Characterization of Polypropylene under DC-Polarity Reversal

In addition to DC stress, common HVDC insulations are subjected to DC Polarity Reversals (DC-PR). DC-PR is used to reverse the direction of power transfer. DC-PR is also used to clear pole faults. It is reported that DC-PR causes reduction in dielectric strength of HVDC insulation systems. The following study reveals the DC-PR breakdown characteristics of polypropylene (PP). A DC-PR generator is constructed to generate the CIGRE (WG-B4.51) defined DC-PR waveform at ±11kV. DC-PR breakdown characteristics of PP (23µm thickness) is obtained for two different reversal times, 500ms and 1.68ms respectively. The effect of reversal time on the breakdown characteristics is studied. The effect of type of reversal (i.e. transition from positive DC to negative DC and vice-versa) is studied. The sample is subjected to standard AC, DC and switching impulse (SI) breakdown tests for comparison. The effect of rate of change of voltage on the breakdown characteristics is analyzed.