Dielectric Strength Ratio Between Alternating and Direct Voltages

High-Voltage insulation testing has been and is usually still done by alternating voltages. High direct voltage was made available for testing purposes by the development of the kenotron tube. When used for testing insulation direct voltage has several advantages over alternating voltage. (1) the power necessary is often much less with direct voltage than with alternating voltage. In apparatus of high electrostatic capacity, such as long high-voltage cables, the size of the alternating-voltage testing transformer becomes excessive, thousands of kilovolt-amperes being necessary. Direct voltages are therefore preferable as they necessitate only a few kilowatts. (2) Excess direct voltage is less likely to permanently damage the insulation than excess alternating voltage. (3) If direct voltage is used conductivity tests can be made and the action of the material on the application of the voltage more thoroughly studied. As the use of high direct voltage for testing purposes is found to be increasing, it is important to determine the relation between the insulation stress produced by direct and that produced by alternating voltages. Little is definitely known of what is called the “dielectric strength ratio of insulation” which is the ratio of the direct disruptive voltage to the crest value of the alternating disruptive voltage. In general, this ratio might be expected to be unity. While such is the case with air some engineers have claimed, however, that some solid insulations stand a higher direct than alternating voltage. Therefore, a very extensive set of investigations was made, with direct and with alternating voltages, on liquid and solid insulations of homogeneous and non-homogeneous structure, over a range of temperature, thickness and rate of voltage application. Their dielectric strength ratios were determined and are given and discussed in the paper. It was found that the dielectric strength ratio may be greater than unity, and sometimes very much so, that is, that the material may stand higher and sometimes very much higher direct voltages than alternating voltages, but also that the ratio with other materials may be less than unity, that is, the material may stand higher alternating than direct voltages. Ratios less than unity were given by oils, petrolatum, powdered glass, etc., that is, they stood higher alternating than direct voltage, though the difference rarely exceeded 10 per cent. Ratios above unity were given by paper, cloth, solid glass and mica, etc., indicating a greater strength for direct than for alternating voltages. The dielectric strength ratio of some materials, such as laminated paper, was found to vary with the condition and in general increase with decreasing temperature, decreasing thickness and increasing rapidity of voltage application. Some materials, such as petrolatum impregnated cable paper, gave a very high ratio, some times exceeding two, while the component materials did not differ much from unity, petrolatum being a little below and air-dry paper a little above unity. It is believed that the observation of the dielectric strength ratio and its changes with the condition of test, will give us a powerful tool for the investigation of insulation, and assist in solving the problem of understanding the mechanism of the breakdown of insulation in an electric field.