The abnormal situation of differential protection of 500 kVmian transformer is introduced in this paper. It is found by inspection at site that over-current traces exist at the fixing bolts and the ground connection in phase C of 5013 SF

6

gas insulated current transformer and surface of insulting spacer has visible crack after disassembly inspection. It is judged by comparing the detection results of the equipment with normal and faulty phase after disassembly as well as X-ray tomography of the insulating spacer that the insulating spacer in phase C has hidden defects of impurities in the production

which causes uneven local internal stress and electric field. During the long-term o

peration

the concentration of stress of the inserts forms cracks and the partial discharge gradually occurs till the penetration breakdown

leading to discharge at the base foundation and the grounding bolt as well as the differential protection action of the main transformer. Accordingly

such preventive measures as strengthening quality control of incoming insulating spacer

strengthening the inspection and maintenance of equipment with the same type and strictly implementing the transportation requirement strictly are proposed so to ensure safe operation of the equipment.