IEEE Transactions on Electrical Insulation Vol. 23 No. 3, June 1988 44 7

Optical Detection of Surface Discharges

F. H. Kreuger, P. H. F. Morshuis, and W. A. Sonneveld

' High-voltage laboratory, Dept. of Electrical Engineering, Technical University, Delft, The

Netherlands

ABSTRACT

This paper describes the use of an optical detection system for measuring surface discharges. A photomultiplier tube (PM) transforms the photon pulses emitted by the discharge process into an electrical signal. To suppress the noise inherent to PM operation, filters were built in the amplitude- and time- domains. Results of discharge measurements showed that in some situations the sensitivity of the optical system exceeds that of electrical methods.

INTRODUCTION HROUGH the years electrical detection has become T the most important means of registration of partial

discharges. Although the sensitivity is good (0.05 pC for small samples), it appeared to be of interest to study the possibilities of optical detection. In the paat, relatively little research has been done on the ultimatt sensitivity of the optical methods. Most of the time optical de- tection is an additional aid in a system with electrical bridge detection [l-61, for example when investigating the initiation of treeing processes [3-61.

Kitamura et al. [2] presented a PM detection sys- tem working a t room temperature without any noise suppressing facilities. It is not clear whether they made use of two PMs, one used as the detector and the other with blinded cathode to filter out common-mode noise by way of a differential amplifier. A sensitivity of a t leaat 0.3 pC was reported. Some authors reduced the noise pulse-count (dark count) by operating the PM tube at very low temperatures, because the noise is exponen- tially related to the temperature, they achieved a dark count down to about 10 s-l [3,5]. However, as it is not always possible to use a cooling system, another way

of suppressing thermal noise was developed. In moat systems a multichannel analyzer was used to perform pulse-height analysis and to compare electrical with o p tical detection signala.

As Brzoeko et al. report [7,8], the radiation emit- ted in the electrical diacharge process can be used as a means for determining the magnitude of the discharge. Measurements on surface discharges resulted in a lin- ear relation between the number of photons emitted per discharge impulse, and the magnitude of the discharge. The radiation can be extremely weak and originates mainly from the UV-part of the electromagnetic spec- trum [1,9,10]. The PM waa chosen as the most suitable optical detector, because of its extremely high gain and good sensitivity in the UV region. The PM delivers an electrical pulse for each photon arriving at its cathode (In practice the efficiency is about 10 to 30%).

In addition to electrical pulses coming from pho- tons, two types of other pulses will arrive at the output of the PM tube, both originating from thermal noise sources within the PM tube. These sources are the pho- tocathode and the dynodes (electron amplifiers). The

0018-Q367/88/0600-447$1.00 @ le88 IEEE

448 Kreuger et al.: Optical detection of surface discharges

former produces noise pulses with the same magnitude as the photon pulses, while the latter produces pulses that are smaller than pulses originating from photons [ll]. Special filters were built to handle these problems.

SIGNAL PROCESSING IGITAL processing was preferred above integration D of the P M output signal, to use as much informa-

tion as possible about the individual P M pulses. After amplification by a high frequency pulse amplifier, dyn- ode originated pulses were filtered out of the original signal by a pulse-height discriminator. By carefully se- lecting the discrimination level only about 1% of the dynode pulses passed the discriminator.

The location of the pulses offers the information for suppressing the cathode originated thermal pulses. The viewpoint here is that there is no correlation between these pulses and the phase of the test voltage, i.e. ther- mal noise pulseg are randomly distributed in time. On the other hand, there is a correlation between the phase of the discharge impulses and the test voltage phase, al- though the correlation factor may be smaller than unity.

This knowledge can be used for discriminating ther- mal noise-pulses by sampling the pulse signal, and count- ing the pulses for a specific number of 50 Hz periods, during a specific interval in phase. Narrowing this in- terval, the noise pulse count will decrease, while the discharge pulse count will stay relatively constant. In the ideal situation, where the discharge pulses are infin- itely narrow, and the phase correlation is equal to unity, the noise pulses can be discriminated fully against the discharge pulses by using an infinitely narrow counting interval.

In practice however, the discharge count will de- crease too, when the counting interval gets too small, because the correlation factor is somewhat smaller than 1.0.

A digital electronic counting system (DECS) was developed to achieve this kind of noise-suppression.

MEASUREMENTS HE optical detection system was tested by putting T a needle a t a cylindrical piece of epoxy, parallel to

the electric field lines. The P M and the test object were installed in a light-tight tank (Figure 1).

m : mirror PM : photomuLti@ier

ED : doctricot detector DIS: dscrimimtor

Figure 1. Laboratory set-up for optical detection

The measurement procedure can be explained as follows: with the use of a HV transformer, the piece of epoxy was electrically stressed until an output was obtained at the optical system. Next, the parameters were set for the P M (amplification), the pulse-height- discriminator (discrimination level), and the DECS (pha- se interval width, number of periods, sensitivity).

Some of the results of measurements on epoxylneed- le can be found in Figure 2(a-c), where R is the repeti- tion rate of a discharge during the preselected number of measuring-periods ( R = 100% when the discharge im- pulse is present each 50 Hz cycle). The magnitude of the discharges has also been measured with the electrical de- tection system when possible (Figure 2(a-c), the lower limit of its sensitivity being 0.05 pC for small samples.

RESULTS

1. There is a strong correlation between the electri- cal and optical detection results regarding the location of the discharges on the sinewave.

2. The optical signal-to-noise-ratio is very good, and improves with increasing discharge repetition-rate.

IEEE Transactions on Electrical Insula tion

Figure 2. Optical detection output for various discharge magnitudes

3. The sensitivity of the optical system depends on the repetition-rate and the variance in the phase-angle of the discharge. Maximal sensitivity can be achieved with a 100% repetition-rate, and appears to be about l ox better than with the electrical detection.

REFERENCES

[l] A. G. Day, “Optical detection of electrical discharges in insulation”, Fourth International Conference on Dielectric Mat., Meas., and Applic., Lancaster, Eng- land 10-13 Sept, pp. 229-232, 1984.

[2] Y. Kitamura, S. Hirabayashi, “Partial discharge de- terioration of epoxy resin for electronic parts”, Conf. on Electr. Insul. and Diel. Phen., IEEE Conf. Rec.85 ch 2165-2169, Annual Report 1985.

[3] S. S. Bamji, A. T. Bulinsky, R. J. Densley, “The role of polymer interface during tree initiation in

Vol. 23 No. 3, June 1988 449

low density polyethylene”, IEEE Trans. on Electr. Insul., Vol. 21 pp. 639-644, 1986.

[4] S. S. Bamji, A. T. Bulinsky, R. J. Densley, “Evidence of near ultraviolet emission during electrical-tree ini- tiation in polyethylene”, J. Appl. Phys. p. 61, 1987.

C. Laurent, C. Mayoux, S. Noel, “Dielectric break- down of polyethylene in divergent field: Role of dissolved gases and electroluminescence”, J. Appl. Phys. p. 54, 1983.

N. Shimizu, H. Katsukawa, M. Miyauchi, M. Kosaki, and K. Horii, “The space charge behavior and lu- minescence phenomena in polymers at 77 K”, IEEE Trans. on Electr. Insul. Vol. 14, pp. 256-263, 1979.

J. S. Brzosko, “The relation between the charge transported and number of photons emitted in par- tial discharges on dielectric surfaces”, J. Phys. D, Appl. Phys., Vol 10, 1977

J. S. Brzosko, W. Zukowski, J. Grudzinski, E. Zukowski and A. Konarzewski, “Statistical model of the creep discharges in air a t the dielectric surface”, IEEE Trans. on Electr. Insul. Vol. 14, pp. 240-249, 1979.

N. Yoshimura, M. Nishida, F. Noto, “Light emis- sion from tracking discharges on organic insulation”, IEEE Trans. on Electr. Insul. Vol. 19, pp. 149-55, Apr. 1984.

[lo] R. W. Engstrom, Photomultiplier Handbook, RCA Solid State Division Electro Optics and Devices, Lancaster, England, 1980.

[ll] H. Raether, Electron Avalanches and Breakdown in Gases, London, Butterworths, pp. 40-49, 1964.

Manuscript was received on 14 Mar 1987, in find form 22 Feb 1988.