EFFECT OF TEMPERATURE ON THE IMPULSE BREAKDOWN VOLTAGE OF SOME SOLID POLYMER DIELECTRICS

V. V. Krivko, Yu. I. Lekht

(Presented by the Seminar of the Research Institute of High Voltages)

Owing to the considerable advances of chemistry in the last decade, insulation and cable engineering have received a large number of new polymer dielectrics with fairly good electrophysical characteristics (polymethyl methacrylate, polyethylene, polytetrafluoroethylene, polystyrene, etc.).

The broad introduction of high-voltage, and especially impulse, technology into various technological processes necessitates the creation of special insulating structures and high-voltage cables using polymer materials.

The range of operating temperatures of a number of high-voltage installations and devices for certain regions of the country may be from −50 to +70°C. Therefore the study of the electrical properties of polymer materials over a wide temperature interval is of scientific and practical interest.

At present there is already a large amount of experimental material [1—4] on the study of the temperature characteristics of various polymer dielectrics. However, data on the effect of temperature on the impulse breakdown voltage of polymers at different durations of voltage exposure are lacking.

The present work is devoted to the investigation of the impulse breakdown characteristics in a nonuniform field of certain polymer dielectrics (polytetrafluoroethylene, low- and high-pressure polyethylene) at temperatures from −40 to +90°C. The results of the work may be useful in designing the insulation of special high-voltage impulse installations and cables.

Experimental Method

Specimens of polymer materials of rectangular shape, with dimensions \(30 \times 30 \times 5\) mm, were cut on a milling machine from sheets and blocks 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 at the breakdown point was taken as 2 mm and was checked with an IZV-1 instrument with an accuracy of up to 0.01 mm. The investigations were carried out in an MPS-500 type thermal chamber with an operating-temperature range from −70° to +100°C and with a specially mounted high-voltage bushing rated at 300 kV. The temperature in the chamber was measured with electric and mercury thermometers with an accuracy of up to 1°C.

A batch of specimens before breakdown at the specified temperature was kept in the thermostat chamber for not less than 30 min.

The breakdown characteristics were taken with “rod—plane” electrodes. The molybdenum rod electrode, 3 mm in diameter, had a 30° cone. The plane, 90 mm in diameter, was made of brass. Single pulses of positive polarity were applied to the rod; the plane was grounded. The electrode device with the specimen was placed in a porcelain vessel 120 mm in diameter, filled with transformer oil having an electric strength of 45 kV in a standard oil tester. This excluded the possibility of surface flashover of the specimens for the selected linear dimensions of the specimens.

The source of single high-voltage pulses was a multistage impulse-voltage generator with a maximum amplitude of 400 kV and a discharge capacitance of 1200 pF. The minimum rise time of the pulse was 0.2 μsec and was increased to 4 μsec by connecting into the circuit the shaping inductances of 6 mH and capacitance of 160 pF. The value of the breakdown voltage was recorded with an OK-19M2 oscilloscope connected through an active divider with a resistance of 8200 ohms.

Breakdown of the specimens was carried out on the front of a single pulse. The breakdown probability was taken as equal to 90%. Each point of the obtained characteristics was constructed on the basis of processing 20 or more breakdown oscillograms.

Experimental results and their discussion.

Figure 1 presents the dependences of the impulse breakdown voltages of polytetrafluoroethylene in a nonuniform field on temperature at pulse-front steepnesses \(a = 500\) kV/μsec (curve 1), \(a = 250\) kV/μsec (curve 2), and \(a = 30\) kV/μsec (curve 3). The pulses were of positive polarity; the thickness of the specimens at the breakdown site was 2 mm.

With an increase in pulse steepness, the breakdown voltage of polytetrafluoroethylene, at constant temperature and with other conditions being equal, increases over the entire investigated temperature range from \(-40\) to \(+90^\circ\)C. For example, for a temperature of \(+20^\circ\)C and steepnesses of 30, 250, and 500 kV/μsec, the breakdown voltages are, respectively, 65, 75, and 90 kV. The increase in breakdown voltage obtained in our experiments with increasing steepness of the pulse front is associated with the phenomenon of discharge lag and agrees well with known data [6–7]. In the temperature interval from 20 to 70–80°C, the breakdown voltages for all steepnesses remain unchanged. With a further increase in temperature above 80°C, there is a certain tendency toward a decrease in the breakdown voltages, the maximum decrease from the value at normal temperature (20°C) not exceeding 7%. The preservation of the breakdown voltage practically unchanged at elevated temperatures is natural, since polytetrafluoroethylene possesses high heat resistance. A noticeable change in physical properties is observed at temperatures above 200°C [2].

Lowering the temperature from 20 to \(-10 \div -20^\circ\)C causes a decrease in the breakdown voltages by, on average, no more than 15%. With a further decrease in temperature (to \(-40^\circ\)C), the breakdown voltages increase to the values at normal temperature.

When the temperature is decreased from normal in polymer dielectrics, the ratio between the crystalline and amorphous phases changes [2]. With an increase in the crystalline phase of the dielectric, its mechanical, as well as electrical, strength increases [5]. On the other hand, when the structure changes, internal mechanical stresses must arise, which lead to a decrease in the breakdown voltages.

It is probable that, in the temperature interval of structural changes in the material \((+20\text{ to }-20^\circ\mathrm{C})\), under the conditions of the given experiment, with a comparatively short holding time of the specimens at an unchanged temperature (30 min), the second factor has the predominant influence. Therefore, in the indicated temperature region a decrease in the breakdown voltages takes place.

Since, during breakdown at lower temperatures, the specimens, as they were cooled, remained in the chamber for a longer time, the mechanical internal stresses could be relieved, and the gradual increase in the crystalline phase of the polymer caused an increase in the breakdown voltages.

Fig. 1. Temperature dependences of the impulse breakdown voltages of polytetrafluoroethylene at different front steepnesses.
1—\(a=500\ \mathrm{kV}/\mu\mathrm{s}\); 2—\(a=250\ \mathrm{kV}/\mu\mathrm{s}\); 3—\(a=30\ \mathrm{kV}/\mu\mathrm{s}\).

breakdown voltages. The explanation given above for the change in the electromechanical properties of the material with temperature is confirmed by our additional experiment. At a temperature of \(-20^\circ\mathrm{C}\), the specimens were held for 3 hours (instead of 30 min). The value of the breakdown voltage at a steepness of \(250\ \mathrm{kV}/\mu\mathrm{s}\) was \(77\ \mathrm{kV}\), i.e., it proved to be practically equal to the voltage at normal temperature.

Figure 2 gives the values of the breakdown voltages of high-density polyethylene (curves 1, 3) and low-density polyethylene (curves 2, 4), respectively, for front steepnesses of 500 and \(30\ \mathrm{kV}/\mu\mathrm{s}\) in the temperature interval from \(-40\) to \(+90^\circ\mathrm{C}\). The thickness of the specimens was \(2\ \mathrm{mm}\); the electrodes were rod–plane; the impulses were of positive polarity.

The dependences of the breakdown voltages of polyethylene and polytetrafluoroethylene on temperature are similar. This is quite natural, since both polymers are nonpolar and have approximately identical electrophysical characteristics \(/ \varepsilon, \tan\delta / [2]\). The principal interpretations of the temperature dependence of the breakdown voltages of polytetrafluoroethylene are also applicable to polyethylene. However, it should be noted that, owing to the reduced

because of polyethylene’s lower heat resistance, its breakdown voltages decrease more steeply with increasing temperature. The temperature interval of unchanged breakdown voltages shifts into the region of lower temperatures (60–70°C).

The experimental results make it possible to draw the following conclusions:

1. In the temperature interval from −40 to +90°C, the breakdown voltages of polytetrafluoroethylene and high-density polyethylene increase as the steepness of the impulse front increases.

Fig. 2. Temperature dependences of the impulse breakdown voltages of high- and low-density polyethylene at different front steepnesses. 1 — high-density polyethylene, \(a = 500\ \mathrm{kV/\mu s}\); 2 — low-density polyethylene, \(a = 500\ \mathrm{kV/\mu s}\); 3 — high-density polyethylene, \(a = 30\ \mathrm{kV/\mu s}\); 4 — low-density polyethylene, \(a = 30\ \mathrm{kV/\mu s}\).

1. The breakdown voltages of polytetrafluoroethylene in the temperature interval from 20 to 80°C, and of polyethylene from 20 to 70°C, remain practically unchanged.

2. Raising the temperature above 70°C leads to a sharp decrease in the breakdown voltage of polyethylene.

3. For the polymers investigated, in the temperature range from 20 to −20°C, when the specimens are held at an unchanged temperature for 30 minutes, a decrease in breakdown voltages occurs, caused by structural changes in the material.

1. A decrease in the breakdown voltages of polyethylene and polytetrafluoroethylene may not be observed if the change in the indicated region takes place over a long period of time, sufficient for the relaxation of internal stresses. Naturally, determining the exact limit of the influence of the time factor, when temperature is varied, on the impulse breakdown voltage of various polymers requires additional experimental investigations.

REFERENCES

1. G. I. Skanavi. Physics of Dielectrics (the Region of Strong Fields). State Publishing House for Physical and Mathematical Literature, Moscow, 1958.

2. A. F. Nikolaev. Synthetic Polymers and Plastics Based on Them. Khimiya Publishing House, Moscow—Leningrad, 1964.

3. J. B. Birks and J. G. Schulman. Progress in Dielectrics, Vols. I and II. Gosenergoizdat, 1962, 1963.

4. A. R. Hippel. Dielectrics and Their Applications. Gosenergoizdat, 1959.

5. A. A. Vorob’ev, E. K. Zavadovskaya. Electrical Strength of Solid Dielectrics. GITTL, 1956.

6. G. A. Vorob’ev. Dissertation, Tomsk, 1956.

7. A. A. Vorob’ev, G. A. Vorob’ev. Izvestiya of the USSR Academy of Sciences, Physical Series, Vol. XXIV, No. 1, 1960.