Partial discharge-induced degradation characteristics of oil-impregnated insulating material used in oil-immersed power transformers

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. This paper was intended to determine the harmful PD level causing the insulation performance of the transformer to be degraded, and investigated the degradation level of an oil-impregnated press-board (PB) used between windings, at which PD is more likely to occur when ac voltage is applied than at any other oil-impregnated insulating paper material used in transformer. The degradation level was quantitatively evaluated through comparison of the lightning impulse (LI) breakdown voltage between the unused PB and what had been exposed to PD. As a result, if the material was exposed to PD of 20,000 pC or more when the voltage was applied for an hour, the residual LI withstand voltage for the insulating material itself tended to decrease along with the magnitude and application time of PD. On the other hand, in the case of the PD charge of 10,000 pC or less, the residual LI withstand voltage barely decreased even if the material was exposed to PD for more than 1 hour. Consequently, the conclusion was reached that the harmful PD level as the insulating material itself used in the oil-immersed power transformer is approximately 15,000 pC. It was also confirmed that PD exceeding this permissive level for several minutes caused the insulation performance to be degraded.