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 a sharply nonuniform field of “point—plane” electrodes, with positive polarity of the point under direct 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: direct, alternating, and impulse. As for the mechanism of the barrier effect, the assumptions about 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 the decisive role in the barrier effect is played by a positive space charge, but that the breakdown voltage is determined by the point–barrier section.

Brago and Stekolshchikov [4] believe that a sufficient condition for the completion of breakdown is the destruction of the barrier by corona branches, and that the role of 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 indicated that, under impulse voltage, barriers made of triacetate film and metal foil in rock salt increase the breakdown voltage—

by 18–20% relative to the breakdown voltage without a barrier. Barriers made of getinax 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 changes in the breakdown voltages of various solid dielectrics when a thin barrier made of a high-strength material is placed in them. The barrier effect was investigated in glass, rock salt, celluloid, and paper-oil insulation.

Experimental procedure

Specimens of glass, rock salt, and celluloid consisted of plates of different thicknesses. The barrier was placed between the plates. The distance from the high-voltage electrode to the barrier was varied by means of combinations of plate arrangements. 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 laid one on another and clamped in an insulating frame. To reduce the probability of air inclusions between the sheets, the specimen assembly 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 barriers made of mica, foil, and triacetate film was 5–7 μm; the thickness of the organic films was 25–30 μm.

The studies were carried out at 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 rated at 100 kV with a discharge capacitance of 11000 pF and at 1000 kV with a discharge capacitance of 22500 pF.

During breakdown under impulses, breakdown occurred on the front of the impulse. The voltage on the specimen was measured with an OK-19M oscillograph. 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; the plane was grounded.

Experimental results

Some dependences obtained in studying the influence of the 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 produced by a barrier made of metal foil (4)—up to \(1.27\,U_0\). In glass, the greatest increase in \(U_{\mathrm{br}}\) 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 \(U_{\mathrm{br}}\) of celluloid with a barrier made of polyethylene film as a function of the position of the barrier at 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 — cellulose triacetate film, 4 — metal foil.

Fig. 2. Effect of a polyethylene-film barrier on the breakdown voltage of celluloid.

1 — direct voltage, points of positive polarity; 2 — direct voltage, points 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. With negative polarity of the point, the barrier does not cause a substantial increase in \(U_{\text{br}}\), and when the barrier is placed directly next to the point, even a decrease in \(U_{\text{br}}\) is observed.

Figure 3 shows the dependence of \(U_{\text{br}}\) of celluloid with a barrier made of fluoroplastic-4 film on the position of the barrier. The specimen thickness is \(d = 8\) mm. The impulse-front duration is \(\tau_{\text{fr}} = 10\) μsec. The maximum increase in \(U_{\text{br}}\) (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 directly next to the point, an increase in \(U_{\text{br}}\) is observed for positive point polarity and is not observed for negative polarity.

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

A study of the influence of the barrier on \(U_{\text{br}}\) for positive and negative impulses made it possible to establish that at times below \(10^{-5}\) sec the maximum barrier effect is approximately the same for both polarities.

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

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

Table 1

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

Discussion of the 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 DC voltage with positive polarity of the point; with negative polarity the barrier does not substantially increase the breakdown voltage and may even reduce the latter.

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 is reduced (Fig. 4), and is consistent with the fact that two barriers placed immediately next to one another lead to a smaller increase in \(U_{\mathrm{br}}\) than barriers located at some distance from one another (Table 1).

The results obtained in the present work and published in the literature [11, 12] show that the use of barriers in a solid dielectric may find broad application in the insulation of high-voltage apparatus and cables. The increase in breakdown voltage due to barriers will make it possible, for the same dimensions, to increase the reliability of equipment operation or to increase 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: DC, AC, and impulse.

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

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

4. For 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 distances 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. Effect of barriers on the electric 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. Effect of barriers on the development of an electric 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. Pre-breakdown currents in large thicknesses of solid dielectrics. Izv. vuzov, 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. Influence of the position of a barrier on the breakdown voltage of solid dielectrics. Izv. vuzov, Energetika, 12, 1961.

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

1. Order 7769.