Effect of Solid Dielectrics on the Breakdown of Transformer Oil Under Impulse Voltage

V. A. Butenko, N. M. Torbin

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

In electrical-insulation structures operating in transformer oil, electrodes that are under high voltage are often arranged in such a way that, when the oil gap breaks down, the discharge partially bridges a solid dielectric.

It is known [1, 2] that, when solid dielectrics are bridged in transformer oil, the breakdown gradients in a nonuniform field, in the absence of a normal component of the electric-field intensity vector to the surface of the dielectric, are close to the breakdown voltages of a purely oil gap.

Under impulse voltage, at short times of voltage application in air, an intersection is observed of the volt-second characteristics for the flashover of a string of insulators and the breakdown of protective gaps [3]. This indicates that the ratio of the breakdown times of equal-strength gaps along the surface of a solid dielectric and of an air gap can vary over wide limits, depending on the steepness of the applied voltage impulse.

We have carried out a study of the influence of solid dielectrics with different dielectric permittivities $\varepsilon$ on the breakdown of an oil gap when a solid dielectric was placed at some distance from the electrodes, and the discharge passed partly through the oil gap and partly along the surface of the solid dielectric.

The sharp–sharp electrodes were positioned above the surface of the solid dielectric at distances $S$ equal to 3, 4, 5, and 6 mm, with distances between the electrodes $d$ from 5 to 25 mm. A rectangular impulse wave of positive or negative polarity with front duration $\tau_{\phi} = 0.17;\ 0.34$ and $1.5\ \mu\text{s}$ was applied to one of the electrodes; the other electrode was grounded. Breakdown occurred on the flat part of the impulse. As solid dielectrics, materials with different dielectric permittivities were used: polymethyl methacrylate $\varepsilon = 3.5$, glass $\varepsilon = 6$, and ticonal ceramic $\varepsilon = 1700$. Depending on the experimental conditions, breakdown occurred either in the oil gap or as a combined breakdown, partly of the oil gap and partly as flashover along the surface of the solid dielectric.

The electrode arrangement (Fig. 1) was placed in a tank made of organic glass, which made it possible to obtain static photographs of the discharge and to observe visually the discharge path. On the basis of 10–15 mea-

the arithmetic mean breakdown delay time and the breakdown path of the gap were determined.

Figs. 2, 3, and 4 show curves of the change in discharge delay time with changes in the distance between the electrodes \(d\) and with different positions of the electrodes relative to the surface of solid

Fig. 1. Electrode arrangement: 1—electrodes, 2—dielectric

dielectrics \(S\), under a negative impulse wave with \(\tau_{\mathrm{f}} = 1.5\ \mu\text{s}\), for polymethyl methacrylate, technical glass, and ticonde ceramic, respectively. It is evident from the graphs that \(t_{\text{del.}}\) of the discharge in the case of poly-

Fig. 2. Dependence of the discharge time \(t_{\mathrm{p}}\) on the distance between the electrodes \(d\) and the distance from the electrodes to the surface of the solid dielectric \(S\). In the case of polymethyl methacrylate, \(\tau_{\mathrm{f}} = 1.5\ \mu\text{s}\). Impulse of negative polarity:
1 — \(S = 3\ \text{mm}\); 2 — \(S = 4.5\ \text{mm}\); 3 — \(S = 6\ \text{mm}\)

methyl methacrylate initially increases linearly with increasing \(d\). Beginning with \(d = 15\ \text{mm}\) (\(S = 3\ \text{mm}\)) and \(d = 20\ \text{mm}\) (for \(S = 4.5\ \text{mm}\) and larger), the growth of \(t_{\text{del.}}\) with increasing \(d\) slows down considerably. For technical glass and especially for ticonde ceramic, as \(d\) increases,

above 10 mm, and in the case where \(S = 3\) mm, at \(d\) above 5 mm, the discharge delay \(t_{\text{del}}\) remains practically the same and, moreover, with an increase of \(d\) above 20 mm (\(S = 6\) mm) even a certain decrease in the discharge delay \(t_{\text{del}}\) is observed (Fig. 5).

Fig. 3. Same as in Fig. 2, for technical glass

Photographs and visual observations made it possible to establish that the change in the discharge delay \(t_{\text{del}}\) is associated with a change in the discharge path between the electrodes. When the discharge path passes through oil, the discharge delay \(t_{\text{del}}\)

Fig. 4. Same as in Fig. 2, for ticonal ceramics

increases approximately linearly with increasing \(d\). When the discharge path passes partially over the surface of the solid dielectric, the growth of \(t_{\text{del}}\) with increasing \(d\) slows sharply, and at \(S = 3\) mm for glass and ceramics it remains practically constant.

As the investigations showed, the change in the breakdown path of the gap depends on \(d\), \(S\), the \(\varepsilon\) of the solid dielectric, the polarity and rate of voltage change on the impulse front \((\tau_f)\), and probably on the physicochemical properties of the liquid dielectric (the latter was not investigated by us).

For the same values of \(S\), with increasing \(d\) the presence of solid dielectrics with large values of \(\varepsilon\) leads to earlier flashover of the solid dielectric and to a reduction in the discharge delay time \(t_{\text{del}}\).

Fig. 5. Same as in Fig. 4, for ticonder ceramic. Negative-polarity impulse, \(\tau_{\phi}=0.17\ \mu\text{s}\);
1 — \(S=4.5\ \text{mm}\),
2 — \(S=6\ \text{mm}\)

Table 1 gives data on the measurement of the discharge delay time \(t_{\text{del}}\) in the presence of solid dielectrics at \(d=20\ \text{mm}\) and \(\tau_{\phi}=1.5\ \mu\text{s}\).

Table 1

As can be seen from Figs. 2–5 and Table 1, with an increase in the dielectric constant \(\varepsilon\), the discharge delay time, for the same parameters \(S\) and \(d\), decreases by more than a factor of 2. The independence of the discharge delay time \(t_{\text{del}}\) at \(d=3\ \text{mm}\) in the case of glass and ceramic is explained by the fact that the discharge passes a large part of its path along the surface of the solid dielectric, where the rate of discharge development is considerably greater than in the breakdown of a purely oil gap.

The deviation of the discharge path from its shortest distance and the reduction in the discharge delay time can be explained by the following circumstances. The presence of a solid dielectric distorts the field in the gap because of the difference between the dielectric constants \(\varepsilon\) of the medium and of the solid dielectric [4]. The greater the difference in \(\varepsilon\) of the media, the more the field strength will increase in the point–surface gap of the solid dielectric, and therefore, at large values of \(S\), the discharge will develop along the surface of the solid dielectric. Since the length of sliding discharges varies approximately as the fifth power depending on the amplitude of the applied voltage [5], when the discharge develops along the surface of the solid dielectric the breakdown time of the gap is significantly shortened. These

these circumstances must be taken into account when designing the insulation of electrical apparatus. The presence of solid dielectrics near high-voltage electrodes may lead to a substantial reduction in the electric strength of the gap under the action of impulse voltages.

REFERENCES

1. Edited by L. S. Sirotinsky, High-Voltage Engineering, Part II. Gosenergoizdat, 1955.

2. Edited by Yu. V. Kornitsky and B. M. Tareev, Electrical Insulating Materials. Gosenergoizdat, 1959.

3. S. Whitehead. Breakdown of Solid Dielectrics. Gosenergoizdat, 1957.

4. L. R. Neiman, P. L. Kalantarov. Theoretical Foundations of Electrical Engineering. Gosenergoizdat, 1959.

5. A. M. Zalessky, N. I. Bachurin. Insulation of High-Voltage Apparatus. Gosenergoizdat, 1961.