Partial discharges in and electric strength of high-voltage polymer insulation at cryogenic temperatures

A study of partial discharges in multilayer polymer insulation in liquid nitrogen was made, for the purpose of determining the effect of overvoltages on initiation of such discharges and the laws of material breakdown at cryogenic temperatures. Specimens of six high voltage insulation materials (polytetrafluoroethylene, polyethylene tetraphthalate, high density polyethylene, polyimide, polypropylene and polypropylene paper) in stacks of 2-12 layers of corresponding 45-520 um total thickness were first tested under alternating voltages with the frequency varied from 50 to 1000 Hz. They were also tested under a 300 Hz oscillatory voltage pulse simulating a switch on surge, with a logarithmic decrement of 3, superposed on a 50 Hz alternating voltage in phase with the positive amplitude of the latter. The data, evaluated in terms of insulation life as function of the electric field intensity and as function of the frequency, serve as basis for accelerated testing within given voltage and frequency constraints which take into account the aging factor. A statistical analysis based on the Weibull distribution and linear refression yields the values of constant coefficients and exponents in those relations.