Study of on-line monitoring method of partial discharge for power transformers based on RFCT and microstrip antenna

It is an important means for partial discharge (PD) on-line monitoring of oil-immersed power transformers in order to evaluating the insulation condition. Radio frequency current transducers (RFCT) are applied to couple the PD pulse current signal, and the difference-balance pairs (one of three-phase bushing-tap grounding terminals respectively corresponded with the neutral or iron core terminal) are selected, and high speed data acquisition card is adopt to sample the PD signal (>10MS/s) from the balance pair, and further amplitude modulation, phase modulation and difference arithmetic is designed, then external electromagnetic interference is effectively suppressed and it is propitious to extracting PD pulses and judge the PD occurrence place. The wideband PD couplers based on microstrip antenna are also applied to detect the high frequency PD pulse signal. Four sensors are installed at the special parts on the transformer where PD electromagnetic wave signal can be received, then the signal is transmitted to the input analog channel of the oscilloscope (>IGS/s). So single PD waveform analysis throughout more wide frequency band can be done, and further PD fault location can also be realized. The PD on-line monitoring system based on the above two monitoring methods for in-service transformer has also been developed and installed at one main transformer in a plant (220/18RV, 360MVA, three phase and dual winding)