ON THE EVALUATION OF IMPULSE BREAKDOWN VOLTAGES OF SOME DIELECTRICS AT VARIOUS TEMPERATURES

V. V. Krivko, Yu. I. Lekht

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

At the present time, high-voltage impulse technology is being successfully introduced into various technological processes. The need to create compact, reliable high-voltage installations and devices prompts research and development of insulation with high electrical characteristics. The most widespread is combined insulation using transformer oil and solid dielectrics.

In view of the possible sufficiently wide range of operating temperatures of special high-voltage installations, the study of the temperature characteristics of electrical breakdown of individual components of complex insulation is of definite interest.

It is known that, with a change in temperature, the density and viscosity of a liquid, the state of moisture, the conditions of emission from the cathode, etc., change. All this, to a greater or lesser extent under different conditions, affects the breakdown voltages of liquid dielectrics. The dependence, as a rule, has a maximum. It is assumed [1, 2] that with a sufficiently long exposure to voltage, thermal breakdown is possible for liquids of technical purification. In this case, the occurrence of breakdown is associated with the process of boiling of the liquids.

In experimental studies of the breakdown of various liquids under pulses with a duration of 1 μsec and more [1—6], a certain decrease in breakdown voltages with increasing temperature has been established. Moreover, the decrease in voltage occurs along the same curve as the decrease in its density.

A whole series of works [7—12] has also been devoted to the study of the influence of temperature on the impulse breakdown voltage of solid dielectrics. However, voltage measurements were carried out in the region of positive temperatures, on pulses with a duration of more than 1 μsec, and mainly for crystalline dielectrics. The data are contradictory in character. In individual works [8—12], temperature maxima of strength were observed.

The present article gives the results of experimental measurements of impulse breakdown voltages of polytetrafluoroethylene, transformer oil, and water, which, under pulses with a steep front, has a strength of several tens of kV/cm. The operating temperature interval for polytetrafluoroethylene and transformer oil was −40 ÷ +100°C. The breakdown voltages for water and transforma-

transformer oil in the temperature interval from +15 to 100°C were measured at various steepnesses of the front of a positive-polarity impulse (from 10 to 1250 kV/μsec).

Experimental procedure

The temperature dependences of transformer oil, technically purified water, and polytetrafluoroethylene were investigated in an MTS-500 thermal chamber with an operating-temperature range from −70 to +100°C, and with a specially mounted high-voltage bushing for impulse voltage of 300 kV. The temperature in the chamber was measured with electric and mercury thermometers with an accuracy of up to 1°C. The breakdown characteristics were recorded on “rod—plane” electrodes. Single positive-polarity impulses were applied to a molybdenum rod electrode 3 mm in diameter with a cone angle at the tip of 30°. The brass plane, 90 mm in diameter, was grounded. The distance between the electrodes was 2 mm. The electrode assembly was placed in a porcelain vessel 120 mm in diameter.

The source of single high-voltage impulses was a multistage impulse-voltage generator with a maximum amplitude of 400 kV and a discharge capacitance of 12000 pF. The magnitude of the breakdown voltage was recorded by means of an OK-19 oscilloscope connected through an active divider with a resistance of 8200 ohms. The strength of the transformer oil in a standard oil breakdown tester was 35 kV. The resistivity of the water was equal to \(1.5 \cdot 10^{-3}\) ohm·cm.

Rectangular specimens of polytetrafluoroethylene with dimensions \(30 \times 30 \times 5\) mm were cut on a milling machine from sheets of one delivery batch. To eliminate the edge effect, a conical recess with an angle of 30° was drilled in the specimens. The thickness of the specimens was checked with an IZV-1 instrument with an accuracy of up to 0.01 mm. To prevent surface flashover of the specimens at the selected linear dimensions, the specimens were placed in transformer oil.

A batch of polytetrafluoroethylene specimens, as well as the liquid dielectrics, was held in the thermal chamber for at least 30 min before breakdown at the specified temperature.

Breakdown of the liquids and of the polytetrafluoroethylene specimens was carried out on the front of a single impulse. Each point of the obtained characteristics was plotted on the basis of processing 20 or more breakdown oscillograms.

Experimental results and their discussion

Figure 1 gives the dependences of the breakdown voltages of transformer oil (curve 3), water (curve 2), and polytetrafluoroethylene (curve 1). The impulses were of positive polarity, the front steepness was 250 kV/μsec. The thickness at the breakdown point was 2 mm.

In the temperature interval from +20 to +90°C, the breakdown voltage of polytetrafluoroethylene under the given experimental conditions remained unchanged because of the sufficiently high heat resistance of the material (maximum operating temperature above 200°C). When the temperature was lowered from +20 to −10°C, the breakdown voltage of polytetrafluoroethylene decreased by 10–15%, and then, with a further decrease in temperature, increased and at a temperature of −40°C became the same as at normal temperature. It may be assumed that a certain decrease in breakdown voltage is associated with the presence of internal mechanical stresses. If polytetrafluoroethylene is held for a longer time at reduced temperatures, the internal stresses associated

associated with a change in the ratio between the crystalline and amorphous phases are removed and do not affect the mechanical or the electrical strength of the material. Thus, when the specimens were held at a temperature of \(-20^\circ\mathrm{C}\) for 3 hours (instead of 30 min), the value of the breakdown voltage was 77 kV, i.e., it proved to be equal to the voltage at a temperature of \(+20^\circ\mathrm{C}\).

Fig. 1. Temperature dependences of impulse breakdown voltages of polytetrafluoroethylene (curve 1), technical-purity water (curve 2), and transformer oil (curve 3). Impulse front steepness \(250\ \mathrm{kV}/\mu\mathrm{s}\).

In the investigated temperature interval, the breakdown voltages of transformer oil and water remain practically unchanged and exceed the breakdown voltage of polytetrafluoroethylene. The latter indicates the possibility of using technical-purity water, for impulses with a steep front (\(250\ \mathrm{kV}/\mu\mathrm{s}\) and more), as an insulating medium. The density of water, as the temperature increases from 15 to \(98^\circ\mathrm{C}\),

Fig. 2. Dependences of the impulse breakdown voltages of transformer oil (curves 1, 2, 3) and technical-purity water (curves 4, 5, 6) on the steepness of the impulse front at various temperatures.
1 and 4 — \(t = 15^\circ\mathrm{C}\), 2 and 5 — \(t = 60^\circ\mathrm{C}\), 3 and 6 — \(t = 98^\circ\mathrm{C}\).

decreases from 0.01 to 0.002 [13]. Such an insignificant change in density is unlikely to have a substantial effect on the discharge time and, consequently, on the magnitude of the discharge voltage [1].

Figure 2 presents the values of the breakdown voltages of transformer oil (curves 1, 2, 3) and water (curves 4, 5, 6) under impulses of positive polarity with different front steepness. The distance between the electrodes at the breakdown point is 10 mm. Operating temperatures: \(+15\), \(+60\), and \(+98^\circ\mathrm{C}\).

It follows from Fig. 2 that, as the steepness of the impulse increases, the breakdown voltages increase at all the temperatures investigated. This is in full agreement with previously known data [1, 3] and is explained by the phenomenon of discharge delay.

A certain decrease in the breakdown voltages (15–20%) of water and transformer oil at elevated temperatures (98 and \(100^\circ\mathrm{C}\)) under impulses with a steepness of less than 100 kV/µsec is probably connected with the fact that, for long times of voltage application, the discharge can develop along gas bubbles. Under steep impulses, gas bubbles, like other inclusions in a liquid, have no effect on the breakdown voltage.

Conclusions

1. Under impulses with a front steepness of 250 kV/µsec and higher, water of technical purity may be used as an insulating liquid.

2. In the temperature interval from \(-40\) to \(+90^\circ\mathrm{C}\) (for water, from \(+15^\circ\mathrm{C}\)), the breakdown voltages of transformer oil, water, and polytetrafluoroethylene remain practically unchanged. This indicates the possibility of reliable operation, within the investigated temperature range, of high-voltage devices using water and transformer oil.

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