PULSED ELECTRICAL BREAKDOWN OF SERIES COMBINATIONS OF TRANSFORMER OIL WITH SOME POLYMERIC DIELECTRICS OVER A WIDE TEMPERATURE RANGE

Yu. I. Lekht

(Presented by the scientific seminar of the Research Institute of High Voltages)

In the practice of high-voltage apparatus engineering, combined insulation is widely used. The widely used paper-oil insulation sharply changes its electrical properties with changing temperature. And the range of operating temperatures of a number of high-voltage installations for individual regions of the country may be \((-50 \div +70)^\circ\mathrm{C}\). According to literature data [1–3], some solid polymeric dielectrics change their electrical properties only slightly over a wide temperature range. Therefore, the study of the pulsed characteristics of the electrical breakdown of combinations of transformer oil with some polymeric dielectrics is of definite scientific and practical interest.

In the present work, combinations of transformer oil with high- and low-density polyethylene and with fluoroplast-4 were investigated at temperatures from \(-40\) to \(+90^\circ\mathrm{C}\).

Samples of polymeric dielectrics of rectangular shape were cut from sheets and blocks of a single batch received. In the case where the solid dielectric was placed at the point, in order to eliminate edge discharges in the samples a conical recess was drilled with an apex angle of the cone of \(30^\circ\). The thickness of the samples at the breakdown point was taken as \(1\ \mathrm{mm}\) and was checked with an IZV-1 instrument with an accuracy of up to \(0.01\ \mathrm{mm}\). In the case where the solid dielectric was placed on the plane, the thickness of the samples over the entire surface was the same and was \(1\ \mathrm{mm}\). The thickness of the samples was checked with a micrometer with an accuracy of up to \(0.01\ \mathrm{mm}\).

The investigations were carried out in an MPS-500-type thermal chamber with a specially mounted high-voltage bushing for \(300\ \mathrm{kV}\) (operating-temperature interval from \(-70\) to \(+100^\circ\mathrm{C}\)). The temperature in the chamber was measured with an electric thermometer with an accuracy of up to \(1^\circ\mathrm{C}\).

Technically pure transformer oil was used as the insulating medium; its electric strength was \(40\ \mathrm{kV}\) in a standard oil tester. The thickness of the oil gap was \(1\ \mathrm{mm}\).

Before breakdown, a batch of samples, as well as the transformer oil in a porcelain vessel, was kept in the thermal chamber at the specified temperature for no less than 60 min. The investigations were carried out in a sharply nonuniform point–plane field on voltage pulses of positive polarity.

The source of single high-voltage pulses was a multistage pulse-voltage generator with a maximum amplitude of 400 kV and a discharge capacitance of 0.012 μF. The magnitude of the breakdown voltage was recorded with an OK-19M2 oscilloscope connected through an active divider with a resistance of 8200 ohms. Breakdown characteristics were taken on the front of a single pulse at rates of voltage rise on the specimen of \(5 \cdot 10^{11}\), \(2.5 \cdot 10^{11}\), and \(0.3 \cdot 10^{11}\) V/sec. Each point of the obtained characteristics was constructed on the basis of processing 20 or more breakdown oscillograms.

Figure 1 presents the temperature dependences of the impulse breakdown voltages of a series combination of transformer oil with low-density polyethylene. The breakdown voltages

Fig. 1. Temperature dependences of the breakdown voltages of a combination of transformer oil with low-density polyethylene: 1, 2, 3—polyethylene is located at the point (\(a = 5 \cdot 10^{11}\), \(2.5 \cdot 10^{11}\), and \(0.3 \cdot 10^{11}\) V/sec, respectively); 4, 5, 6—polyethylene is located on the plane (\(a = 5 \cdot 10^{11}\), \(1.5 \cdot 10^{11}\), and \(0.3 \cdot 10^{11}\) V/sec, respectively).

increase with increasing steepness of the pulse front in the investigated temperature range both in the case where the solid dielectric is located at the point and in the case where it is placed on the plane. It was noted that, in the range of the investigated exposures, the breakdown voltages are practically independent of temperature in the interval from \(+90\) to \(-40^\circ\)C. At the same time, the breakdown voltages when the polyethylene is located at the electrode with the smaller radius of curvature are higher than when the oil layer is placed in the region of higher field intensities. Moreover, as the rate of voltage rise on the specimen decreases, this difference increases. For example, at pulse-front steepnesses of \(5 \cdot 10^{11}\), \(2.5 \cdot 10^{11}\), and \(0.3 \cdot 10^{11}\) V/sec, the difference between the breakdown voltages was 7%, 11%, and 25%, respectively. This is explained by the fact that, in the breakdown of series combinations, the decisive role is played by the electric strength of the dielectric located in the region of higher field intensities. It is known [4, 5] that at short voltage exposure times the electric strength of transformer oil is higher than the strength of polyethylene. As the voltage exposure increases, the strength of transformer oil decreases more rapidly than the strength of polyethy-

Fig. 2. Temperature dependences of the breakdown voltages of a combination of transformer oil with high-density polyethylene: 1, 2, 3 — polyethylene is located at the point \((a = 5 \cdot 10^{11},\ 2.5 \cdot 10^{11}\ \text{and}\ 0.3 \cdot 10^{11}\ \text{V/sec},\ \text{respectively})\); 4, 5, 6 — polyethylene is located on the plane \((a = 5 \cdot 10^{11},\ 2.5 \cdot 10^{11}\ \text{and}\ 0.3 \cdot 10^{11}\ \text{V/sec},\ \text{respectively})\).

Fig. 3. Temperature dependences of the breakdown voltages of a combination of transformer oil with fluoroplastic-4: 1, 2, 3 — transformer oil at the point \((a = 5 \cdot 10^{11},\ 2.5 \cdot 10^{11}\ \text{and}\ 0.3 \cdot 10^{11}\ \text{V/sec},\ \text{respectively})\); 4, 5, 6 — fluoroplastic-4 at the point \((a = 5 \cdot 10^{11},\ 2.5 \cdot 10^{11}\ \text{and}\ 0.3 \cdot 10^{11}\ \text{V/sec},\ \text{respectively})\).

...of polyethylene. In our experiments, at a voltage rise rate of \(5 \cdot 10^{11}\ \mathrm{V/s}\), the strength of polyethylene is only slightly higher than the strength of transformer oil. Therefore, the difference between the breakdown voltages of the two combinations is small. In the case of a pulse-front steepness of \(0.3 \cdot 10^{11}\ \mathrm{V/s}\), breakdown of polyethylene occurs at considerably higher voltages than breakdown of transformer oil. Hence there is such a large difference (25%) between the breakdown voltages of the combinations studied.

All that has been set forth above can also be applied to the combination of transformer oil with high-pressure polyethylene (Fig. 2).

Figure 3 presents the temperature dependences of the breakdown voltages of a series combination of transformer oil with fluoroplastic-4. In this case as well, the breakdown voltages, with increasing pulse-front steepness in the temperature range studied, increase. However, at pulse-front steepnesses of \(5 \cdot 10^{11}\) and \(2.5 \cdot 10^{11}\ \mathrm{V/s}\), the breakdown voltages when the oil layer is located at the point are higher than when fluoroplastic-4 is located at the point. Moreover, in the first case a monotonic decrease in the breakdown voltage with increasing temperature is observed. This is explained by the fact that, for the above-mentioned exposures of the applied voltage, the strength of transformer oil is higher than the strength of fluoroplastic. The strength of the oil decreases with increasing temperature, whereas the strength of the fluoroplastic changes practically not at all. Therefore, the difference between the breakdown voltages of the two combinations decreases with increasing exposure and temperature. For example, at \(t = 40^\circ\mathrm{C}\) and \(a = 5 \cdot 10^{11}\ \mathrm{V/s}\), the breakdown voltages when the transformer oil is located at the point are 35% higher than when fluoroplastic is located at the point. At \(t = +90^\circ\mathrm{C}\) this difference is 30%. And at \(a = 2.5 \cdot 10^{11}\ \mathrm{V/s}\) this difference is 26% at \(t = -40^\circ\mathrm{C}\) and 20% at \(t = +90^\circ\mathrm{C}\).

The work carried out makes it possible to conclude that, when selecting combined insulation for the region of elevated field intensities, it is necessary to place the dielectric with the greater electrical strength.

REFERENCES

1. A. F. Nikolaev. Synthetic Polymers and Plastics Based on Them. Moscow—Leningrad, 1964.
2. A. A. Tager. Physical Chemistry of Polymers. Moscow, 1963.
3. G. I. Skanavi. Physics of Dielectrics (Region of Strong Fields). GIFML, 1958.
4. A. T. Chepikov, A. I. Limasov. Volt-second characteristics of organic glass and fluoroplastic-4 at large thicknesses. Izv. SO AN SSSR, No. 7, 1962.
5. A. T. Chepikov, A. I. Limasov. Volt-second characteristics of polar and nonpolar liquids. Izv. SO AN SSSR, No. 8, 1962.