Oil-pressboard interface charges under combined AC/DC electric field: Their characteristics and influence on surface flashover voltage

Under AC/DC combined electric field, the oil-pressboard insulation of the converter transformer accumulates space charges to generate a partially distorted non-uniform field and thus affects the electric strength distribution of the insulation system. The present research uses the needle-plate electrode to simulate the distorted non-uniform electric field fostered by the space charges and adopts the electrostatic capacitive probe method to record the changing patterns of the interface charges in the AC/DC combined electric field, and studies the impact of interface charges on flashover voltage, aiming at providing concrete references for the design of converter transformer. The research results indicate that, under AC/DC combined electric field where the ratios between AC voltage equivalent value and the DC voltage are recorded as 1:1 and 1:5, the interface charges accumulated at the upper layer of the pressboard in the non-uniform electric field possess the same polarity as that of the DC proportion in the applied voltage complex. The charge density under the combined electric field superimposed by AC on negative DC voltage appears larger than that in the context of the positive DC voltage. The interface charge density increases with the duration of voltage application and also fluctuates with the AC instantaneous field in the similar manner as power frequency voltage. The larger the DC proportion in the combined electric field, the less the fluctuation appears. Due to the reverse effect of the homopolar charges to the electric field, the surface flashover voltage in the combined electric field superimposed by AC voltage on the negative DC voltage is 1.3 times greater than that in the scenario of positive DC voltage.