ANALYSIS OF ONSITE FAILURE OF PERMA-WOOD RING OF THREE-PHASE AUTO TRANSFORMER AT 400KV TRANSMISSION SUB-STATION

Abstract

This paper presents an analysis of solid insulation perma-wood ring failure of 315MVA, 400/220/33kv transformer at site and its on-site replacement procedure in details. In power transformers, insulation of high voltage (HV), low voltage (LV) and tap windings are designed in which all the insulation co-ordination i.e. phase to phase and phase to earth, partial discharge and stray & eddy current losses are considered and calculated. The basic insulations i.e. perma wood ring insulation, lead insulation, Inter turn insulation , insulation between respective winding , insulation with respect to tank , yoke & core etc are designed and placed as per voltage level of transformer. For uniform pressing of winding coils, Perma wood ring are used in each phase of core coil assembly (active part) of transformer for uniform pressing of winding coil and also for providing insulation between coil & core clamp.