Breakdown Voltage of Polymeric HVDC Insulation at DC Stress and Superimposed Lightning Impulse Voltages

Abstract

Space charge formation makes it difficult to predict the resulting electric field distribution within HVDC cable insulation. The main purpose of the work presented here has been to experimentally study how the lightning impulse breakdown strength of thin (0.1 mm thick) insulation foils of polypropylene (PP) and Polyethylene Terephthalate (PET) are affected by DC prestress, polarity of the impulse voltage and the temperature. Tests were performed using parallel plate electrodes and prior to DC prestressing the lightning impulse breakdown strengths were found to be approximately 17.5 kV for samples of both PP and PET. Prior to superimposing lightning impulse breakdown voltages, a DC voltage of 8 kV was applied for 60 seconds. The effect of DC prestressing was to significantly increase the impulse breakdown strength during aiding and reduce the breakdown strength during opposing impulse polarity. This strongly support the assumption of homo-charge formation close to the metal electrodes during DC stress. The results showed higher and more temperature dependent charge accumulation in samples of PET. It is indicated that at 22 °C the homo-charge formed during DC stressing approximately reduced the electric field at the electrodes from 100 to 48 kV/mm in the PP samples and from 80 to 22 kV/mm in the PET samples, respectively