Flashover voltage

Knowledge of the flashover voltage of the necessary insulators in helium at low temperatures in addition to the breakdown voltage will be important. Flashovers can occur wherever surfaces of solid insulators are parallel to the electric field lines or their components. 

Fig. 2. Flashover voltage vs. pressure for high-density polyethylene.

By increasing the electrode distance to 8 mm and, with this, decreasing the electric field strength, higher flashover voltages could be attained, however, not to the same degree as the electric field strength decreased (see Fig, 1),... 

Generally, it can be said that at low temperatures the flash-over voltages of various insulation materials are markedly below those of the breakdown voltage, as in the conventional electrotechnology, and that increase of the flashover distance has less effect on flashover voltage than the increase of insulator thickness has on breakdown voltage. 

Since in superconductive equipment, temperature increase of short duration is unavoidable in the case of a failure, the effect of the helium temperature on flashover voltage was investigated, too. 

As shown in Fig, 4, the flashover voltage declines with increasing temperature, and this effect is greater the greater the electrode distance is. This is probably due to the decrease of the density of the helium with increasing temperature at constant pressure, and with this, a greater mean-free path. 

Because of the lower minimal values in all tests, compared with the measured mean values of the flashover voltage, it appeared to make sense to observe also the ground current of the sample. It has already been shown that before the occurrence of the continuous... 

Figure 7 shows the dependence of the flashover voltage upon the product of the dielectric constant and the tangent of the loss angle ( r> tan 6). The dielectric loss in the insulation material and the dielectric loss at its boundary layers with the helium each are proportional to this product. These losses lead to heating of the helium and a decrease of its density, and, in further sequence, to flashover. These measured values also show deviation and lie within the dashed lines. But here, too, decrease of the... 

Fig, 6, Flashover voltage vs. dielectric constant for various materials. 

Improvements of flashover voltage were also achieved by Suitable shaping of the annular insulators, especially at the mentioned spots. 

Flashover voltage The voltage between conductors at which hashover just occurs. 

Flashover voltage can be increased by raising the gas pressure, by utilizing electronegative gases like SFe with a high electric strength, and by immersion in... 

Geometry affects the flashover voltag. For example, a copper rod placed inside a plastic tube distorts the electric field (9) and thereby reduces the flashover voltage. 

The effect of contamination, moisture, and spacing on flashover voltage is shown in Figures 10a and 10b the very low values at 4.2 MHz are noteworthy (Fig. 10b). 

Under voltage stress, scintillation (tiny arcs) may occur on surfaces covered with a wet conducting contaminant. Under such conditions progressive electrical surface failure may take place by tracking. With serious contamination, a phenolic resin-paper laminate fails over a 12.5-mm spacing at only 115 V (162-V peak), a much lower voltage than for the Paschen flashover voltage (Fig. 9). 

Fig. 10. Dependence of flashover voltage on spacing and contamination of plastic laminates at (a) 60 Hz and (b) 4.2 MHz. A, At 50% rh B, at 100% rh with contaminated surface.-Average value.—Minimum value.

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