1002 IEEE Transactions on Electrical Insulation Vol. 28 No. 6, December 1993

The Effect of the Definition Used in Measuring Partial Discharge

Inception Voltages

M. Pompili, C. Mazzetti, M. Libotte Dept . Electrical Engineering, University of Rome,

Italy

and E. 0. Forster

Dept . Physics, Rutgers University, Newark N J

ABSTRACT At present several definitions of partial discharge inception voltage (PDIV) have been advanced by various interested groups. To establish the value of each of these definitions, four prac- tical electrical insulating fluids were selected and their PDIV were determined according to three of these different specifica- tions. The results of this study are presented and they suggest that the use of PDIV as a quality criterion generally is not warranted. The significance of these findings is discussed.

1. INTRODUCTION

HE detection and measurement of partial discharges Tw, is difficult because P D are random events, the temporal characteristics of which are not well known. In principle, PD are manifestations of some limited charge displacement which occurs within a void or low-density region located somewhere in the dielectric [l]. The charge displacement is believed t o be the result of an electric field induced ionization process within the void. The random- ness of this process can be attributed to either a distri- bution of voids which randomly ionize or to a sequence of ionizations within the same void. In either case the time scale in which such events occur are believed to be in the range of 1 to 50 ns [2].

Little is known about the size, shape and number of these voids, their location with respect to the electrodes,

and the local field conditions that could induce such ion- ization processes in solid dielectrics or mixtures of solids with liquid or gaseous dielectrics. The situation is much simpler when dealing with liquids and still simpler in gases. It is, therefore, not surprising that much more is known about PD in gases than in any other environ- ment [3]. The reason for this simplification resides most likely in the fact that both liquids and gases, in contrast to solids, conform well t o the metal surface. The lack of conformity leads then to the formation of voids a t the interface where the local electric field is highest so that ionization of these voids becomes highly probable [4].

Conventional detection techniques, commonly employed in the study of these events, operate on a time scale of 1 to 10 ps [5]. The information produced by such equip- ment does not reflect directly on individual discharges but might rather indicate an envelope over a sequence of PD. Further difficulties arise in the calibration of such equip- ment and in their inability t o detect P D of amplitudes

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IEEE nansactions on Electrical Insulation Vol. 28 No. 6, December 1003 1003

liquid Density p Permitt. tan6 rc/cm3 * cSt c’ at 1 kHz low4

< 0.1 pC [6]. Finally, one has to realize that the P D measurements are commonly done under ac conditions which favor space charge generation and hence local field distortion. The assumption that the contributions to the total P D count come equally from the positive and neg- ative voltage cycles, particularly under nonuniform field conditions, is difficult to substantiate.

Vb ac, kV *

For the above reasons it is very difficult, if not im- possible, to reproduce P D amplitude distributions [7,8]. These experimental difficulties have been recognized by various research groups, notably CIGRE Study Group 15 and IEC Technical Committee 10. These groups looked for means to evaluate the tendency of the recently intro- duced new transformer and capacitor fluids towards P D development [9]. In this effort investigators were guided by the idea of measuring the voltage a t which the first P D was detected. At this voltage, no steady regime of P D activity could be observed. A steady regime usual- ly could be detected a t somewhat higher voltages. The question then arose about the definition of such a regime. For example in [9] this condition is defined as ‘. . . when discharges with a magnitude > 500 pC occur’. It is fur- ther stated that ‘. . . under these conditions, the average number of discharges per minute is 10 to 20 . . .’. Actu- ally, under an applied ac voltage producing P D of such high amplitude, hundreds of P D of smaller amplitude may be detected. This definition of the inception voltage ap- peared to be not workable and other definitions were ad- vanced. Three of these definitions are compared here and the results they produce are discussed. In the next Sec- tion the experimental conditions used in this investigation are described briefly. In Section 3 the results so obtained are presented. In the last Section the significance of these results is discussed and some conclusions are drawn.

2. EXPERIMENTAL

The liquids used in this study included samples of the following oils: UGILEC C-101, a mixture of mono and dibenzyl toluene (MDBT) produced by Prodelec (France), Baysilone M-50, a poly(pheny1 methyl siloxane) (PPMS), produced by Bayer and two experimental products XAD and HT-40, both perfluoro polyethers (PFPE) of 800 and 2000 molecular weight, respectively. These oils were used as received without further purification. The basic phys- ical and electric properties of these four fluids are sum- marized in Table 1; the parameters E ’ , tan6 and ac break- down were measured as reported in [lo].

A detailed description of the experimental setup used in the detection and measurement of P D in pure liquids is given in [7].

Table 1. Physical and electrical properties of the test fluids.

V I

123% I 23% I Ugilec C-101 I 1.006 I 6.5 I 2.650 I 4.4 I 74 f 4

HT-40 I 1.860 I 50 I 2.082 I 1.7 I 64 f 6 * IEC 156. Vb is the breakdown voltage, p is the viscosity.

The cell used for P D measurements is similar to the one described by Fallou et al. [9] and consisted of a cylindrical vessel made of Plexiglasm. A sharp steel needle with radius of curvature of - 30 p m and a stainless steel ball of 12.5 m m diameter were used as electrodes.

The detection and measuring circuit used for detecting P D signals follows IEC Standard 270, in which the mea- suring impedance Z with a 150 kHz resonance frequency and a Q factor of 0.4, was inserted in series with a cou- pling capacitor of 150 pF. The test object was put in par- allel to the above branch and to a 0.22/60 kV discharge- free testing transformer. A wide band Biddle amplifier, centered on the above resonance frequency, was used ei- ther as P D signal integrator or as power frequency shunt. The amplified P D signals were then applied to the in- put of a 4096 channel analyzer (MCA by Silena mod. 7423/UHS) so that the digital acquisition and data pro- cessing of signals were achieved easily by using a PC. The MCA allows to discriminate and count the peak values of sequential pulses not closer than 2 ps from each other. The MCA does not give any information on the shape of each pulse.

Each of the 4096 channels of the MCA had a width of 5 0.12 pC so that P D amplitudes of - 450 pC could be counted and stored. All da t a reported in the next Section include background noise, unless stated otherwise.

Before starting any P D measurement, the background noise was determined. I t involved typically < 100 counts of low amplitude pulses that could be stored in the first 11 channels. This irreproducible pattern corresponds to a random charge deposition between 1 and 2 pC.

After background determination, the voltage was raised in 2 kV steps, each lasting 2 or 5 min. Measurements of P D activity lasted 15 s and they were performed either a t the beginning or the end of the voltage step. Experiments had shown that similar results were obtained by either approach. Voltage steps of longer duration produced also

1004 Pompili et al.: The Definition Used in Measuring P D Inception Voltages

Fluid type

similar results except that the total count was higher. Voltage steps of > 2 kV proved less precise in defining the inception voltage. The results obtained under these experimental conditions are reported in the next Section.

Crit. #1 Crit. #2 Crit. #3 PDIV, kV PDIV, kV PDIV, kV

max. amplitude, pC count/l5 s

3. RESULTS

Baysilone XAD

HT-40

Three criteria were used t o define the PDIV. The first one used the lowest voltage a t which a given amplitude (charge) was recorded while the second referred to the lowest voltage a t which a certain P D counts per 15 s oc- curred.The third criterion dealt with the voltage a t which the total P D count and the channel number in which a t least three or more of the highest pulses were stored changed significantly from the preceding voltage step. Ta- ble 2 shows typical measurement results to which these definitions were applied.

30 34 38 34 36 38 28 40 40 42 < 38 44 > 44 42 28 40 40 28 40 40 25

Table 2. MCA record of PD counts. (Ugilec Oil; Gap: 15 mm; 2 kV/step each lasting 5 min; measuring time 15 s; MCA channel width: - 0.12 pC)

v (kV)

24 26 28 30 32 32 30 28 26 24

tot. count

4736 7007 6723 4558

979 146

tot. charge

141 179

81122 325839 452817 455059 304176

160 1232 145

(PC) channel # for

3 + highest P D 12 11

1118 2441 2896 2821 2383 11 17 11

For comparison purposes three levels were selected for the first two criteria proposed for determining the PDIV, namely the voltage required to observe a maximum charge of > 20, 100 and 200 pC and the total counts obtained in 15 s of > 100, 1000 and 10000 counts, respectively. Us- ing these criteria and the procedure outlined below, the results shown in Table 3 were obtained. The background noise was excluded for criteria #1 and #2 to conform with IEC recommendations. In all cases, the distance between the needle and the sphere was 15 m m and the voltage was raised and lowered in the manner described earlier starting a t x 50% of breakdown voltage. Using the MCA, the total number of P D as well as the ampli- tude distribution were recorded and stored in the memory of the PC. This procedure was repeated three times and

42 44 46 k V 36 38 40

Figure 1. Total PD count and total charge as a function of applied voltage in HT-40 (gap: 15 mm; 2 kV step test each lasting 2 min; measuring time 15 s).

the results averaged. All measurements were performed ,at room temperature a t approximately 50% relative hu- midity. The reason for repeating the measurements only three times is based on a compromise between statisti- cal requirements and chemical degradation caused by the PD. Changing the oil after each test does affect repro- ducibility of the data , increases the time requirements and utilizes too large a volume of test oil.

As mentioned in the Introduction, raising the applied voltage caused an increase in the total P D count (using the above mentioned measurement approach). This in- crease was not proportional t o the voltage as shown in Figure 1. In this Figure is shown also the total charge deposited in the liquid a t each voltage. There exists no obvious correlation between these two quantities; both in- crease with increasing voltage but in different, superlinear ways.

Table 3. Partial discharge inception voltages (kV) of test oils using three different definitions.

) > 2 0 l > 1001 > 200 I 100 ~1000~10000~ Ugilec I 28 I 30 I 32 I 24 I 28 I 32 I 28

The effect of electrode separation (gap) on PDIV was

IEEE Transactions on Electrical Insulation Vol. 28 No. 6 , December 1003 1005

also investigated. As shown in Table 4, changing the gap from 10 to 25 m m did not affect, within experimental er- ror, the inception voltage using criterion 3. Each data point represents the average of three determinations us- ing the measuring technique described in the preceding section.

Table 4. The effect of electrode separation on PD incep- tion voltage (2 kV/step each lasting 2 min; 15 s measuring time)

I 2; I ;: I

Inspection of the da t a shown in Table 4 strongly sup- ports the idea that the local field rather than the exter- nally applied field is controlling P D inception and hence P D growth. Table 2 illustrates the randomness of the P D pulses as the applied voltage is increased (or de- creased). This randomness of the total number of PD disappears somewhat when the applied voltage approach- es the breakdown voltage [7]. I t appears, therefore, that the applied voltage cannot be used to quantify the initi- ation of the P D process.

The third conclusion deals with the meaning of the PDIV. From the da t a shown in Table 3 it appears that the PDIV is rather sensitive to the definitions used. This may indicate that PDIV does not properly characterize the P D process in agreement with the second conclusion. I ts use should, therefore, be questioned and possibly dis- continued, as suggested in an earlier study [5].

The fact that by changing the interelectrode distance over the the above range does not affect change the PDIV

dius of curvature 30 pm), the local field, does not change. The significance of these results is discussed in the next Section.

Nevertheless, the third criterion, which establishes the at which One Observes a dramatic

appears to have somewhat more merit than the other definitions of PDIV, particularly if and when more ~ensi t ive measuring systems become available.

PDIV as the significantly, indicates that the field a t the needle tip (ra- change Of the PD activity from that Of the background

One aspect left out of this study is the role of the sen- sitivity of the detection equipment one uses. The above remarks apply to the use of presently available commer- cial equipment operating in the 150 to 450 kHz bandwidth range. Perhaps the use of more sophisticated instruments

NSPECTION of the results shown in Table 3 suggests having much shorter rise times and broader bandwidths I t h a t none of the three criteria provides a unique and in the 100 to 500 MHz range would yield more meaningful meaningful means for the establishment of a character- results.

4. DISCUSSION AND CONCLUSIONS

istic PDIV. Three important conclusions can be drawn from these observations. The first one deals with the irre- producible nature of the background noise. This irrepro- ducibility is not a function of the measuring equipment which, under controlled input conditions, is capable of re- producing da ta to 32%. I t is quite possible that sudden external noise sources may become active and interfere with ongoing measurements. These fluctuations make the use of statistical treatment of the noise problematic; they also do not permit the exclusion of background noise from the measured values and, occasionally, they may confuse the results.

From the above findings and conclusions it appears that P D measurements provide no real means for dis- tinguishing quality differences among various dielectric liquids. Furthermore, PDIV should not be used as di- agnostic tool, even in well defined systems using liquids free of voids and particulate impurities because of the un- certainties surrounding its determination. If one were to use P D measurements as a diagnostic tool, i t would be necessary to have proper information on earlier PD activ- ity under identical conditions. With such da ta it would then perhaps be possible to have useful information on the operating conditions of the system by comparing da- t a collected under identical conditions a t various times so as to be able to note significant changes in PD activity. The second conclusion deals with the randomness of the

PD initiation process. As shown in Figure 1, an increase in voltage does not lead to a proportional increase in the total P D count. The reason for this behavior may well be the origin of the PD, the role played by the local field and hence by the local space charge distribution. No quantitative information is available on either subject.

ACKNOWLEDGMENT

The authors wish to acknowledge the support of the ‘40% Fund’ of the Italian Ministry of University, Scientific

... .

1006 Pompili et al.: The Definition Used in Measuring P D Inception Voltages

and Technological Research. They also wish to thank Messrs. A. Albujar, L. Caroli and S. Patrissi for their help in collecting the experimental data.

[6] F. Oswath, E. Carminati and A. C. Gandelli, “A Contribution to the Traceability of Partial Discharge Measurements”, IEEE Trans. Electr. Insul., Vol. 27,

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This paper is based on a presentation given at the 1992 Volta Colloquium on Partial Discharge Measurements, Como, Italy, 26-28 August 1992.

Manuscript was received on 9 September 1992, in final form 1 2 July 1993.