500 kV transformer is the key equipment in substation and the bushing is the important component for the stable operation of the main transformer. According to relative requirements for 500 kV high voltage bushing

the real-time monitor to the bushing’s operation condition is made based on online monitoring system. It is found that the dielectric loss of the bushing in phase B increases significantly

but its capacitance is not abnormal. In order to verify the accuracy of the data and considering that the real operation condition of the bushing can not be reflected by routine dielectric loss measurement under 10 kV

a series resonant voltage based on frequency conversion principle is used to measure the dielectric loss under higher voltage

and it is found that the dielectric loss of phase B increases as the voltage increase. The measured value is mutated in a voltage interval and the increasing slope is obviously larger compared to phase A and phase C

whilst the capacitance almost keeps the constant. The results show that the high voltage bushing of phase B has potential defects and it is confirmed by disassemble of the bushing. An accident is avoided.