The Effect of Transient Voltages on Dielectrics

Abstract

In practise, failures of dielectrics are generally caused by transient voltages. It is, therefore, of great practical importance to determine the various phenomena affecting the strength of dielectrics and means of protecting them when they are subjected to known transient voltages. An impulse generator from which impulse voltages of any given wave front, length of tail, etc., can be obtained is described. Energy is required to rupture gaseous, liquid and solid dielectrics; this introduces a time element. Thus, on account of this time lag, when voltage is applied at a very rapid rate, as by an impulse, spark-over does not occur when the continuously applied breakdown voltage is reached. The voltage ``over-shoots'' or rises above this value during the time rupture is taking place. This excess, or rise, in voltage above the continuously applied breakdown value is greater the greater the rate of application. The time depends upon the nature of the dielectric, the dielectric field, the shape and spacing of the electrodes, initial ionization, etc. The strength of air between spheres and needles for impulses of different front, length of tail, etc., is given, as well as the time in micro-seconds and the voltage required to rupture air between spheres and needles on the front of waves rising at various rates. Transient spark-over and corona voltages for wires, surface spark-over, insulator spark-over, effects of polarity, air density, practical application, etc., are given. Transient spark-over voltage and time are recorded for oil, and various solid dielectrics.