Abstract
Space charge effect on DC breakdown strength of Ethylene Vinyl-acetate Copolymer (EVA) was studied in the region of cryogenic to ambient temperatures. DC breakdown strength and pre-stressing effect on it were measured in the region of cryogenic to ambient temperatures. In addition, direct space charge measurement was carried out in the correlating temperature region using the pulsed electroacoustic method.
DC breakdown strength below 170 K was almost constant, however, it slightly increased with increasing temperature in the region of 170-230 K. This slight rise in electric strength would derive from the formation of homo space charge in front of the cathode. Above 230 K, it showed a significant drop with increasing temperature. This would be due to the formation of hetero space charge in front of the cathode.
The direct space charge measurement by pulsed electroacoustic method showed no space charge at 180 K, however, significant amount of positive charge was seen at 290 K. This positive charge would enhance the cathode field and reduce, the DC breakdown stress. The "ratio of electrode signals"; was defined in order to assess the field enhancement at cathode. The "ratio";showed almost the same temperature dependence as the breakdown strength, suggesting that the relaxation and enhancement of the cathode field by space charges significantly affect on the breakdown characteristics.
