In view of an abnormal capacitance and dielectric loss during a routine test of a 500 kV capacitor voltage transformer (CVT)

the fault diagnosis is systemetically developed. It is preliminarily judged through diagnostic tests

comparison of test data of equipment horizontally

and analysis of historical data of delivery test

handover test and routine test vertically that there is an internal insulation defect in the middle section capacitor unit in phase B. It is confirmed through high - voltage dielectric loss test and oil chromatography analysis that excessively high energy discharge is occurred inside the capacitive voltage divider. Finally

it is found through disassembly inspection that the 5th capacitor element leads to partial discharge due to burrs on the electrode plate

resulting in electrode breakdown and insulation oil pyrolysis. Moreover

its is confrmed that the fault is from both the local field strength concentration due to the burrs on the electrode plate stamping and the dielectric damage due to defect of the insertion process

which lead to breakdown of the capacitor element plate during long - term operation. Finally

the comprehensive prevention and control measures

containing the improvement of test diagnostic methods

optimization of manufacturing processes

and strengthening of operation and maintenance strategies

are propsoed so to ensure the enhanced reliable operation of CVT.