IEEE Transactions on Electrical Insulation Vol. 24 No. 4, August 1989 681

Statistical Variation of ac Corona Pulse Amplitudes in Point-to-plane

Air Gaps

A. A. AI-Arainy, N. H. Malik and M. K. AI-Bahloul

Elect. Eng. Dept., College of Engineering, King Saud University, Riyadh, Saudi Arabia

ABSTRACT Using a multichannel analyaer, statistical variations of corona pulse amplitudes are studied for ac corona in point plane air gaps. It is shown that, for each half cycle, corona pulse am- plitudes generally follow a Gaussian type of probability distri- bution when the applied voltage is near the corona inception level. However, as the applied voltage is further increased, the distributions show deviations from this behavior. Typical pulse amplitude parameters such as the average value, the standard deviation, and the coefficient of variation are measured for dif- ferent experimental conditions and the results for corona under ac voltages are compared with previously obtained results for dc corona.

INTRODUCTION

HE phenomenon of corona in nonuniform field air T gaps is of considerable interest. In air, several coro- na modes exist depending upon the experimental condi- tions [l-31. These modes could broadly be classified as pulseless (or glow corona) and pulse type of corona. For the pulse-corona mode, the corona pulses are generally random in both amplitude and repetition rate. Thus, for a comprehensive understanding of the corona phe- nomenon, a knowledge of the statistical distribution of corona pulse parameters is essential.

A review of the existing literature indicates that the statistical properties of corona pulses have been thor- oughly investigated using pulse-height analysis (PHA) techniques for corona in SFG, and its mixtures with N2

and water vapor [7-91. PHA techniques have also been used to investigate statistical properties of partial dis- charges inside insulating cavities and other equipment such as oil-paper and XLPE insulated power cables [lo].

However, for corona occurring between metallic elec- trodes in atmospheric air, limited and sometimes con- flicting information is available about the random as- pects of corona pulse parameters [4,5,10].

In an earlier paper [6] the statistical distributions of corona pulse amplitudes were reported for dc corona in point-to-plane air gaps. The influences of applied voltage, gap length, as well as polarity on the corona pulse height distributions were examined in detail. It was shown that for dc corona, in general, pulse ampli- tudes follow a Gaussian type of probability distribution. However, exceptions to this behavior were observed near the corona onset level, near the pulse-to-glow transi- tion level, or in more uniform field gaps. The corona pulse amplitudes were reasonably reproducible for posi- tive corona, whereas, for negative corona, the pulse am- plitudes were very sensitive to the applied voltage level and were less reproducible.

This paper reports on the statistical distributions of corona pulse amplitudes for ac corona in point to plane

0018-9307/89/0800-081$1.00 @ 1989 IEEE

682 AI-Arainy et al.: Corona Pulse Amplitudes in Air Gaps

air gaps. The influence of applied voltage level, gap length, as well as polarity during each half cycle of the supply voltage on the corona pulse height distributions is examined and the results for ac voltages are compared with those obtained for dc applied voltages.

150 X I 2 gap = 20mm

n EXPERIMENTAL SYSTEM 10 -

In 8 - IGURE 1 shows a schematic diagram of the exper- U)

F i m e n t a l set-up. The power source was a 20 kVA, 2 6 - 200 kV rms, 60 Hz discharge free test transformer (Hafely, LL Model 720-386). The corona geometry consisted of a 0 4 - point-plane air gap. The plane was a well-polished, Ro- $

15 mm diameter cylindrical steel rod with one side ta- pered to a sharp point having an angle of 90 * was the point electrode. The test gap was assembled vertically,

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gowski profiled, 150 mm diameter, steel electrode. A 2 -

10 20 30 40 with the point electrode being at the top and connected to the HV source. The plane could be moved up and down, thereby varying the gap length, which could be measured by using a vernier scale built into the test set up. A current limiting resistor of 280 Mil was used in series with the HV electrode.

PULSE AMPLITUDE ( m A )

( a )

gap = 60 mm t

Figure 1. Experimental set up. 1: Regulating transformer; 2: 200 kV, 60 Hz, ac transformer; 3: voltage di- vider; 4: damping resistor; 5: corona geometry; 6 : detection impedance; 7: cathode ray oscillo- scope; 8: multichannel analyzer.

An impedance in series with the low-voltage electrode was used to detect the corona pulses. This impedance consisted of a 50 il resistor and a coaxial cable. For neg- ative ac corona, the corona pulses were fed to a variable gain pulse amplifier having a maximum gain of 100. The gain adjustment of the amplifier determined the overall sensitivity of the measurement system. For positive ac corona, the pulse amplitudes were sufficient and no am- plification was required. The pulse height distributions (PHD’s) were measured using a multichannel analyzer

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( b ) Figure 2.

Pulse height distributions for positive ac corona in point-plane air gaps. Sampling time: 200 s.

(MCA) which accepts 0 to 10 V pulses of either polarity and sorts them according to their relative amplitudes, thereby, providing a histogram of the number of pulses

IEEE Transactions on Electrical Insulation Vol. 24 No. 4, August 1989

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Figure 3. Amplitude probability distributions for positive ac corona.

vs. the channel number. The MCA used had 1024 chan- nels and was calibrated by applying voltage pulses of known amplitude and polarity across the detection im- pedance.

RESULTS AND DISCUSSION

HE ac corona pulses in point-to-plane air gaps have T different magnitudes for both half cycles. During each half cycle, the pulse amplitudes are influenced by the applied voltage as well as the gap length. When the corona pulses were 'directly' applied to the MCA,

it accepted the positive corona pulses only. To inves- tigate the corona pulses occurring during the negative half cycle of the supply voltage, the output of the detec- tor circuit was fed to the MCA through a pulse amplifier which accepted negative pulses only. Thus, it was possi- ble to investigate individually the statistical properties of corona pulse amplitude for the positive as well as for the negative half cycle of the ac supply voltage. The re- sults for both types of corona are given in the following sections.

POSITIVE CORONA

When the applied voltage was near the corona onset level, the positive ac corona had PHD curves which were

684 AI-Arainy et al.: Corona Pulse Amplitudes in Air Gaps

26

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Figure 3 shows the cumulative probability distribu- tions for corona pulses which occur during the positive half cycle of the supply voltage. This Figure clearly shows that, near the inception voltage, (i.e. lowest volt- age shown in each Figure) the plots are more or less straight lines. However, as the applied voltage is further increased, the deviations from straight line become ev-

more or less bell shaped. However, the PHD's had a ten- dency to deviate from the bell-shaped curves at higher voltages. Figure 2 shows the typical PHD curves ob- tained for two gaps and different applied voltage levels. The results for other gaps in the range of 20 to 80 mm were generally similar to those shown in Figure 2.

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ident. Thus, the distributions which are approximately Gaussian near the corona inception level become non

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gap = 60 mm

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( b ) VOLTAGE ( k V Peak)

Figure 4. Effect of applied voltage on average pulse ampli- tude (Iam), standard deviation (U) and coefficient of variation (Cvan) of pulses for positive dc and positive ac corona.

Gaussian as the peak applied stress is progressively increased. However, for a given value of the applied voltage, such deviations from straight line become less pronounced as the gap length is increased.

From numerous measurements, the average amplitude i.e. the pulse amplitude with 50% cumulative probability

IEEE fitinsactions on Electrical Insulation Vol. 24 No. 4, August 1989

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Figure 6. Amplitude probability distributions for negative ac corona pulses

(Ia,,), the standard deviation of the pulse amplitude (0) as well as the coefficient of variation (CVAR = cr/I,,) of the pulses were calculated. Figure 4 shows the variation of these parameters with applied voltage for different gap lengths for positive ac corona. The results for posi- tive dc corona are also included in Figure 4 for a compar- ison. It is clear from these results that, near the onset voltage, I,,, U as well CVAR are similar for positive dc as well as positive ac corona. However, a s the applied voltage is further increased, the average pulse ampli- tude for positive dc corona is slightly higher than that for the positive ac corona. This is due to the fact that ac corona occurs at different instantaneous values of the applied voltage and consequently the pulses would be of different amplitudes. However, since the peak values of ac and dc excitations are the same, the probability of larger pulses is higher under the dc voltages compared to the ac applied voltages. Consequently, the average pulse amplitude for the dc corona is expected to be larger than the corresponding value for the ac corona. In this

connection, it is useful to remember that generally the positive corona pulse amplitude increases with the ap- plied voltage [l]. Figure 4 clearly indicates that as the applied voltage is increased, I,, increases for both types of applied voltages. However, it seems that applied volt- age magnitude does not affect ac corona average pulse amplitude as much as it does to the dc corona pulse am- plitude. It was also observed that, for the same applied voltage level, as the gap length is increased the average pulse amplitude decreases for both dc as well as ac anode corona. Figure 4 further shows that the pulse standard deviation and the coefficient of variation for the posi- tive ac corona are larger than the corresponding values for positive dc corona. The standard deviation increases with the applied voltage for both types of corona. How- ever, for a given value of the applied voltage, the stan- dard deviation decreases as the gap length is increased. The values of U for positive ac corona are larger than the corresponding cr values for positive dc corona. Figure 4

686 AI-Arainy et al.: Corona Pulse Amplitudes in Air Gaps

further shows that, as the applied voltage is increased, the coefficient of variation of the pulse amplitudes for ac corona is also increased. However, as the gap length is increased, this coefficient decreases for a given value of the applied voltage suggesting that less uniform gaps generate corona pulses which are better reproduced as far as amplitude is concerned.

NEGATIVE CORONA

Figure 5 shows the pulse height distribution curves for negative ac corona for two gap lengths. It is clear from this Figure that, similar to the positive ac corona, the PHD curves for negative ac corona are roughly bell shaped when the applied voltage is near the inception level. However, a t higher applied voltages, there are de- viations from this behavior. Moreover, for some cases, the curves show two peaks indicating the instability of the negative ac corona. A similar observation was previ- ously reported for negative dc corona [6]. Thus, near the corona inception vohages, the pulse amplitudes follow the Gaussian type of probability distribution. However, as the applied voltage is further increased, the distribu- tions become non Gaussian as shown in Figure 6. This is true irrespective of the gap length.

Figure 7 shows the effect of applied voltage on average pulse amplitude, standard deviation and the coefficient of variation of the pulses for dc and ac cathode coro- nas for different gap lengths. This Figure shows that for both gap lengths, the standard deviation and the coefficient of variation of the pulses are always higher for negative ac corona than those obtained for nega- tive dc corona. In order to understand this behavior, it should be remembered that, in general, the amplitude of negative corona pulses decreases with an increase in the applied voltage [l]. For similar peak values, ac volt- ages will have instantaneous values lower than the peak over most of the cycle and therefore will produce corona pulses of different amplitudes. Consequently, the statis- tical spread will be larger in ac corona than in dc corona and this spread is expected to increase as the ac applied voltage is increased. This is due to the fact that, for a given gap, the difference between the peak voltage and the corona inception voltage will increase a s the applied voltage is increased. Therefore, upon increasing the ap- plied voltage, the pulses of different magnitudes are ex- pected to be more common for ac than for dc corona. Consequently, the pulse standard deviation and the co- efficient of variation of ac corona will increase with the applied voltage. On the other hand, these parameters

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Figure 7. Effect of applied voltage on average pulse ampli- tude ( I a w ) , standard deviation (a) and coefficient of variation ( C v a ~ ) for negative dc and negative ac corona.

are not expected to change significantly for dc voltages. It was further observed that for most of the gaps stud-

IEEE Transactions on Electrical Insulation Vc

ied, the average pulse amplitude was somewhat higher for ac cathode corona as compared to the negative dc corona as shown in Figure 7(a). However, for larger gaps e.g. 80 mm, the opposite was the case. The reasons for this behavior are not clear.

In negative ac corona, for a given gap, when the ap- plied voltage increases, t he average pulse amplitude de- creases while the standard deviation increases. For a given voltage, as the gap length is increased, the pulse standard deviation decreases. It is important to note that, in dc corona, the variations in pulse amplitudes are due to statistical nature of the corona phenomena only. However, in ac corona, the sinusoidal variation of the applied stress introduces further spread in the corona pulse amplitudes. Therefore, the net result is that the ac corona pulse parameters exhibit higher sta- tistical variations as compared to the dc corona.

CONCLUSIONS

OR positive and negative ac corona in point-plane F air gaps, the pulse amplitudes follow Gaussian type of probability distribution when the applied voltages is near the corona inception level. However, as the applied voltage is progressively increased, the deviations from Gaussian type of probability distribution also increase for positive as well as negative ac corona. The standard deviation and the coefficient of variation for pulse am- plitudes have much higher values for ac excitations as compared to the dc applied voltages. For ac voltages, the standard deviation as well as the coefficient of vari- ation increase with the applied voltage during both half cycles of the supply voltage. Negative ac corona exhibits higher values of coefficient of variation as compared to the positive ac corona.

d. 24 No. 4, August 1989 68 7

REFERENCES

[l] H. Loeb, Electrical Coronas - Their Basic Physical Mechanism, University of California Press, 1963.

[2] J . M. Meek and J . D. Craggs (Editors), Electrical Breakdown of Gases, John Wiley, 1978.

[3] M. N. Hirsh and H, J . Oskam (Editors), Gaseous Electronics, Vol. I, Electrical Discharges, Academic Press, 1978.

[4] M. M. Khalifa, A. A. Kamal, A. S. Zeitoun, R. M. Radwan and S. El-Bedwainy, “Correlation of Ra- dio Noise and Quasi peak Measurements to Corona

Pulse Randomness”, IEEE Trans., Power App. and System, Vol. 88, pp. 1512-1519, 1969.

[5] J . P. M. Schmitt and C. H. Gary, “Radio Noise Characteristics of Single Corona Source: Amplitude, Threshold Gradient, Random Aspects” IEEE/PES Conf. Paper C72-194-4, Winter Power Meeting, New York, 1972.

[6] N. H. Malik and A. A. Al-Arainy, “Statistical Vari- ation of dc Corona Pulse Amplitudes in Point-to- Plane Air Gaps”, IEEE Trans. Electr. Insul., Vol. 22, pp. 825-829, 1987.

[7] R. J. Van Brunt and D. Leep, “Characterization of Point-Plane Corona Pulses in SFs”, J . Appl. Phys., Vol. 52, pp. 6588-6600, 1981.

[8] R. J. Van Brunt, S. H. Hilten, and D. P. Silver, “Par- tial Discharge Pulse Height Distributions and Fre- quencies for Positive and and Negative dc Corona in SFG and SF6-Nz Mixtures”, in Gaseous Dielectrics 11, Pergamon Press, New York, pp. 303-311, 1980.

[9] R. J . Van Brunt, “Effects of HzO on behavior of SF6 Corona”, Proc. 1982 IEE Conference on Gas Discharges and Their Applications, U. K.

[lo] R. Bartnikas, “Corona Pulse Counting and Pulse Height Analysis Techniques”, in Engineering Di- electrics, Corona Measurement and Interpretations, Vol.1 ASTM, Philadelphia, pp. 285-326, 1979.

Manuscript was received on 14 Jul 1988, in revised form 9 Jan 1989.