In this paper

the reason on higher secondary output voltage at 500 kV capacitor voltage transformer during normal operation is analyzed in detail in accordance with result of disassembly inspection together with similar fault of 500 kV capacitor voltage transformer from the same manufacture

same type and same batch.It is shown by the result that since environmental cleanliness does not meet requirement of cutting aluminum foil

small particle dust and other foreign matters are adhered on the surface of aluminum foil during cutting period.Under action of long term operation voltage

partial discharge occurs due to concentration of partial field strength

causing oil to be deteriorated and produce wax

which leads to more concentration of field strength and form vicious cycle and

finally causes breakdown of film and aluminum foil.Moreover

corresponding preventive measures are proposed based on the operation condition.