The soundness of an oil-immersed power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, only insufficient basic data backing this criterion are available. In addition, the harmful level of the insulation performance has not necessarily been perceived in such a manner as the present one, for example, only the insulation breakdown, instead of the insulation performance degradation, has been assumed to be harmful. In light of the consideration presently paid, it is necessary to determine the PD level that causes the insulation performance to be degraded in order to confirm the soundness by the test using an actual transformer. As the initial step of the study for this purpose, the previous paper investigated the PD and the insulation damage level using oil-impregnated press-boards (PBs) used in the transformer, at which PD is more likely to occur when ac voltage is applied than at any other parts, and evaluated the harmful PD level for insulating materials themselves. To investigate the harmful PD level for the actual transformer, this paper studied the harmful PD level that decreases the residual lightning impulse (LI) withstand voltage using the main insulation model, which is the insulating structure constituting the oil-immersed power transformer. As a result, black discoloration occurred on the surface of the PB and the residual LI withstand voltage also tended to decline when exposed to ac PD of 20,000 pC or more. On the other hand, when it had been exposed to ac PD of 10,000 pC, no visible damage was observed and there was no drop in the residual LI withstand voltage either. Consequently, the harmful ac PD level for the oil-immersed transformer can be determined at between 10,000 pC and 20,000 pC. This result coincided with that for insulating materials themselves, hence it was revealed that the harmful PD level be set to 10,000 pC on a conservative side.
