Capacitor voltage transformer is major substation equipment.Measuring their dielectric loss angle and capacitance can help one timely detect insulation defects in the equipment.However

its special insulation structure makes the diagnosis more complicated.In this paper

a fault case of capacitor voltage transformer is analyzed based on the principle of measurement and the structure of measuring device

and the result indicates that the on-line monitoring data of dielectric loss factor and capacitance change obviously when a fault happens in capacitor

while the off-line detecting data of dielectric loss factor keeps stable but capacitance changes clearly.The reliability and effectiveness of the on-line monitoring method for capacitor voltage transformer is verified.Some countermeasures against the cause corresponding to the clear variation of capacitance are suggested.