Abstract: This paper analyzes a 3U0 over line alarm event of capacitive voltage transformer. The operation principle, common faults and test methods of capacitive voltage transformer are analyzed. Through the analysis of specific cases, through the analysis of insulation resistance test data, dielectric loss and capacitance test data, the diagnostic test of 220 kV auxiliary bus voltage transformer is carried out. The test results show that the dielectric loss and capacitance change do not meet the specified requirements, Through further analysis, it is clear that the 3U₀ line crossing alarm of the capacitive voltage transformer is caused by the obvious deterioration of the insulation of the capacitor unit on the capacitive voltage transformer, and the equipment is replaced later. The event reminded relevant staff to pay attention to the quality problems of power measurement equipment and carry out regular defect elimination and maintenance to avoid inaccurate measurement data and misjudgment of power system due to the measurement device itself.