Reliability of GIS EHV epoxy insulators: the need and prospects for more stringent acceptance criteria

More demanding partial discharge (PD) acceptance criteria for gas-insulated switchgear (GIS) insulators are required at the higher voltage classes to maintain high levels of reliability. In addition to increased operating stresses and greater manufacturing difficulties, fundamental limitations exist which lessen PD detection sensitivity of large physical objects. PD scaling relationships based on a recently developed theory that enable prediction of PD magnitude in equivalent defects in different insulators are presented. For example, PD magnitudes of 2 to 5 pC (the current acceptance criterion) in a 138 kV insulator correspond to magnitudes of 0.5 to 1.2 pC for 550 kV insulators, based on certain assumptions. In effect, defects with greater severity would thus be passed in present factory tests for 550 kV insulators. To maintain the high levels of reliability that manufacturers and users have come to expect, PD acceptance criteria of 0.5 to 1.0 pC are proposed for 550 kV insulators to obtain a standard of severity equivalent to that in existence at 138 kV