Dielectric Breakdown Characteristics of Oil Impregnated Pressboard according to Frequency Change under AC-DC Combined Voltage

Since the converter transformer in the HVDC system is affected by both AC and DC voltages differently from the conventional converter transformer, a new isolation problem appears. As the DC voltage is applied to the valve winding of the converter transformer, the press board insulation on the valve side of the converter transformer must withstand the mixed AC voltage and DC voltage. In addition, high frequency harmonics are generated in the process of converting DC voltage to AC voltage through the converter transformer. High frequency AC voltage is mixed with DC voltage, eventually the mixed voltage can cause insulation breakdown in the press board. However, many studies have been conducted on the insulation characteristics of press boards under AC or DC voltage, but research on the insulation characteristics of press boards under AC-DC combined voltage are insufficient. Therefore, it is necessary to study the insulation characteristics of pressboard under AC-DC combined voltage. In this paper, frequency and ripple factor were selected to investigate the insulation characteristics of pressboard under high frequency and AC-DC combined voltage. Experiment results, confirmed that the dielectric strength decreased linearly as the ripple factor increased, and the breakdown voltage decreased as the frequency increased. Consequently, it has been analyzed that the valve-side insulation of the converter transformer is more affected by frequency and AC-DC combined voltage.