IMPULSE BREAKDOWN OF COMPOSITE INSULATION IN A STRONGLY NONUNIFORM FIELD

A. T. Chepikov, B. V. Semkin

(Presented by the scientific seminar of the Department of High-Voltage Engineering)

In various high-voltage electrical-engineering devices, composite insulation consisting of solid and liquid dielectrics is widely used. Under normal operating conditions or in transient processes, such insulation may be subjected to voltage impulses of various durations.

The literature contains extensive information on the impulse strength of both solid and liquid insulation. Data on the impulse breakdown of composite insulation [1, 2] are limited and pertain to thicknesses not exceeding 1 mm. It was of interest to study the impulse breakdown of the combination “solid dielectric—transformer oil” at various ratios of their thicknesses and at various exposure times of the impulse voltage.

The studies were carried out using an impulse-voltage generator rated at 480 kV with a discharge capacitance of 0.016 μF. The voltage was measured by means of a capacitive voltage divider and an OK-19M impulse oscillograph, modified for measuring times from 0.1 to 12 μsec.

Breakdown was produced on the front of a single impulse in a strongly nonuniform field. The electrodes used were a steel rod 8 mm in diameter with a sharpening angle of 30° and a brass plane 140 mm in diameter. Specimens of fluoroplastic-4 were cut from sheets of one batch and had thicknesses of 20; 7.5; 5; 3.3 mm, with a specimen area of \(50 \cdot 50\ \mathrm{mm}^2\).

When the thickness of the fluoroplastic specimens was changed, the total length of the gap between the electrodes remained constant and was equal to 10 mm. At

\[ \frac{T}{S} = 1 \]

(Fig. 1), that is, in the absence of an oil layer, the specimen thickness was 20 mm; a conical recess 10 mm deep was drilled in the specimens, and the recess was coated with Aquadag. In this case as well, the interelectrode distance was 10 mm. The relative position of the specimen and the oil layer is shown in Figs. 1 and 2. The electric strength of the transformer oil was 30 kV in a standard breakdown apparatus.

Each point of the curves was obtained on the basis of breakdown of 20 specimens. The scatter of individual values from the mean breakdown-voltage value was \(\pm 2\%\). In constructing the curves, the breakdown probability was taken as \(\Psi = 50\%\).

Figure 1 shows the dependence of the breakdown voltage of the composite insulation “fluoroplastic-4—transformer oil” on the ratio of the thickness of the solid dielectric to the interelectrode spacing, for positive polarity of the rod. The dependences were obtained for voltage exposure times of 0.1; 1; 10 μsec. Figure 2 shows the analogous dependence obtained for negative polarity of the rod. From Figs. 1 and 2 it is seen that, at an exposure of \(10^{-5}\) sec, the breakdown voltage of transformer oil is lower than that of fluoroplastic-4 by 27% for both polarities of the rod. As the exposure is decreased, this difference

Fig. 1. Dependence of the breakdown voltage of the composite insulation “fluoroplastic-4—transformer oil” on the ratio of the thickness of the solid dielectric to the interelectrode spacing, for positive polarity of the rod. Voltage exposure time:
● — \(10^{-7}\) sec,
▲ — \(10^{-6}\) sec,
■ — \(10^{-5}\) sec.

Fig. 2. Same as Fig. 1, but for negative polarity of the rod.

decreases, and already at \(t = 10^{-6}\) sec the electric strength of transformer oil is higher than the strength of fluoroplastic-4 by 47% and 38% for positive and negative polarity of the rod, respectively.

The breakdown voltage of the composite insulation over the entire range of investigated exposures occupies an intermediate position between the breakdown voltages of the solid and liquid insulation.

At short voltage exposure times (\(10^{-7}\) sec), the liquid dielectric has a greater influence on the breakdown voltage of the composite insulation than the solid one. Thus, at

\[ \frac{T}{S} = 0.5 \]

the breakdown voltage of the composite insulation is lower than \(U_{\mathrm{br}}\) of the liquid insulation by 8.5% and 10% for positive and negative polarities of the rod, respectively. At the same time, the breakdown voltage of fluoroplastic is lower than \(U_{\mathrm{br}}\) of the composite insulation by 21.4 and 25.3%.

Conclusions

1. The increase in the electric strength of composite insulation at short voltage exposure times, as compared with the electric strength of solid dielectrics, can be used in electrical engineering devices whose insulation operates under the action of voltage pulses of duration \(1 \cdot 10^{-6}\) sec and less.

2. Under the action of pulses of longer duration \((10^{-5}\) sec), the impulse strength of composite insulation is lower than that of solid dielectrics.

References

1. K. Kojima and S. Tanaka. “Research on the Impulse Breakdown Mechanism of the Dielectric as Used in Oil—Filled Cables,” IEEE Transactions on Power Apparatus and Systems, No. 5, May, 1964.

2. W. G. Standring, M. A. and R. C. Hughes, B. Sc. “Breakdown under Impulse Voltages of Solid and Liquid Dielectrics in Combination.” The Proceedings of the Institution of Electrical Engineers, part A, December, 1956.