Long-term performance of solid insulators in gas insulated systems under HVDC stress

The results of DC long-term tests with typical GIS insulators are reported in this paper. More than 100 insulators made of different materials are under test at 500 kV DC voltage of both polarities. Compared to AC service conditions (RMS value) the electric field strength inside the epoxy material is increased up to 3 times. Until now, more than 16.000 testing hours were achieved for a large number of insulators. No puncture occurred even at the highly stressed insulators. General aspects to be considered for DC testing of HVDC insulating systems are presented. Based on several investigations of the short-time and long-term performance, conclusions for the development of reliable insulators for DC equipment are given.