As for the secondary protection action caused by instable rise of secondary voltage of phase A of 500 kV capacitor voltage transformer（CVT）

it is found through diagnosis and test of phase A of CVT that the dielectric loss factor

the dielectric resistance and capacitance of the upper coupling capacitor segment are severely unqualified.It is pointed in the preliminary analysis that the failure of seal cover gradually leads to inner damp

causing break-down or short-circuit of some inner capacitance sections.According to disassembly inspection

it is proved that un-tight sealing cover leads to moisture in such batch of CVT in 1 year’s continuous operation and the trace of breakdown or short-circuit of some capacitance sections exists.It is pointed out in the analysis that the breakdown or short-circuit of some capacitance sections causes the change of voltage ratio

which rises the secondary voltage and leads to the protection action.