Capacitor voltage transformer is an important protection and metering equipment in power system and its reliable operation has an important role for safety and stability of power grid. Therefore

the state evaluation and fault analysis of the capacitor voltage transformer are of great significance to maintain the safety of power grid. The abnormal secondary voltage of one 220 kV Capacitor voltage transformer is investigated in this paper. It is concluded through structural analysis

diagnostic test and disassembly inspection that the cause of the abnormality is the breakdown of capacitor unit in the lower section of main capacitor

resulting in variation of division ratio of main and dividing capacitance and thus causing abnormal secondary voltage of capacitor voltage transformer. Finally

the corresponding preventive measures are given so to provide reference for the manufacturing process and fault diagnosis of the same type capacitor voltage transformer.