EFFECT OF FILM BARRIERS ON THE IMPULSE BREAKDOWN VOLTAGE OF GLASS

D. D. Rumyantsev

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

During the breakdown of gaseous and liquid dielectrics in a nonuniform field, the so-called barrier effect is observed, i.e., an increase in the electric strength of the insulating gap when a thin layer of a solid dielectric, customarily called a barrier, is placed between the electrodes [1, 2].

In solid dielectrics an increase in breakdown voltage is also observed if a thin layer of another dielectric with greater electric strength is placed between layers of one dielectric [3, 4, 5].

The present work is devoted to investigating the influence of barriers made of films of high-polymer materials on the breakdown voltage of technical glass. The investigations were carried out under impulse voltage in the sharply nonuniform field of “point—plane” electrodes. The front duration of the impulses was varied from \(10^{-7}\) sec to \(2 \cdot 10^{-5}\) sec. Breakdown of the specimens occurred on the front of the impulses. The specimens consisted of glass plates of different thickness and, in most experiments, consisted of three plates, whose total thickness was 5.2 mm (1.3 mm; 1.4 mm; 2.5 mm). The barrier was placed between the plates at different distances from the point. Films of polyethylene (PE), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PETP), and polystyrene (PS), with thicknesses from 20 to 200 μ, served as barriers.

Fig. 1. Electrode device: 1, 2—electrodes; 3—specimen; 4—barrier; 5—fixed frame; 6—movable frame; 7—stops; 8—springs.

During breakdown the specimens were placed in transformer oil. The electrode device, a sketch of which is shown in Fig. 1, made it possible to press the glass plates against one another with a constant force, thereby ensuring approximately the same thickness of the oil interlayer between the plates in different experiments.

Below are the results of the investigations carried out. Figure 2 presents the dependence of the breakdown voltage \((U_{\text{br}})\) of glass on the position in the specimen of a barrier made of PE-50 film (here and fur—

the following digits after the letters denote the film thickness in microns) for different pulse polarities. The thickness of the specimens was 5.2 mm. \(S\) is the distance from the “point” electrode to the barrier.

It is seen from Fig. 2 that the maximum \(U_{\mathrm{br}}\) is exhibited by specimens in which the barrier is located at a distance of 1.3 mm from the point, i.e., between plates 1 and 2. The relative increase in \(U_{\mathrm{br}}\) due to the barrier is greater for positive polarity than for negative polarity. An increase in the \(U_{\mathrm{br}}\) of glass is also observed when PTFE, PETF, and PS films are used as the barrier.

When a film barrier is placed in the specimen between the plates, an “additional” layer of liquid dielectric is formed. To avoid the formation of this “additional” layer, a thin layer of Bakelite varnish applied to the surface of the glass was used as the barrier. In this case as well, an increase in the \(U_{\mathrm{br}}\) of glass was observed when the barrier was located near the “point” electrode.

To determine the effect of the thickness of the transformer-oil interlayer on the \(U_{\mathrm{br}}\) of glass specimens, a hole 20 mm in diameter was cut in the film barrier. The thickness of the barrier, and consequently the thickness of the interlayer, varied from \(\approx 20\) to 200 \(\mu\). The barrier with the hole was placed at a distance of 1.3 mm from the “point” electrode.

Fig. 2. Dependence of the breakdown voltage of glass on the position of the barrier in the specimen: 1—pulses of positive polarity; 2—pulses of negative polarity; \(1'\) and \(2'\)—breakdown voltage of specimens without a barrier. Pulse-front duration \(10^{-5}\) sec.

As a result of the experiments it was established that changing the thickness of the transformer-oil interlayer from 20 to 200 \(\mu\) does not lead to any noticeable increase in the \(U_{\mathrm{br}}\) of glass. On this basis, and taking into account that the electric strength of transformer oil at thicknesses up to 500 \(\mu\) is \(10^5\) V/cm [6], i.e., more than an order of magnitude lower than the electric strength of the films (barriers), it may be concluded that the increase in the \(U_{\mathrm{br}}\) of glass occurs only due to the barrier. Therefore, in the subsequent discussion we shall not take into account the presence of an interlayer of liquid dielectric between the layers of solid dielectrics.

During breakdown of specimens with a barrier, the discharge time lag increases sharply; moreover, the difference in the time lags during breakdown of specimens with and without a barrier depends on the rate of voltage rise—the steepness of the pulse. Fig. 3 presents the dependence

difference in the discharge delay times in specimens with a PE-50 barrier and without a barrier on the steepness of the pulse.

From Fig. 3 it is evident that the difference in the delay times decreases as the steepness of the pulse increases. Since the statistical delay time in solid dielectrics is small and amounts to \(10^{-9} \div 10^{-8}\) sec [7], it may be considered that the increase in the discharge delay time during breakdown of specimens with a barrier occurs because of an increase in the discharge-formation time.

The increase in the breakdown voltage of a solid dielectric in the presence of a barrier can be explained as follows. When the critical field strength is reached at the “point” electrode, a discharge begins to develop in the dielectric and reaches the barrier. But since the electrical strength of the barrier is considerably higher than that of the principal solid dielectric, the field strength at the head of the discharge channel is still insufficient for discharge development in the barrier to begin. Further growth of the discharge into the depth of the principal dielectric does not occur until the field strength at the point of contact between the developing discharge and the barrier reaches a value sufficient for discharge development in the barrier; after this the discharge develops further and breakdown occurs.

Fig. 3. Dependence of the difference in discharge delay times in specimens with a barrier and without a barrier on the steepness of the pulse. Specimen thickness—5.2 µm. Barrier—polyethylene, 50 µm.

In Fig. 4 a schematic time sweep is presented for the development of a discharge in a specimen without a barrier (1) and with a barrier (2) in a sharply nonuniform field.

To clarify the mechanism by which the breakdown voltage of glass is increased by a barrier, the influence of the electrical strength of the barrier on \(U_{\text{br}}\) was investigated. As a result of the experiments it was established that the increase in \(U_{\text{br}}\) is proportional to the electrical strength of the barrier and does not depend on the magnitude of the breakdown voltage of the barrier itself. Thus, a PTFE-30 barrier, having \(E = 464\ \text{kV/mm}\) and \(U_{\text{br}} = 5\text{–}7\ \text{kV}\) in a sharply nonuniform field, increases the breakdown voltage of glass by 44%, whereas a PTFE-200 barrier, having \(E = 224\ \text{kV/mm}\) and \(U_{\text{br}} = 10\text{–}12\ \text{kV}\) in a sharply nonuniform field, raises the breakdown voltage of glass by 22%.

Since the principal role in increasing \(U_{\mathrm{br}}\) is played by the electric strength, and not by the thickness of the barrier, it may be assumed that a high-strength barrier with a thickness of only a few microns will also lead to an increase in \(U_{\mathrm{br}}\).

However, no increase in \(U_{\mathrm{br}}\) will be observed if the electric strength of the barrier and of the main dielectric differ only slightly. Thus, a barrier made of PE-50 does not lead to an increase in \(U_{\mathrm{br}}\) for specimens composed of two plates of organic glass.

Fig. 4. Diagram of discharge development in a solid dielectric without a barrier (1) and with a barrier (2): \(t_{\mathrm{f}}\) — discharge-formation time, \(\Delta U\) — increase in breakdown voltage due to the barrier, \(\Delta t\) — increase in the discharge-formation time due to the barrier.

Figure 5 presents the dependences of \(U_{\mathrm{br}}\) for specimens of technical glass without a barrier (1) and of the magnitude of the increase in \(U_{\mathrm{br}}\) due to a PE-50 barrier—\(\Delta U\) (2)—on the duration of voltage application. From Fig. 5 it is seen that \(\Delta U\) changes only slightly up to \(t \geq 2 \cdot 10^{-6}\) sec, and then decreases sharply as the time to breakdown decreases; at times on the order of \(10^{-7}\) sec the barrier has practically no effect on \(U_{\mathrm{br}}\) of the glass. The value of \(U_{\mathrm{br}}\) for specimens without a barrier at times of \(2 \cdot 10^{-6}\) sec and less increases sharply as the voltage exposure is shortened, and at \(t\) on the order of \(10^{-7}\) sec \(U_{\mathrm{br}}\) increases by a factor of 2 in comparison with \(U_{\mathrm{br}}\) at \(t = 10^{-5}\) sec. Consequently, for voltage exposures on the order of \(10^{-7}\) sec, the field strength at the head of the discharge developing in the glass is sufficiently high that, upon reaching the barrier, the discharge begins to develop in the latter without any time delay. This may be regarded as the main reason why, at \(t = 10^{-7}\) sec, barriers made of polymer films do not increase the breakdown voltage of glass.

Fig. 5. Dependence of \(U_{\mathrm{br}}\) for glass specimens without a barrier (1) and of the magnitude \(\Delta U\) (2) on the duration of voltage application.

On the basis of the foregoing, the following conclusions may be drawn.

1. In a sharply nonuniform field, the impulse breakdown voltage of glass increases substantially when a thin layer of another, stronger dielectric—a barrier—is placed in it.

1. The increase in the breakdown voltage of a solid dielectric due to a barrier depends on the ratio of the electric strengths of the barrier and the main dielectric; the greater the difference between the electric strength of the barrier and that of the main dielectric, the more significant the increase in breakdown voltage.

2. The increase in the discharge delay time during the breakdown of glass with a barrier depends on the steepness of the voltage impulse and, at a steepness on the order of \(10\ \text{kV}/\mu\text{s}\), reaches 5 microseconds or more.

REFERENCES

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

2. A. A. Shavlan. Investigation of methods for increasing the strength of liquid and gaseous insulation by using barriers. Proceedings of the LPI, 1, 1954.

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. D. D. Rumyantsev, N. M. Torbin. Influence of barriers on the breakdown voltage of certain solid dielectrics. Breakdown of Dielectrics and Semiconductors. Collection of reports of the IV Interuniversity Conference, 1964.

5. D. D. Rumyantsev, N. M. Torbin. On the barrier effect in solid dielectrics. Izv. TPI, vol. 149.

6. D. A. Kaplan. Electric strength of transformer oil at small distances between electrodes. Elektrotekhnika, No. 10, 1964.

7. M. A. Mel’nikov. Dissertation, Tomsk, 1959.