Volt-second characteristics of impulse flashover of some materials

ON THE BARRIER EFFECT IN SOLID DIELECTRICS

D. D. Rumyantsev, N. M. Torbin

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

Increasing the breakdown voltages of air gaps or liquid dielectrics in the presence of a barrier is widely used in practice. It is known that, with a certain arrangement of the barrier in a nonuniform field in air [1, 3, 5] and in a liquid dielectric [5, 7], the breakdown voltage of the gap can be increased by 50–200% relative to the breakdown voltage without a barrier. The greatest increase in breakdown voltage in air and transformer oil is observed in the sharply nonuniform field of “point—plane” electrodes, with positive polarity of the point under dc voltage, when the barrier is located at 10–25% of the gap length from the point. The barrier effect is observed under all types of voltage action: dc, ac, and impulse. As for the mechanism of the barrier effect, the assumptions concerning it are highly contradictory.

Voloshenko [2] believes that the mechanism of the barrier effect consists in the transformation of the electric field by a space charge concentrated on the barrier, and that the breakdown voltage of the “point—plane” gap is determined by the barrier—plane section, where the field approaches a uniform one.

Komelkov and Lifshits [3] also believe that a positive space charge plays the decisive role in the barrier effect, but that the breakdown voltage is determined by the point—barrier section.

Brago and Stekolshchikov [4] believe that a sufficient condition for completion of breakdown is the destruction of the barrier by the branches of the corona, while the role of the space charge in the barrier effect is insignificant.

Nikolaevskaya [6] reduces the role of the barrier to a “mechanical obstacle” for developing streamers. Thus, there is as yet no sufficiently clear understanding of the physical nature of the barrier effect in air and liquid dielectrics.

Experimental results on the breakdown of solid dielectrics under impulse voltage, obtained recently [8—10], have made it possible to establish a number of analogies in the development of discharge in solid and gaseous dielectrics.

It has been shown [11, 12] that an increase in breakdown voltage in the presence of a barrier is also observed in solid dielectrics. In [11] it is stated that, under impulse voltage, barriers made of triacetate film and metallic foil in rock salt increase the breakdown voltage—

by 18–20% relative to the breakdown voltage without a barrier. Barriers made of getinaks and textolite in MK-45 cable compound and in pure paraffin increase the breakdown voltage to 165–170% [12].

In the present work, studies were carried out of the change in the breakdown voltages of various solid dielectrics when a thin barrier made of a high-strength material was placed in them. The barrier effect was investigated in glass, rock salt, celluloid, and paper-oil insulation.

Experimental Method

Specimens of glass, rock salt, and celluloid consisted of plates of various thicknesses. The barrier was placed between the plates. The distance from the high-voltage electrode to the barrier was varied by means of combinations in the arrangement of the plates. The total thickness of the specimen remained unchanged during the experiment.

In studying the influence of barriers on the electric strength of paper-oil insulation, the specimens were assembled from sheets of cable paper 0.12 mm thick. The paper was impregnated with transformer oil. Sheets measuring 20 × 20 cm were placed one on another and clamped in an insulating frame. To reduce the probability of air inclusions between the sheets, assembly of the specimen was carried out under oil. The barrier was placed between the sheets. Films of organic materials (fluoroplastic-4, polyethylene, polystyrene), mica, triacetate film, and metal foil were used as barriers. The thickness of the mica, foil, and triacetate-film barriers was 5–7 μm; the thickness of the organic films was 25–30 μm.

The investigations were carried out under direct, alternating, and impulse voltages. The source of alternating and direct voltages was a 150 kV test transformer and a rectifier installation assembled according to a voltage-tripling circuit. The source of impulse voltage was voltage generators of 100 kV with a discharge capacitance of 11000 pF and of 1000 kV with a discharge capacitance of 22500 pF.

In breakdown under impulses, breakdown occurred on the front of the impulse. The voltage on the specimen was measured with an OK-19M oscilloscope. Specimens that had flashed over or partially flashed over were rejected. Breakdown of the specimens was carried out in transformer oil in a sharply nonuniform field of “point–plane” electrodes. The point served as the high-voltage electrode, and the plane was grounded.

Experimental Results

Some dependences obtained in studying the influence of a barrier on the impulse breakdown voltage of glass (1,2) and rock salt (3,4) are given in Fig. 1. The specimen thicknesses were \(d = 3\) mm and \(d = 5\) mm. As is seen from Fig. 1, the barrier effect decreases as the barrier is moved away from the point. The maximum strengthening is provided by a barrier of metal foil (4)—up to \(1.27\,U_0\). In glass, the greatest increase in \(U_{\text{br}}\) is observed with a mica barrier (2)—up to \(1.19\,U_0\), where \(U_0\) is the breakdown voltage without a barrier.

Figures 2 and 3 present the results of studying the barrier effect in celluloid. Figure 2 shows the change in \(U_{\text{br}}\) of celluloid with a barrier of polyethylene film as a function of the position of the barrier under direct (1,2) and alternating (3) voltages. The specimen thickness was \(d = 1.5\) mm.

Fig. 1. Effect of the position of the barrier on the breakdown voltage of glass and rock salt.
Barrier in glass: 1 — polyethylene film, 2 — mica; barrier in rock salt: 3 — triacetate film, 4 — metallic foil.

Fig. 2. Effect of a barrier made of polyethylene film on the breakdown voltage of celluloid.
1 — direct voltage, point of positive polarity; 2 — direct voltage, point of negative polarity; 3 — alternating 50 Hz voltage.

As follows from Fig. 2, the maximum increase in breakdown voltage (up to \(1.7\,U_0\)) is observed under dc voltage when the point has positive polarity. When the point has negative polarity, the barrier does not cause a substantial increase in \(U_{\mathrm{br}}\), and when the barrier is placed close against the point, even a decrease in \(U_{\mathrm{br}}\) is observed.

Figure 3 presents the dependence of \(U_{\mathrm{br}}\) of celluloid with a barrier made of fluoroplast-4 film on the position of the barrier. The specimen thickness is \(d = 8\) mm. The impulse front duration is \(\tau_{\mathrm{fr}} = 10\) μsec. The maximum increase in \(U_{\mathrm{br}}\) (up to \(1.35\,U_0\)) is observed when the barrier is located at \(0.25\,d\) from the point and does not depend on the polarity of the impulse. It should be noted that when the barrier is placed close against the point, an increase in \(U_{\mathrm{br}}\) is observed for positive point polarity and is not observed for negative polarity.

The results of the investigation of the barrier effect in paper-oil insulation are presented in Fig. 4 and in Table 1. Figure 4 shows the dependences of the increase in \(U_{\mathrm{br}}\) of cable-paper specimens on the position of fluoroplast-4 barriers under impulse (1, 2) and alternating (3) voltage. As can be seen from Fig. 4, the greatest increase in breakdown voltage (up to \(1.24\,U_0\) under alternating voltage and up to \(1.14\,U_0\) under impulse voltage) is observed when the barrier is located at \(0.20\text{--}0.40\,d\) from the point. The barrier effect is higher for impulses of longer duration.

Investigation of the influence of the barrier on \(U_{\mathrm{br}}\) under positive and negative impulses made it possible to establish that, at times less than \(10^{-5}\) sec, the maximum barrier effect is approximately the same for both polarities.

The barrier effect in paper-oil insulation was investigated with barriers made of polystyrene and polyethylene. The character of the change in \(U_{\mathrm{br}}\) is analogous to curve 2 in Fig. 4. The maximum increase in \(U_{\mathrm{br}}\) is \(1.16\,U_0\) for polystyrene and \(1.10\,U_0\) for polyethylene.

The results of investigating the influence of two barriers on \(U_{\mathrm{br}}\) of cable-paper specimens are given in Table 1.

Table 1

As can be seen from the table, the maximum increase in \(U_{\mathrm{br}}\) is observed when the barriers are located at 0.1 and 0.35 of the specimen thickness from the point. Other variants of barrier arrangement give a smaller increase in \(U_{\mathrm{br}}\).

Discussion of results

The results obtained show that the barrier effect is observed in various solid dielectrics: glass, rock salt, celluloid, and paper-oil insulation.

Fig. 3. Effect of a barrier made of fluoroplastic film on the breakdown voltage of celluloid.

Pulses with \(\tau_{\mathrm{fr}} = 10^{-5}\ \mathrm{sec}\): 1—positive polarity; 2—negative polarity.

Fig. 4. Effect of a barrier made of fluoroplastic film on the breakdown voltage of cable paper.

Pulses of positive polarity: 1—\(\tau_{\mathrm{fr}} = 0.5 \cdot 10^{-6}\ \mathrm{sec}\), 2—\(\tau_{\mathrm{fr}} = 10^{-5}\ \mathrm{sec}\), 3—alternating voltage, \(50\ \mathrm{Hz}\).

A certain analogy is observed in the barrier effect in a solid dielectric and in air: the maximum barrier effect is observed when the barrier is located near the point (Figs. 1–4); the greatest increase in breakdown voltage in the presence of a barrier occurs under direct voltage with positive polarity of the point; with negative polarity the barrier does not substantially increase the breakdown voltage and may even reduce it.

The increase in \(U_{\mathrm{br}}\) of a solid dielectric due to a barrier can be explained by the fact that the presence of a thin layer of a high-strength dielectric makes it possible to concentrate on it a space charge which, being distributed over the barrier, reduces the nonuniformity of the field. This assumption is confirmed by the fact that the barrier effect decreases as the duration of voltage application decreases (Fig. 4), and is consistent with the fact that two barriers placed close to one another lead to a smaller increase in \(U_{\mathrm{br}}\) than barriers located at some distance from each other (Table 1).

The results obtained in the present work and those published in the literature [11, 12] show that the use of barriers in a solid dielectric may find wide application in the insulation of high-voltage apparatus and cables. The increase in breakdown voltage due to barriers will make it possible, with the same dimensions, to increase the reliability of equipment operation or to raise the operating gradients of the insulation.

Conclusions

1. An increase in breakdown voltages in the presence of a barrier is observed in various solid dielectrics under all types of voltage: direct, alternating, and impulse.

2. The barrier produces the maximum strengthening when it is located at \(0.1\)–\(0.4\) of the specimen thickness from the point.

3. The greatest barrier effect is observed under direct voltage with positive polarity of the point.

4. At times less than \(10^{-5}\) sec, the maximum increase in breakdown voltage does not depend on the polarity of the impulse.

5. In the presence of two barriers, the maximum barrier effect is observed when the barriers are located near the point at a distance of \(0.1\) and \(0.35\) of the specimen thickness from one another.

6. There is an analogy in the barrier effect in solid dielectrics and in air.

References

1. N. F. Voloshenko. Influence of barriers on the electrical strength of air gaps. Elektrichestvo, No. 3, 1946.

2. N. F. Voloshenko. On the question of the mechanism of the barrier effect. Elektrichestvo, No. 6, 1947.

3. V. S. Komelkov, A. M. Lifshits. Influence of barriers on the development of an electrical discharge in long gaps. Izv. AN SSSR, 10, 1950.

4. E. N. Brago, I. S. Stekolnikov. Investigation of the nature of the long spark, VI, Izv. AN SSSR, OTN, 11, 1958.

5. L. I. Sirotinskii (ed.). High-Voltage Engineering. Part 1, GEI, 1951.

6. N. N. Nikolaevskaya. On the question of the mechanism of the barrier effect in air under impulse voltage. Tr. LPI, 1, 1954.

7. A. A. Shilvan. Investigation of methods for increasing the strength of liquid and gaseous insulation by the use of barriers. Tr. LPI, 1, 1954.

8. A. A. Vorob’ev, G. A. Vorob’ev. Regularities of impulse breakdown of solid dielectrics. Izv. AN SSSR, ser. fiz., 1, 24, 1960.

1. N. M. Torbin. Prebreakdown currents in large thicknesses of solid dielectrics. Izv. vyzov, Energetika, 10, 1960.

2. T. N. Barchenko, M. A. Melnikov. On the possibility of a streamer mechanism of discharge in solid dielectrics. In: Physics of Dielectrics, 247, Publishing House of the Academy of Sciences of the USSR, 1960.

3. Yu. N. Leont’ev, N. M. Torbin. The influence of barrier position on the breakdown voltage of solid dielectrics. Izv. vyzov, Energetika, 12, 1961.

4. B. Mitov. Influence of a barrier in breakdown voltage on solid dielectrics (Bulg.). Yearbook of the Machine-Electrotechnical Institute, 6, 2, 27, 1959.