Methods and Devices for Equalizing Voltage Distribution on the Surface of a Solid Dielectric under Pulses

A. A. Vorob'ev, V. S. Dmitrevsky

Studies [1] have shown that the voltage distribution along the surface of a solid dielectric under pulses is more nonuniform than under direct current. Therefore, the problem of equalizing the voltage distribution over the surface of a solid dielectric under pulses is especially urgent. Methods for equalizing voltage distribution under pulses have scarcely been investigated.

General Requirements for Devices for Equalizing Voltage Distribution over the Surface of a Dielectric

In order to increase the voltage at which partial and complete discharges appear along the surface of insulation, various devices are used by means of which a more uniform voltage distribution is created along the axis of the insulating structure.

The following requirements are imposed on devices for equalizing the voltage distribution over the surface of an insulating structure:

1. The device must equalize the voltage distribution over the surface of the dielectric under direct, alternating, and impulse voltages.

2. The use of a device that equalizes the voltage distribution over the surface of a dielectric must not reduce the discharge voltage for any of the types of voltage (direct, alternating, and impulse).

3. The device used must make it possible to obtain a practically rectilinear voltage distribution along the length of the dielectric.

4. The installation of devices that equalize the voltage distribution must not lead to an increase in the electrical and geometrical dimensions of insulating structures. This requirement is especially important for structures operating in a closed vessel (for example, oil circuit breakers, transformers, electric generators operating under gas pressure, etc.).

5. The devices used must not reduce the discharge distances over the surface of the dielectric.

Let us consider the methods currently used for equalizing voltage distribution from the standpoint of satisfying the requirements listed above.

The following methods are known for equalizing voltage distribution over the surface of a dielectric:

1) Semiconducting coatings.
2) Rings and shields installed at the electrodes.
3) External auxiliary electrodes installed along the surface of the dielectric.
4) Internal electrostatic shields installed at the high-voltage electrode.

Semiconducting Coatings

Semiconducting coatings are widely used to equalize voltage distribution along the surface of a dielectric under alternating and direct current and give satisfactory results. The effectiveness of this method of equalizing voltage distribution under pulses is small, owing to the slight influence of active conductivity on the voltage distribution along the surface of the dielectric. Experiments by R. T. Levshunov [2] showed that semiconducting coatings make it possible to obtain a rectilinear voltage distribution along the surface of a dielectric under alternating current, and have very little influence on the voltage distribution along the surface of a dielectric under pulses. This is a major disadvantage of the method of semiconducting coatings.

Rings and Shields Installed at the Electrodes

Rings and shields installed at the electrodes are widely used to equalize voltage distribution along a string of insulators under alternating current. The effectiveness of this method of equalizing voltage distribution is sufficient for short strings (6–7 elements), but insufficient for long strings, as shown in our work [3]. The action of shields and rings under pulses and alternating current is approximately the same. Our experiments [3] showed that, for long strings of insulators, it is impossible to obtain a rectilinear voltage distribution along a long string of insulators by means of rings and shields.

External Auxiliary Electrodes

External auxiliary electrodes installed along the surface of a dielectric are widely used to equalize voltage distribution over the surface of support columns [4]. We investigated the influence of external auxiliary electrodes on the voltage distribution along the surface of a dielectric under pulses having a wave shape of 0.15/32.

Figure 1 presents a cross section of the specimen on which the investigations were carried out. Tinfoil strips, which were glued to the surface of a Plexiglas plate, were used as auxiliary electrodes. The width of a tinfoil strip was 2 mm. The length of the specimen was 815 mm. The tests were carried out at a pulse-voltage magnitude of 68.8 kV. Figure 2 gives the results of the measurements performed. It is seen from Fig. 2 that, when the distance between the auxiliary electrodes located at the high-voltage

electrode, the voltage distribution becomes more uniform. Fig. 3 gives the dependence of the field strength at the high-voltage electrode on the distance between auxiliary electrodes located at the high-voltage electrode.

Fig. 1. Specimen with external auxiliary electrodes

The grounded electrode is shown on the left; the high-voltage electrode is shown on the right.

From Fig. 3 it is seen that as \(a_1\) increases, the field strength at the high-voltage electrode also increases. The same results were obtained by us for a getinax plate 750 mm long and 10 mm thick. The investigations carried out show that the installation of external auxiliary electrodes leads to equalization of the voltage distribution over the surface of the dielectric under pulses. The equalizing action of the auxiliary electrodes depends on their arrangement. By arranging the auxiliary electrodes in the most advantageous manner, one can obtain a practically rectilinear voltage distribution along the surface of the dielectric. Thus, the use of external auxiliary electrodes makes it possible to obtain a rectilinear variation of voltage (direct, alternating, and pulsed) along the length of the dielectric. However, installing auxiliary electrodes on the external surface of the dielectric leads to an increase in the electrical dimensions of the setup, to a decrease in the discharge distances along the surface of the dielectric, and to a decrease in the distances between neighboring parallel supports. These drawbacks considerably reduce the value of the method of external auxiliary electrodes.

Fig. 2. Voltage distribution over the surface of a Plexiglas plate 815 mm long with external auxiliary electrodes.
\(*\)—specimen in the absence of auxiliary electrodes; \(x\)—auxiliary electrodes installed only at the high-voltage electrode,
\(o\)—\(a_1 = 5\) mm, \(a_2 = 50\) mm;
\(\odot\)—\(a_1 = 10\) mm, \(a_2 = 50\) mm;
\(\times\)—\(a_1 = 20\) mm, \(a_2 = 50\) mm.

Internal Electrostatic Shield

An internal electrostatic shield, installed at the high-voltage electrode, is recommended for increasing the discharge voltage along the surface of a supporting insulator under alternating current and under

Fig. 3. Dependence of the field strength at the high-voltage electrode on the distance \(a_1\)

positive-polarity impulses [4]. Experiments by G. A. Lebedev [4] showed that the use of an internal electrostatic shield leads to a reduction in the discharge voltage under negative-polarity impulses. We carried out special experiments in order to determine the reason for the reduction in flashover voltage under negative-polarity impulses. Figure 4 shows the design of the specimen on which the influence of the internal electrostatic shield on the voltage distribution over the dielectric surface under impulses was studied. In

Fig. 4. Design of a specimen with an internal electrostatic shield.

Fig. 5 the change in the voltage distribution along the surface of a solid dielectric under impulses is given as a function of the length of the internal shield. From Fig. 5 it is seen that the greatest voltage drop with the internal shield installed occurs at the end of the shield. Figure 6 gives the distribution of the field strength along the dielectric surface for an internal electrostatic shield, installed at the high-voltage electrode, of length 200 mm. As is seen from Fig. 6, at the end of the electrostatic shield the field strength has its greatest value. The presence of the greatest field strength at some distance from the high-voltage electrode creates favorable conditions for the development of a negative streamer. The presence of the internal electrostatic shield, as it were, neutralizes the influence of the positive space charge that hinders the development of the negative streamer. As a result—

As a result, the discharge voltage is reduced for pulses of negative polarity.

Thus, the use of the method of an internal electrostatic shield cannot be recommended for equalizing the voltage distribution over the surface of insulating structures that may be subjected to the action of negative pulses (for example, for installations that may be subjected to the action of atmospheric overvoltages).

Fig. 5. Dependence of the voltage distribution over the surface of a getinax plate on the length of the internal electrostatic shield: \(o\)—without shield, \(\times\)—\(\Delta=750\) mm; \(\oplus\)—\(\Delta=600\) mm; \(\odot\)—\(\Delta=400\) mm; \(*\)—\(\Delta=200\) mm.

Fig. 6. Distribution of the field strength over the surface of the dielectric at \(\Delta=200\) mm.

Conclusions

The analysis carried out of several common methods for equalizing the voltage distribution over the surface of a dielectric has shown that none of them is sufficiently satisfactory. Of all the methods considered, the method of external auxiliary electrodes deserves the greatest attention; with its help it is possible to obtain a practically uniform voltage distribution along the surface of the dielectric under pulses and alternating current. Other methods do not provide such a possibility. However, this method too has the serious shortcomings listed above. Therefore, undertaking investigations aimed at finding new methods for equalizing the voltage distribution over the surface of a dielectric, which would not have the shortcomings of the methods being used, is timely.

References

1. Dmitrevskii V. S. Voltage distribution along the surface of a dielectric under pulses (present collection).

2. Levshunov R. T. Investigations of insulators coated with semiconducting glaze. Elektricheskie stantsii, No. 4, 36, 1954.

3. Vorob’ev A. A., Dmitrevskii V. S. On the question of the distribution of voltage along a string of insulators. Elektrichestvo, No. 10, 75, 1954.

4. High-Voltage Engineering, under the general editorship of Professor L. I. Sirotinskii, Gosenergoizdat, 1951.