Partial discharge signal propagation characteristics inside the winding of oil-immersed power transformer - using the three-winding transformer model in air

The soundness of a power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. To establish clear control criteria for this PD test, it is necessary to study PD propagation characteristics inside the transformer windings. In this paper, a study was conducted on PD signal propagation characteristics for an oil-immersed transformer. This involved a winding model of a three- winding transformer, or the structure of an actual transformer being produced to investigate the PD signal propagation characteristics relative to the occurrence position, measurement position, and front time of the PD as parameters. Subsequently, since a preceding study on a gas-filled transformer confirmed that an equivalent circuit of the winding could be used for analytical evaluation, an analysis model was also constructed for a three-winding model of the oil-immersed transformer in the present study to investigate the propagation characteristics. Consequently, it emerged that an analysis using an equivalent circuit could also effectively simulate the propagation characteristics, waveform shape, and transmission rate for an oil- immersed transformer. As for the transmission rate, it was found that the signal was detected as significantly damped to a level of several percent depending on the occurrence and measurement positions of the PD, as was also the case for a gas-filled transformer.