FLASHOVER OF CENTERING INSULATORS OF VARIOUS SHAPES IN A COAXIAL FIELD

V. S. Dmitrevskii, G. E. Kurtenkov

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

It is known that the reliability of high-voltage coaxial systems with gas or liquid insulation is determined by the flashover conditions of the centering insulators.

Flashover of dielectrics in a coaxial electrode system in an atmosphere of insulating gases has been investigated in [1–3].

In [1], data are given on the flashover of ebonite in compressed nitrogen at an alternating voltage of 50 Hz in small interelectrode gaps.

The authors of [2, 3] investigated coaxial systems with nitrogen and sulfur hexafluoride gas. Washers made of polyethylene, polyvinyl chloride, polymethyl methacrylate, polytetrafluoroethylene [3], and porcelain and an epoxy compound [2] were used as centering insulators. The studies established that such coaxial systems can be used at \(U = 275 \div 400\) kV.

The purpose of the present work was to obtain experimental data on the flashover of solid dielectrics with different dielectric constants in transformer oil, since the absence of these data causes certain difficulties in designing coaxial oil-filled cables and other high-voltage equipment.

To investigate the flashover of solid dielectrics, the source of single voltage pulses was a pulse-voltage generator rated at 1 million volts with a discharge capacitance of 28,000 pF. The voltage was measured by means of an OK-19-M2 oscilloscope and a capacitive voltage divider.

In the experiments an electrode system was used consisting of inner and outer coaxially arranged replaceable electrodes and two insulating centering covers with openings that served for filling the system with liquid dielectric and for the escape of gas bubbles from the interelectrode space. In assembled form, the electrode system was placed in a bakelite-paper tank into which 50 liters of transformer oil were poured.

The flashover voltage was investigated for specimens of three shapes (Fig. 1), which are the most characteristic from the standpoint of the presence or absence of tangential \(E_t\) and normal \(E_n\) components of the electric-field strength. For specimens of the “washer” type (Fig. 1, a), the absence of the normal component of the field strength is characteristic—

field intensity, while for specimens of the “cone” type (Fig. 1c) there is the simultaneous presence of tangential and normal components of the field intensity. Specimens of the “step” type (Fig. 1b) are characterized by the absence of a tangential component of the field in the cylindrical part of the specimen.

Surface flashover was produced on the front of a single positive-polarity pulse. The wave-front length was \(0.4\ \mu\text{s}\). In the experiments, the transformer oil had a breakdown voltage in a standard breakdown gap of \(27 \div 32\ \text{kV}\).

The work investigated surface flashover of specimens made of high-density polyethylene \((\varepsilon = 2.2)\) and polymethyl methacrylate \((\varepsilon = 3.5)\) in three electrode systems. The inside diameter \(D\) of the outer electrodes was, respectively, 38, 56, and 80 mm. The diameter of the inner electrodes was varied in the range from 6 to 60 mm.

Fig. 1. Shapes of centering insulators

Figure 2 presents the dependences of the flashover voltage along the surface of washers in transformer oil on the diameter of the inner electrode at \(D = 38\ \text{mm}\) (curves 1, 2) and \(D = 56\ \text{mm}\) (curves 3, 4). Curves \((1, 3)\) show the dependence of the flashover voltage of polyethylene washers, and curves \((2, 4)\) are for washers made of polymethyl methacrylate. The data shown in the figure are average values of \(12 \div 15\) measurements.

After several flashovers, discharge traces appeared on the surface of the specimens, and the flashover voltage decreased sharply. Therefore, in order to obtain correct values of the flashover voltage, the polyethylene specimens had to be removed after \(1\text{–}2\) flashovers. The polymethyl methacrylate specimens withstood a somewhat larger number of flashovers (up to \(2 \div 4\)) without a sharp decrease in the flashover voltage.

It follows from Fig. 2 that the flashover voltage has a maximum value corresponding to the ratio

\[ \frac{D}{d} \simeq e. \]

For comparison of \(U_{\text{fl}}\) of washers with \(U_{\text{br}}\) of transformer oil, control experiments were carried out on breakdown of transformer oil (without a specimen) under identical conditions at

\[ \frac{D}{d} = \frac{56}{24} \quad \text{and} \quad \frac{D}{d} = \frac{56}{29}. \]

It was established that the flashover voltage of polyethylene washers is approximately \(10 \div 15\%\) lower than the breakdown voltage of transformer oil.

It is characteristic that in some experiments breakdown of the medium, i.e., of the transformer oil, occurred at a distance of \(3 \div 20\ \text{mm}\) from the surface-

Fig. 2. Dependence of the flashover voltage of washer-type specimens on the diameter of the inner electrode

Fig. 3. Dependence of the average flashover field strength of washer-type specimens on the diameter of the inner electrode

section, which obviously indicates the presence of a comparatively small field distortion.

It also follows from Fig. 2 that \(U_{\text{fl}}\) of polymethyl methacrylate washers has a somewhat lower value than \(U_{\text{fl}}\) of polyethylene washers, which may be explained by field distortion at the boundary of the “solid dielectric—oil” system owing to the higher dielectric permittivity of polymethyl methacrylate in comparison with polyethylene.

In Fig. 3 the values of the average flashover field strength of the washers are compared as a function of the diameter of the inner electrode. The flashover field strength was calculated from the formula:

\[ E_{\text{av}}=\frac{2U_{\text{fl}}}{D-d},\ \text{kV/mm} \tag{1} \]

Curves (1, 3, 5) show the dependence \(E_{\text{av}}=f(d)\) for polyethylene washers, and curves (2, 4, 6) for polymethyl methacrylate washers for coaxial systems with the inner diameter of the outer electrode equal, respectively, to 38, 56, and 80 mm.

With an increase in \(D\), the average flashover field strength decreases, which is associated with an increase in the nonuniformity of the field.

In Fig. 4 the dependences \(U_{\text{fl}}=f(d)\) are presented for conical specimens (\(D=38\) mm). Flashover of conical specimens made of polyethylene (curve 1) and polymethyl methacrylate (curve 2) was investigated.

Fig. 4. Dependence of the flashover voltage of conical specimens on the diameter of the inner electrode

Fig. 5. Dependence of the average flashover field strength of conical specimens on the diameter of the inner electrode

The specimens had a cone angle \(\alpha=30^\circ\). In the system \(D=56\) mm, specimens with cone angle \(\alpha=45^\circ\) were investigated, and in the system \(D=80\) mm, with cone angle \(\alpha=56^\circ\). The values of the flashover voltages of the specimens at

\[ \frac{D}{d}=\text{const}\simeq e \]

are summarized in Table 1.

In Fig. 5 the dependences of the average discharge field strength on the diameter of the inner electrode \(d\) are presented for the system \(D=38\) mm.

The designations in the figure are the same as in Fig. 4. The average field strength was calculated by the formula:

\[ E_{\mathrm{av}}=\frac{U_{\mathrm{fl}}}{l_{\mathrm{fl}}},\ \mathrm{kV/mm} \tag{2} \]

\[ l_{\mathrm{fl}}=3+\frac{D-(d+b)}{2\sin\alpha},\ \mathrm{mm} \tag{3} \]

where \(l_{\mathrm{fl}}\) is the flashover path length, mm;
\(\alpha\) is the cone angle of the specimen.

The data presented indicate that the average discharge gradient along the surface of conical specimens has a value smaller than \(E_{\mathrm{av}}\) for washers.

The experimental data confirm Toepler’s theory that the values of the average discharge gradients are lowest (see Table 2) in fields in which, along with the tangential component of the field \(E_t\), there is also a normal component \(E_n\), which increases as the cone angle decreases.

Table 1

Table 2

Under the action of the tangential component of the field, ions are displaced along the surface of the insulator, while under the action of the normal component of the field their motion occurs with friction. The result of friction is the release of heat and local heating of the surface to temperatures at which thermal ionization is possible, facilitating the development of discharge along the insulator surface.

Flashover of stepped specimens (Fig. 1, b) was investigated in two coaxial systems with \(D = 38\) mm and \(D = 56\) mm. The length of the protruding part of the specimen was 15 mm, and the diameter of the step was

\[ d_1 = d + \frac{D - d}{2}, \text{ mm} \tag{4} \]

The values of the flashover voltages and the mean discharge gradients of the stepped specimens are summarized in Tables 1 and 2. The mean discharge gradient for the stepped specimens was calculated by formula (1), since in most experiments flashover began at the end face of the specimen on the inner electrode and ended through the oil. There were cases in which the discharge traveled completely along the surface of the specimen.

It follows from the data given in Table 2 that stepped specimens, in contrast to conical and washer-shaped specimens, have the highest flashover voltage, since the discharge, having begun on the surface of the specimen, terminates through the stronger medium, transformer oil, which has high electric strength under impulse-voltage action. As a result, \(U_{\mathrm{fl}}\) of the stepped specimens approaches \(U_{\mathrm{br}}\) of transformer oil at the same interelectrode distance.

Conclusion

In the present work, the values of the flashover voltages of solid dielectrics located in transformer oil in a coaxial field have been determined for the first time. It has been established that the value of \(U_{\mathrm{fl}}\) has a maximum when the ratio

\[ \frac{D}{d} \simeq e. \]

It has been shown that, as the dielectric permittivity of the insulator material increases, the discharge gradients decrease, which is caused by distortion of the field between the electrodes due to the presence of a dielectric with higher dielectric permittivity.

The presence of a normal component of the electric-field strength together with a tangential component leads to a reduction in the mean discharge field strength along the surface of centering insulators.

The insulators are subjected to strong action by surface discharges, associated with carbonization of the discharge traces and penetration of discharge channels into the body of the insulator, which leads to premature failure of the insulator (breakdown of the insulator).

References

1. B. M. Vul, I. M. Goldman. Breakdown and flashover of solid dielectrics in compressed nitrogen. Proceedings of the Academy of Sciences of the USSR, No. 4, 1936. Physical Series.

2. Ikeda Giichi, Yoshida Yasuhisa, Haka Koshi. A spacer for a new gas insulated cable. Sumitomo Electric Technical Review, No. 7, 11–12, 1966.

3. D. I. Skipper, P. J. McNeal. “Impulsestrength measurements on compressed-gas insulation vor extrahighvoltage power cables,” Proceedings of the Institution of Electrical Engineers, 112, No. 1, 103–108, 1965.