Conceptual Study of a SF sub 6 Impregnated High Voltage Capacitor Bank. Final Report

摘要

Capacitor units with polypropylene film dielectric were wound, and impregnated with SF sub 6 gas at pressures from one to ten atmospheres. The discharge inception voltage (DIV) and the level of partial discharges as a function of voltage and pressure were measured. In the first series of tests the DIV for flattened rolls was found to be lower than expected. Stepped stress tests to breakdown were made to provide an indication of the appropriate voltage level for longer term tests. Accelerated life tests at 5 and 10 atmospheres were performed at room temperature and a time to failure as a function of stress was established. Failed units were dissected to determine the location of the breakdown and to search for microscopic evidence of damage in a scanning electron microscope. Failures were primarily located at the edges of the aluminum foil electrodes or around tabs (leads). Breakdowns in the bulk of the capacitor were concentrated in the inner half of the roll. This part of the capacitor undergoes the greatest amount of distortion during the flattening process. A theoretical analysis of voltage division between the solid dielectric and the gas gap indicates that the observed DIV can most likely be explained by field enhancement from edges (leads or foil) or particles. Tests with unflattened rolls with a retained arbor showed that omitting the forming step and increasing the tightness of the winding improved performance significantly. Failure sites for these capacitors were more evenly distributed. Conceptual designs for a large capacitor array in a pressurized tank and for a protective fuse element are presented. (ERA citation 08:000534)