Effects of negative direct voltage pre-stressing on the breakdown of conductor-rod gaps under positive impulse voltages

I impulse generatoi

N. L. AI len D.E. Gourgoulis P.N. Mi kropoulos C.A.Stassinopoulos C.G. Ya ki nt hos

indexing terms: Conductor-rod gaps, impulse voltage, Direct voltage pre-stress, Gap geometry, 50% breakdown voltage, Humidity, Spark paths

Abstract: The sparkover of a 50cm conductor- rod gap under positive impulses is investigated when a pre-stressing negative direct voltage is applied to the rod. Under lightning impulses the pre-stress has a small effect on the breakdown voltage of the gap. For long-front impulses, a considerable increase in the dielectric strength of the gap can be observed. Several parameters affect this behaviour, like the value of the pre- stressing direct voltage, the diameter of the energised conductor, the position of the rod with regard to the conductor and the waveshape of the applied impulse.

List of symbols

q LI = lightning impulse (l.2/5Op) SI = switching impulse (25012500p) d = diameter of conductor U,, = negative direct voltage Us0 = 50% sparkover voltage from a breakdown prob-

CT = standard deviation as a percentage of Us0 via

k, = humidity correction factor for Us0 L = average length of the projection of the spark

= front duration of the applied voltage

ability distribution

breakdown probability distribution

path on the plane of the gap

1 Introduction

Environmental and economic necessities have encour- aged the electric power utilities to seek improved design

0 IEE, 1998 iEE Proceedings online no. 19981964 Paper first received 17th June 1997 and in revised form 27th January 1998 N.L. Allen and P.N. Mikropoulos are with UMIST, Department of Elec- trical Engineering & Electronics, High Voltage Laboratory, PO Box 88, Manchester M60 lQD, UK D.E Gourgoulis, C.A. Stassinopoulos and C.G. Yakinthos are with the Aristotle University of Thessaloniki, Department of Electrical & Compu- ter Engineering, High Voltage Laboratory, Building D, New Egnatia Str., Thessaloniki 540 06, Macedonia, Greece

approaches to make transmission lines more effective. To improve the insulation design, results pertaining to breakdown of gaps with switching surges superimposed on DC voltages have been reported [1-4]. In some cases the sparkover voltage was greater when the gaps were pre-stressed with DC voltage than for the switch- ing surge alone, and this was attributed to space charge modification of the electric field distribution.

The aim of the present study is to investigate the impulse breakdown of a conductor-rod gap when the rod is pre-stressed with negative direct voltage. It was found that the effect of pre-stressing depends on the diameter of the conductor, on the position of the rod, on the waveshape of the applied impulse voltage, on humidity and on the value of the direct voltage.

2 Experimental set-up

The experimental layout is displayed in Fig. 1. The 50cm conductor-rod gap was vertically mounted and the axis of the gap was perpendicular to the ground. The rod was 80cm long having a diameter of 2cm and a hemispherical end. Two 4m-long conductors with diameters d = 2cm and d = 3cm have been used alter- natively. Two types of gap configurations (axial and nonaxial) have also been used: at the axial configura- tion the rod was set beneath the midpoint (Fig. 1, posi- tion a); at the nonaxial configuration the rod was set 1 m from the midpoint (Fig. 1, position b); the conductor (Fig. 1) was always energised through a lead at mid- point.

P E L & .... ... ..&---L./ -

Fig. 1 Schematic representation ofgap type

IEE Proc-Sci. Meas. Technol., Vol. 14S, No. 3, May 1998 105

A 4-stage, 560kV, IkJ Marx generator produced impulse voltages with the following waveshapes: LI, i.e. 1.2/50ps, 211700p, 25/1900p, 50/1950p, 100/2200p, 150/2200p, and SI, i.e. 250/2500p. The pre-stress direct voltages UDc were produced by a direct voltage generator and had values of 75, 100 and 125kV. An auxiliary sphere gap, set to break down at a voltage slightly above the UDo was used for the protection of the direct voltage generating unit in cases of sparkover of the main gap (Fig. 1). The humidity in the labora- tory was not interfered with artificially and was found to vary between 6 and 20gm-3.

Breakdown probability distributions were obtained through the multiple level test method [5]. Each voltage level consisted of 20 impulses applied at time intervals of - 30s. From each distribution the values of 50% sparkover voltage Us, with their confidence limits and standard deviation CT were computed in accordance with IEC [5]. In the cases of pre-stressing the value of Us, is the algebraic sum of the components of the applied voltages. All the U,, were corrected for air den- sity [5].

Markings on the surface of the conductor at 5cm intervals helped determine by visual observation the position of the spark paths to this electrode.

Photomultiplier measurements were obtained via a Thorn EM1 9781R photomultiplier mounted inside a metal housing with a narrow horizontal slit positioned so as to view either the area immediately above the rod or the one immediately below the conductor. The height of the area viewed was - 5cm and the width - 80cm. The photomultiplier output was connected to a digital oscilloscope.

3 Experimental results

3. I Breakdown probability distributions The breakdown probability distributions for the pre- stressed gaps were plotted on normal probability paper. It was found that they were mostly linear; however, in some cases they tended to be nonlinear in the sense that they deviated considerably from the cumulative normal distribution. Typical examples of breakdown probability distributions, both linear and nonlinear, can be seen in Fig. 2.

99[ 99F

I L I L 320 360 400 440340 380 420 460

a b Fig. 2 Breakdown probability distributions with impulse shape and humidity as parameters a Impulse shape: U;, kV, U,, = 100kV, nonaxial, d = 2cm, 12 g w 3 (i) 1.215Op; (ii) 100/2200p; (iii) 250/2500p b Humidit U;, kV, (iv) 8 gm-{:(v) 14.5 gm- ; (vi) 19.5 gm-3

= 7SkV, axial, d = 2cm, 100/2200p

The nonlinear cases are more frequent under wave- shapes with long front durations (Fig. 2a) than they are with the non-prestressed gaps [6]. There was a ten- dency also for nonlinear distributions to occur more frequently with pre-stressed rather than non-pre- stressed gaps and when humidity increased (Fig. 26).

106

3.2 Influence of gap type, impulse shape and uDC The average values of U,, for all the measured humidi- ties were plotted against the front duration of the applied impulse tf with UDc as a parameter. This is shown in Fig. 3 for the axial geometry with d = 2cm, Fig. 4 for the nonaxial geometry with d = 2cm and Fig. 5 for the nonaxial geometry with d = 3cm. The points relating to the standard impulse LI although presented in the Figures were not taken into account in the drawing of the curves, which thus refer only to impulses with long wavetails.

,0°[ 4

300 I I

Fig.3 as parameter, axial, d = 2cm

Vertical bars represent 90% confidence limits of U;,

Average value of U,, (kV) for all humidities against ?with U,,

-0- U,c=125kV -.- UD,= l00kV A Uoc= 75kV - -0- - non-pre-stressed gaps [6]

500 6oo 1

2 1 10 10

4,PS

Fig.4 Average value o U,, (kV) for all humidities against with U,, as parameter; nonaxial, df= 2cm

-.I-. UDc = lO0kV - -0- - non-pre-stressed gaps [6] Vertical bars represent 90% confidence limits of U;,

4- U,,= 125kV ....A .... l J D c = 15kV

6oo F t

400 1

i" 2

1 10 10

4>PS

Fig.5 Average value o U,, (kV) for all humidities against with U,, as parameter; nonaxial, df= 3cm -0- UDc= 125kV -..-. Uoc = l0OkV - -0- - non-pre-stressed gaps [6] Vertical bars represent 90% confidence limits of Us0

....A. .. U,, = 75kV

IEE Proc.-Sci. Meas. Technol., Vol. 145, No. 3, May 1998

Concerning the influence of tf on U50, it can be seen from Figs. 3 and 4 that for pre-stressed gaps with d = 2cm Us0 increases with increasing tfi especially for tf > 25p . This is more obvious for axial geometry (Fig. 3) and when U,, > 75kV. For pre-stressed gaps with d = 3cm (Fig. 5), irrespective of U,, and gap geometry U,, is almost constant with 9.

The influence of U,, on U50 for d = 3cm is negligible especially if the standard deviations are considered (Fig. 5). The same is true for gaps with d = 2cm but only under LI and short-fronted impulses, 2/1700p and 2511900p (Figs. 3, 4). However, for axial geometry and under long front impulses, U50 increases gradually with increasing UDc (Fig. 3); this is not in evidence for nonaxial geometry at least for U,, between 75 and 125kV (Fig. 4).

Fig. 6 shows variations in CJ with 9. For d = 2cm the values of CJ are large when there is no pre-stress or when the pre-stressed gap is subjected to long-fronted impulses (Fig. 6), whereas for d = 3cm CJ was not affected significantly. There is no systematic variation with gap type but there is some evidence that at both high and low humidities CJ is larger in axial than in nonaxial geometry. Furthermore, CT increases with increasing humidity, particularly in the axial case.

1.0 e 2

0.5

0 1 10 10

t f4S

Fig.6 parameter; axial, d = 2cm -E U,, = 125kV ....... U,, = 75kV -..-. UDc = lOOkV - -0- - non-pre-stressed gaps [6] Vertical bars represent 90% confidence limits of o

Average value of (T (%)for all humidities against t/ with U,, as

- A 6 ............................. ...... .......... -x.--. &---:. A

a n 8 0

............................... 1- -a.. 5-L -: . - ... - .. - . . . - - 0 - - - 0 0 _- - - _ -

3.3 Influence of humidity Linear regressions of Us0 over humidity have been cal- culated and in general U50 was found to increase with humidity. The percentage of correction of the U50 per gm-3 of absolute humidity has been computed by find- ing a per cent correction factor k, from the equation

S A x 100 k, =

un50

where sA is the slope of the straight lines of the linear regression of U,, over humidity and U,,, the values of U50 under normal humidity (1 1 gm-3) obtained from these linear regressions.

The values of ks were related to 9. In Fig. 7 (nonax- ial, d = 2cm) k, decreased with increasing pre-stress voltage. In Fig. 8 (axial, d = 2cm) pre-stressed gaps show a similar trend but the non-pre-stressed gap shows smaller values.

For U,, = 125kV and the gap type as parameter (Fig. 9) k, increases with increasing d, especially for nonaxial geometry for all values of q.

Furthermore, from the study of the breakdown prob- ability distributions it was found that the influence of humidity is not equal throughout the full probability

IEE Proc.-Sci. Meas. Technol., Vol. 145, No. 3, May 1998

distribution but it tends to influence the lower break- down probabilities less.

2'ol 1.5

-0.5 L 1 10'

Fig.7 -0- U,,= 125kG .... .... UDc = 75kV -..-.

k, against t , with U,, as parameter: nonaxial, d = 2cm

U,, = lOOkV - -0- - non-pre-stressed gaps [6]

I 2'ol .5

I "

Fig. 8 -% U,,= 125k6 .-a- U,, = 75kV

k, against t with U,, us parumeter; axial, d = 2cm

U,, = lOOkV - -0- - non-pre-stressed gaps [6]

2'o I e.. ..-

-... '4 . , ..-- .,' .... . ,,'

- - 5 -._ - _. _ _ _ - - - _ _ - - _ _ - - . 0

D ................................................. 0 0

......... 0 .................. v 0.0 0

2 -0.5 I

1 10 10 tf!PS

Fig.9 -E+ axial, d = 2cm -...O.... nonaxial, d = 2cm - -.- - axial, d = 3cm nonaxial, d = 3cm

k, against 9 with gap type as parameter, U,, = 125kV

T

2 10 lo t+m 1

L (em) against 4 axial geometry, with U,, as parameter, d = Fig.10 2cm -0- U,, =: -125kV

- -0- - Vertical bars represent standard deviations (cm) of L

....A .... UD,::-7skV non-pre-stressed gaps [6]

107

Table 1: Average voltage outputs of photomultiplier (mV1 I

U,, = 0 kV U,, = 75 kV U,, = 100 kV U,, = 125 kV I

I Conductor Rod Conductor Rod Conductor Rod Conductor Rod

Axial, d = 2cm 5.0 5.1 21.4 77.2 28.2 775.6 36.5 3312.0

Axial, d = 3cm 5.2 5.2 8.6 82.3 10.5 780.3 15.1 3352.0

Nonaxial, d = 2cm 5.3 5.2 18.1 83.4 22.9 762.4 33.8 3378.0

Nonaxial, d = 3cm 5.3 5.1 9 1 79.6 15.2 777.6 24.5 3395.0

I

3.4 Spark paths For gaps with axial geometry the impact points of the sparks at the conductor were noted. From this the pro- jection of the spark path on the plane of the gap has been computed, and its average length L under UsO, for the full range of absolute humidity, was plotted against tf with U,, as parameter. A typical plot is presented in Fig. 10 for d = 2cm and from this Figure it can be seen that for pre-stressed gaps both L and its standard deviation increase with increasing tf and U,,, especially for tf > 25p.s. This is not so for d = 3cm, where no dis- tinct influence of 9 and UDc on spark paths was found to exist.

3.5 Photomultiplier measurements The average value of the voltage output of the photom- ultiplier (mV) under the various pre-stress voltages is shown in Table 1 both when the photomultiplier looks at the vicinity of the conductor or at the vicinity of the rod.

When no pre-stress was applied the small values (- 5mV) recorded are due to background illumination. For the pre-stress voltages the output of the photomul- tiplier increases at both ends of the gap because a DC corona occurs both at the rod and at the conductor. The light output of the DC corona increases with U,, and is much stronger at the rod. At the conductor it is stronger for the conductor with the smaller diameter (d = 2cm).

4 Discussion

The behaviour of non-pre-stressed positive conductor- rod gaps at U50 has been explained in terms of a nega- tive impulse corona starting first from the rod influenc- ing the emergence and development of the much more complex and longer positive impulse discharge. The lat- ter plays a more important role in breakdown because of its higher conductivity and greater length [6],

For pre-stressed conductor-rod gaps, before the application of the positive impulse, the negative U,, was found to give rise to DC corona from both elec- trodes. This results in the effusion of space charges in the vicinity of the electrodes which modifies the respec- tive field values, tending to increase the midgap field. Thus there exist space charges homopolar to each elec- trode, in effect extending them. Assuming that the light output is related to space charge, it can be deduced (Table 1) that the amount of these charges is strictly related to the value of Ubc and to the gap type, i.e. the diameter of the conductor and the position of the rod with regard to the high voltage lead of the conductor.

When a positive impulse is applied to the conductor, a negative impulse corona can be assumed to start first from the rod [6, 71, influencing the emergence and the subsequent deveiopment of the positive impulse dis- charge. However, during the front of the applied

108

impulse the field at both electrodes increases and if the time is long enough it results in an increase and a big- ger expansion of the space charges associated with the DC coronas. These space charges decrease the diver- gence of the field, thus the field at the gap becomes more ‘uniform’ as the midgap field increases; this results in higher breakdown voltages (Figs. 3, 4). For the present experiments, the higher the U,, and/or the longer the front duration the stronger the inhibitory effect of the DC coronas on breakdown.

For pre-stressed gaps breakdown occurs when the applied voltage is sufficient to increase the field at the electrodes to overcome the inhibitory effect of the space charges. This compensates the acquired ‘uniform- ity’ of the gap. For nonaxial geometry this condition is required when U,, is higher than 75kV (Fig. 4). For axial geometry, which is characterised by smaller field values [6, 71, the applied impulse voltages must be increased to reach this condition; this explains the higher breakdown voltages and the stronger effect of DC pre-stressing on breakdown (Figs. 3, 4).

For gaps with d = 3cm U,, does not seem to be affected either by pre-stressing or by gap geometry (Fig. 5 referring to nonaxial geometry). This is possibly due to the large diameter of the conductor resulting in a small positive DC corona at its vicinity (Table 1). If, however, higher direct voltage values are applied it has also been shown that there is a tendency for increased U,, when positive direct voltage pre-stressing is applied at the conductor [8].

In Fig. 6 both the smaller B for pre-stressed gaps rel- ative to the non-pre-stressed gaps and its reduction with increasing U,, can be explained by the tendency for a breakdown mechanism like the one of a more ‘uniform’ field gap which is characterised by smaller G.

It is known that, as the field at the positive electrode becomes higher, humidity has a smaller influence on the emergence of the positive discharge [9, 101. Further- more it is well established that humidity has a very small influence on the breakdown mechanism of ‘uni- form’ field gaps. These explain the smaller values of k, with increasing U,, (Figs. 7, 8). For the same reasons k, is smaller for d = 2cm than for d = 3cm, especially for nonaxial geometry (Fig. 9). This mechanism does not apply to non-pre-stressed gaps, which show a dif- ferent behaviour.

Space charge concentrations during the breakdown process are encountered also for gap configurations involving insulating barriers between the electrodes. There, too, breakdown occurs whenever the field at the electrodes overcomes the inhibitory action of the space charges. It can, therefore, be said that the effect of DC pre-stressing on breakdown can also be called ‘barrier effect without a barrier’.

IEE Pvoc -Sei Meas Technol, Vol 145, No 3, May 1998

5 Conclusions

Under long-fronted impulses the negative direct voltage at the rod may cause a significant increase in the dielec- tric strength of the gap. This depends on the value of U&, the diameter of the conductor, the position of the rod with regard to the conductor and the front dura- tion of the applied impulse. For the cases where the increase in Us0 is observed, humidity is seen to exert a smaller influence on the dielectric strength of the gap.

All this is explained in terms of the various DC and impulse coronas that occur at the conductor and the rod which tend to create a more ‘uniform’ field distri- bution in the gap.

6 Acknowledgments

The authors wish to thank the Department of Electri- cal and Computer Engineering of the Aristotle Univer- sity of Thessaloniki for the use of the High Voltage Laboratory.

7 References

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2 KNUDSEN, N., and ILICETO, F.: ‘Flashover tests on large air gaps with DC voltage and with switching surges superimposed on DC voltages’, IEEE Trans., 1970, PAS-@, pp. 781-788 MOSCH, W., LEMKE, E., and FAHD, I.: ‘Breakdown tests on inhomogeneous air gaps with switching surges superimposed on DC voltages’. Proceedings of 1st I.S.H., Munchen, 1972, pp. 319- 326

4 FESER, K.: ‘Das Durchschlagverhalten von Luftfunkenstrecken mit inhomogenem Feld bei Mischspannungen’, Bull. SEV, 197 1,

IEC: ‘High-voltage test techniques, Part 1 : General definitions and requirements’. Publication 60-1, 1989

OULOS, CA., and YAKINTHOS, C.G.: ‘Behaviour of positive conductor--rod gaps stressed by impulse voltages in atmospheric air’, IEE Proc., Sei. Meas. Technol., 1997, 144, (S), pp. 209-214

7 ALLEN, M.L., BOUTLENDJ, M., and STASSINOPOULO- S, CA.: ‘ Gap factor and breakdown mechanism in the conduc- tor-rod geometry’. Proceedings of 9th I.S.H., Graz, 1995, Subject 2, Paper 20.96

8 ALLEN, N.L., HUANG, C.F., CORNICK, K.J., and GREAVES, D.A.: ‘The sparkover of air gaps under composite impulse/direct voltages’. Proceedings of 10th I.S.H., Montreal, 1997 MIKROPOULOS, P.N., and STASSINOPOULOS, CA.: ‘Influ- ence of humidity on the breakdown mechanism of medium length rod-plane gaps stressed by positive impulse voltages’, ZEE Proc., Sei. Meas. Technol., 1994, 141, (S), pp. 407-417

10 ALLIBONE, T.E., DRING, D., and ALLEN, N.L.: ‘Influence of humidity on the sparkover voltage of rod-rod gaps of several geo- metrical forms subjected to positive impulse voltages of varying waveshapes’, IEE Proc., 1979, 126, (S), pp. 462466

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62, (6), pp. 920-923 5

6 GOURGOULIS, D.E., MIKROPOULOS, P.N., STASSINOP-

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