High-voltage D-C flashover of solid insulators in compressed nitrogen

HIGH-voltage apparatus insulated by compressed gas requires the use of solid insulating supports. It is generally recognized that even in a uniform field the flashover voltage of such solid insulation is considerably lower than for the gas-filled gap of equal length. An increase in the allowable gradient along such insulators would in general result in a reduction in the size, weight, and cost of gas-insulated apparatus, and must therefore be considered an important and fundamental objective of high-voltage engineering. It is probable that the flashover strength of solid insulators may be brought closer to that of the gases in which they are immersed, or to their inherent volume breakdown strength, through a better understanding of the influence on insulator flashover of such factors as material, surface condition, corrugations, and gas pressure. Most of the scant literature on this subject is restricted to relatively low alternating voltages, small samples of solid dielectrics, or to special insulator shapes in non-uniform fields.¹ Reher studied the flashover of plain cylinders of ebonite, paraffin, and porcelain in dry air at pressures up to 15 atmospheres, using alternating voltages up to 140 kv maximum.² Goldman and Wul³ have investigated the flashover of plain ebonite cylinders in nitrogen with alternating voltages up to 125 kv maximum and pressures up to 20 atmospheres. The present paper reports d-c flashover values up to 250 kv across short cylindrical insulators placed in the uniform field between metallic electrodes. These studies were made on both plain and corrugated cylinders of three dissimilar insulating materials immersed in dry nitrogen at gauge pressures up to 400 lbs. per square inch.