754 IEEE lkansactions on Dielectrics and Electrical Insulation Vol. 1 No. 4, August 1994

COMMUNI C AT1 0 N

High-speed Photography of PMMA Breakdown under ns Pulse Voltages

Hiroshi Yamada, Tamiya Fujiwara

Department of Electrical and Electronic Engineering, lwate University, Morioka, Japan

Koji Tamada

Department of Electrical Engineering, Tokyo National College of Technology,

Hachioji, Japan

ABSTRACT The electrical breakdown process in polymethyl methacrylate for a point-to-plane gap configuration has been studied using an image convertor camera and a nanosecond rectangular HV pulse generator. The propagation velocity of the breakdown channel for positive polarity is estimated to be - 80 km/s. The velocity is of the same order as that of secondary fast streamers in several hydrocarbon liquids.

1. INTRODUCTION

HE phenomena of electrical breakdown in solid di- T electrics have been of great interest for practical use. Nevertheless, there is still no breakdown theory gener- ally accepted for solid dielectrics, including polymethyl methacrylate (PMMA). This is partly because there are very few accurate time-resolved da ta on the breakdown process. However, the breakdown of solid dielectrics oc- curs with a very short time, - 1 ns to 10 ps after a voltage application [I , 21. To record these ultrafast events with- out disturbance from the electromagnetic radiation asso- ciated with voltage application, optical measuring tech- niques are most attractive. The transparency of PMMA is relatively high compared with that of other solids, and thus PMMA is a suitable test sample for observing elec- trical events in the solid.

0

0 760 ns

2. EXPERIMENTAL Figure 1. Applied voltage trace.

The experimental setup is essentially the same as that reported previously [3]. It is composed of three basic parts: a test cell, a fast HV pulser and an optical ob- servation system. The brass test cell has a polymer HV

1070-9878/94/ $3.00 @ 1994 IEEE

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 4, August 1994 755

-.-

8 0 r- I+

- (b)

Figure 2 (a) . The sequential records of breakdown process in PMMA obtained using an image converter camera, +50 kV point voltage, 1.70 mm gap, 10 ns exposure time, 40 ns between frames. (b). The corresponding breakdown channels viewed by an optical microscope.

bushing and an optically flat glass window a t each side to observe the test sample. The solid dielectric test sam- ple was a 5 x 5 ~ 1 0 n1m3 PMMA block (Mitsubishi Ray- on Ltd., Acrylitem). A point-to-plane gap configuration was used for the test. The point electrode was a sewing needle with a tip radius of - 10 pm. Little difference in shape and radius between these needles was observed, using an optical microscope. The point electrode was in- serted into the solid a t 190°C; then the sample was cooled down to room temperature a t the rate of 1.25'C/min to reduce internal mechanical stress. The bottom surface of specimen wa.s painted with conducting paint, as a plane electrode. This plane electrode contacts a flexible con- ductive wire, connected to the ground through a resistor, thus avoiding the mechanical stress induced in the solid by rigid contact. Proper contact was ensured by direct inspection through the windows. The test sample was immersed in transformer oil to prevent surface flashover. A 96 kR resistor was connected between the plane elec- trode and the ground to limit the gap current. Rectangu- lar voltage pulses of 50 kV amplitude, 20 ns rise time and 760 ns duration were used. The pulses were obtained by a coaxial line pulser of the self-matched type [4]. A typ- ical voltage pulse shape is shown in Figure 1. An image converter camera (Imacon 790) with 10 ns exposure time and 40 ns framing intervals was used to obtain sequential images of a breakdown channel on a single photograph- ic record. Each measurement, with a fresh sample and electrode, was carried out at room temperature. After voltage application, the gap spacing was measured using

an optical microscope.

3. RESULTS AND DISCUSSION The largest gap spacing for total breakdown at 50 kV

amplitude with positive and negative points are 1.81 and 0.47 mm, respectively. Since gap spacings > 1 mm give better resolution in the photographs, the test was carried out under positive point conditions. Typical sequential records of the breakdown process for a positive point and 50 kV application are shown in Figure 2(a). The number a t the edge of each frame indicates the fram- ing sequence. The second frame is taken 30 ns after the leading edge of pulse voltage. The streamer starts to de- velop at the time between the first and second frame. To- tal breakdown takes place between the second and third frame. The second frame shows an aspect of breakdown channel propagating across the gap. The intensity of the channel luminosity is considerably higher than that for liquid dielectrics [5,6]. The Figure indicates that the channel propagates a distance of - 1.5 mm within 40 ns. The propagation velocity of the breakdown chan- nel can be deduced from these values to be > 40 km/s. Breakdown channels shown in Figure 2(b), obtained us- ing the optical microscope, were formed by the break- down event corresponding to Figure 2(a). Sixteen pic- tures, which show the breakdown channel propagating from the positive point to the plane electrode as seen in Figure 2(a), could be taken out of 50 shots for a gap spacing of - 1.5 mm. From this result, the propagation velocity of the breakdown channel for a positive point in

75 6 Yamada et al.: PMMA Breakdown under ns Pulse Voltages

PMMA can be roughly estimated to be 80 km/s. The velocity is of the same order as that of secondary fast streamers in several hydrocarbon liquids [6,7],

Under positive point conditions, the electric field a t the tip is so high that electrons are probably pulled from neighboring molecules. They in turn can pull electrons from other molecules, and field ionization occurs [8]. This hole injection means that molecules give up electrons to the anode and become positive ions to produce high posi- tive space cha.rge density 191. I t seems that such electron hopping lea,ds to the high propagation velocity of the brea.kdown channel in the order of 10 km/s. Because the channel conta.ins a high concentration of positive charge near the point electrode, its conductivity is high enough for the tip to be of almost the same potential as the elec- trode. Thus, the channel can be considered to be an extension of the point electrode into the liquid [lo].

REFERENCES [l] I. Kitani and K. Arii, “Impulse Tree and Discharge

Light in PMMA Subjected to Nanosecond Pulse”, IEEE Trans. Elec. lnsul., Vol. 19, pp. 281-287,1984

under Microsecond Pulse Voltage”, IEEE Trans. Elec. lnsul., Vol, 26, pp. 708-714, 1991.

[4] M. Ishii and H. Yamada, “Self-matched HV Rectangular Wave Pulse Generator”, Rev. Sci. lnstrum., Vol . 56, pp. 2116-2118, 1985.

[5] R. E. Hebner, E. F, Kelley, E. 0. Forster and G. J. FitzPatrick, “Observation of Prebreakdown and Breakdown in Liquid Hydrocarbons 11, Non- uniform Field Condition”, IEEE Trans. Elec. lnsul., Vol. 20, pp. 281-292, 1985

[6] G. J. FitzPatrick, P. J . McKenny and E. 0. Forster, “The Effect of Pressure on Streamer Inception and Propagation in Liquid Hydrocarbons”, IEEE Trans. Elec. Insul., Vol. 25, pp. 672-682, 1990.

[7] E. 0. Forster, “Progress in the Field of Electri- cal Breakdown in Dielectric Fluids”, IEEE Trans. Elec. lnsul., Vol. 20, pp. 905-912, 1985

[8] B. Halpern and R. Gomer, “Field Ionization in Liq- uids”, J . Chem. Phys., Vol. 51, pp. 1048-1056,1969

191 W. G. Chadband, “On Variations in the Propa- gation of Positive Discharges between Transformer Oil and Silicone Fluids”, J. Phys. D, Vol. 13, pp.

[2] H. Yama.da., T. Fujiwara, K. Tamada, S. Kimura 1299-1307, 1980 and T‘ Measurement Of Break- down Proc.ess in Polymethylmethacrylate under Nanosecond Pulse Voltage Application”, Trans. IEE Japan, Vol. 112-A, pp. 237-243, 1992 (in Japanese).

[3] H. Yama.da, T. Murakami, K. Kusano, T. Fujiwara and T. Sato, “Positive Streamer in Cyclohexane

[lo] J . C. Devins, S. J . Rzad, and R. J . Schwabe, “Breakdown and Prebreakdown Phenomena in Liquids”, J. Appl. Phys., Vol. 52, pp. 4531-4545, 1981

Manuscript was receivedon 15 January 1994.