(Fig. 4e);and finally breakdown is introduced.

Phase IV: While, using low enough applied voltages phaseIII can last until breakdown (curve 3, phase III2), with possiblyeven a decrease in the readings of the instruments at the end

of phase III, using higher voltages phase IV follows phase III,during which mixed discharges appear again, i.e. inner andsurface discharges in the plates as well as discharges in the oil(see also Fig. Af). The phenomenon becomes unstable and thereadings of the instruments vary continuously betweenrelatively wide limits.

4.2 Time variation and magnitudes of discharge quantitiesIt should be pointed out that the magnitudes of the variousdischarge quantities varied during the discharge developmentby several orders of magnitude; their highest values alwaysappearing during phase I. To give an idea of these magnitudes,some characteristic values (measured maximum and minimumvalues) selected from a large number of measurements arepresented in Table 2. It should be emphasised that phase I isexcluded, i.e. the values refer to phases II, III or IV only. InTable 3, the maximum and minimum values of the samequantities which were measured in the whole investigationare given; phase I is also excluded here.

It can be said that there is a general trend towards anincrease in the magnitudes of the discharge quantities withthe applied voltage. However, a decrease of the magnitudesby increasing voltage is also possible (compare, for example,the Up values for plates 8 and 17), owing to the formation ofonly volume partial discharges. The existence of high values

148 IEE PROC, Vol. 130, Pt. A, No. 3, MA Y1983

Table 3: Measured maximum and minimum values of discharge quantities in the insulating plates— phase I has been excluded

Discharge

quantity

'mpQ. M AQm.nCW,nJ0p**. mV

Plates with thickness = 1 mm

max.

3424.8

2483169

30000

mm.

0.100.090.89

14< 1

Plates with

max.

6532.5

32550809000

thickness = 2 mm

mm.

0.350.090.88

177.5

Measuring resistance = 60 fl

of the discharge quantities is most probably due to strongdischarges during phase II; this sometimes (especially withlow Ua values) leads to an increase of the life duration tb ascan be seen in the cases of plates 1 and 5 or 6.

5 Optical examination of the stressed plates

To find out if there were any discharge elements visible,the plates subjected to breakdown were examined opticallyand photographs of them were taken. It is, in fact, possibleto recognise discharge channels, not only on the surface of theplates (to the side and in the vicinity of the rod electrode), but,for plates of 1 mm thickness, also inside them if the side ofeach plate opposite to the rod is illuminated by means ofsome sufficiently strong light source.

In most cases discharge channels can be seen, startingfrom the rod tip and running on the surface of the plates fora certain distance, then entering the plate and continuingperpendicularly as far as the plate electrode. For example,in Fig. 5a, one can see at least three channels (numbered3, 4, 5) starting at the rod tip and coverging to the pointwhere they entered the plate. At this point (numbered 2 inthe Figure X) a part of the solid material is missing, havingbeen mechanically broken at the breakdown instant. InFig. 5 b no channel whatsoever connecting the rod tip withthe breakdown trace can be seen on the surface or in thevolume of the relevant plate; it seems that here the breakdown

channel was completely in the oil. Such cases are seldomand it is more probable that a discharge channel runs partiallyon the surface of the plate and partially in the oil. Aninteresting case can, finally, be seen in Fig. 5c. Two channels(numbered 3 and 4) have entered the plate exactly under-neath the rod tip, run inside the plate parallel to its surface,met again at point 2 (at a distance of 10.1 mm from the rodtip) and struck downwards towards the plane electrode.

The distance lb of the breakdown trace from the rod tipdoes not show any dependency on certain variables. It can,using the same applied voltage Ua, vary from plate to plate;this apparently depends on some local conditions which mayappear in each case.

Nevertheless, a general trend can be seen of a proportionalitybetween lb (in mm) and Ua (in kV), which is

mm kV(2)

Whereas the coefficient 0.28mm/kV was found to be valid,on average, for both thicknesses of the plates, the pro-portionality was more evident for the plates of 1 mm. In anycase, as the voltages applied here attained values of up to27 kV, lb values of up to an average of 8 mm should beexpected; in fact, values even higher than 10 mm were measured(compare with Fig. 5 c).

5mm 5mm 5mm

Fig. 5 Pictures of discharge channels in plates in which breakdown has occurred

a, b, c Ua= 14.4 kV {k = 1.2), d = 1 mm1 Trace of rod electrode, 2 trace of breakdown,3, 4, 5 discharge channels

t Actually the diameter of the hole at the breakdown trace, unlesssome mechanical damage caused by the breakdown has taken place,has a diameter of only 0.1 mm. To make it more visible, it was widenedmechanically.

IEEPROC, Vol. 130, Pt. A, No. 3, MAY 1983 149

6 Measurements in uniform electric fields

By substituting the usual test object with one of a uniformelectric field and applying the same test voltages as above,discharge activity in the oil was completely missing; con-sequently, phases I and II of the discharge development werealso missing. Volume partial discharges only appeared in theplates of a relatively low but constant magnitude; the cor-responding impulses falling near the zeros of the appliedvoltage. This means that the discharge development wasdirectly introduced with phase III. Under these circumstances,the life duration for a given applied voltage was much longerthan that corresponding to the usual test object.

7 Life duration as function of applied voltage

This function was first investigated with TA2. It was foundthat the mean life duration tb depends on the appliedvoltage Ua (effective value) according to the well knownexponential law [13, 14]:

tb = to (3)

f0, Uo being constants depending on the characteristics ofthe materials involved and the geometry of the test objects.Taking into account that pressboard is an inhomogeneousmaterial, this is a very interesting result. For the twothicknesses of the plates, we have:*

for plates of 1 mm

t'bl = 5 . 8 7 8 x l O

for plates of 2 mm

t'b2 =_ i n8 ,,-0523 Ua

(4a)

(4b)

The last two functions are represented graphically in thesemilogarithmic plot of Fig. 6 (straight lines l' and 2').

Now, as measurements of life durations were also actuallydone with TA1, two more functions, similar to the above,have resulted as follows:

for plates of 1 mm

tbl = 1.85 x 10s e-°

3MUa

for plates of 2 mm

tb2 = 5.29 x io 3 e- O i 8 7 £ / a

(5a)

(5b)

these are also plotted in Fig. 6.Every two characteristics for the same plate thickness,

referring to the two test arrangements, intersect at a point;points Alt A2 for plates with thicknesses lmm and 2mm,

Respectively. The relevant voltages UaAl and UaA2 (in kV),for both thicknesses of the plates, are approximately 2/3of the corresponding breakdown voltages Ub. For voltages,lower than 6 ^ and U^i, the life durations t'b (in minutes)measured in TA2 were longer than tb (in min) measured withTA1. It is, furthermore, very important to note that thecharacteristics referring to TA2 seem to be only valid forvoltages lower or equal to those of the points of intersection,being truncated at these points and then, for higher voltages,coinciding rather with the characteristics referring to TA1.

This diverse behaviour in the two test arrangements can beattributed to the only existing difference in the two TAs,i.e. the magnitude of the impedance of the test-voltage sourcewhich was substantially lower in TA1 than in TA2. Hence,at least according to the existing results, for relatively low

#To be able to distinguish easily between quantities referring to oneor other of the test arrangements, we designate fj, and t'b as the lifedurations measured with TA1 and TA2, respectively.

applied voltages, the life duration increases, by increasingthe impedance of the test-voltage source. This could meanthat, for lower voltages, the discharge development is dueto processes (for example, the deterioration of the surfaceof the plates being a thermal-chemical process) demandinga certain amount of energy which has to be delivered by thevoltage source. However, by increasing the voltage, the purelyelectrical processes sustained by the electric field attaingreater importance, so that the role of the impedance of thevoltage source becomes less decisive, and, in this way, itcould also explain why the characteristics from both TAscoincide when the voltage is sufficiently high. In any case it isobvious that, for practical purposes, only the characteristicsreferring to TA1 and expressed by eqns. 5 should be considered,giving, for lower voltages, shorter life durations. A moredetailed investigation of the various factors affecting thelife duration, among them also the impedance of the voltagesource, is planned for the near future.

~U 6 810 2| | U 6 8102 2t b A 2

6 8103 2 A 6 810^

Fig. 6 Life duration tb against applied voltage Ua (effective value) ofthe pressboard plates

1 d= lmm, TA1; 2 d = 2mm, TA1;l' d = l m m , TA2; 2' d = 2 mm, TA2

8 Conclusions

The results of the investigations reported here reveal thatpressboard is a very good insulating material, suitable forhigh-voltage applications; it shows some remarkablecharacteristics, for example, high enough electric strengthand behaviour against stresses similar to those of homogeneousmaterials. As it is foreseen to co-operate with mineral oil,it is of great importance that its surface resistivity againstpartial discharges should be increased as much as possible;this is necessary for two reasons, first, that the dischargesalways start in the oil (being electrically less rigid than thepressboard), as, of course, near the solid insulation thereare metallic pieces with small radii of curvature under highpotential, and, secondly, because it seems that thedeteriorationof the surface of the pressboard is in most cases a necessarycondition in order that dangerous partial discharges will appearin its volume, causing the breakdown of the material.

9 References

1 SAKKAS, G.C.: 'Partial discharges in insulating materials underimpulse voltages'. Ph.D. thesis, National Technical University,Athens, Greece, 1979 (in Greek)

2 NIKOLOPOULpS, P.N., and SAKKAS, G.C.: 'Der Durchschlagvon Haxtpapier in Ql unter Stosspannung' (To be published)

3 DIAMANTOPOULOS, El N : 'Partial discharges in inhomogeneousinsulations under alternating voltages of industrial frequency!Ph.D. thesis, National Technical University, Athens, Greece, 1981(in Greek)

4 DAKIN, T.W., and MALINARIC.P.J.: 'A capacitance bridge methodfor the measurement of integrated corona charge transfer and powerloss per cycle', Trans. Amer. Inst. Elec. Engrs., 1960, 79, pt. Ill,pp. 648-653

150 IEE PROC, Vol. 130, Pt. A, No. 3, MAY 1983

5 BOEK, W.: 'Verfahren zur Ladung- und Leistungsmessung beiKoronaentladungen an Isolierstoffen', ETZ-A, 1963, 84, pp. 752—756

6 MOLE, G.: 'Design and performance of a portable A.C. dischargedetector'. Electrical Research Association, Report V/T115,CIGR£,Paper 105,1952

7 MOLE, G.: 'ERA discharge detector, model III'. F.C. Robinsonand Partners Ltd., Cheadle

8 'Methods of measuring radio noise' (NEMA Publication 107, NewYork, 1964)

9 'Vorschriften fur Funkstor-Messgerate'. VDE 0876/12.55

10 HYLTEN CAVALLIUS, N., and FRYXELL, J.: 'Corona in powertransformers and associated test-problems'. CIGRE, Paper 104,1962

11 KIND, D.: 'Grundlagen der Messeinrichtungen fur Korona-Iso-lationspriifungen', ETZ-A, 1963, 84, pp. 781-787

12 GEM ANT, A., and PHILIPPOFF.: 'Die Funkenstrecke mit Vorkon-densator'.Zr. F. Techn. Physik, 1932, 13, pp. 425-430

13 BUI AI, LACOSTE, R., GRUYETAND, A.,andGALAND,J.: 'Etudedes phenomenes de decharges partielles', Rev. Gen. Electr., 1971,80, pp. 229-235

14 KURAHASHI, A.: 'Behavior of oil corona and damage of trans-former insulation', IEEE Trans., 1971, PAS-90, pp. 2330-2338

ErratumJONES, N.B., and LAGO, P.J.A.: 'Spectral analysis and theinterference EMG', 1982, 129, (9), pp. 673-678:

The authors wish to make the following correction:

In Fig. 7, p. 676, Figs. 1b and 1c should be transposed butthe caption kept the same, as follows:

Fig. 7 Models of action potentials, after Maranzana, Fingini andFabro [22]

a Normal b Myopathic c Initial neurogenic

ETC 101A

IEEPROC, Vol. 130, Pt. A, No. 3, MAY 1983 151