Breakdown characteristics of oil-paper insulation under ac and polarity reversal voltage

In HVDC transmission systems, the main insulation of the converter transformer on valve side is subjected to an AC+Pr (AC and polarity reversal) voltage. In order to ensure the safety operation of the converter transformer, it is necessary to research breakdown characteristics of the oil-paper insulation under this voltage waveform. A set of AC+Pr high voltage test equipment is developed, and breakdown experiments on the oil-paper insulation are carried out under the voltage waveform with the different Pr component in this paper. Results indicate that the breakdown voltage of the oil-paper insulation increases along with the increase of the Pr component, and so does the breakdown probability during polarity reversal. AC currents through the insulation are not affected by the Pr component and have linear relationship with the AC component voltage. There are obvious polarization currents after polarity reversal. Their initial currents have nonlinear relationship with the Pr component voltage, and the rate of decay does not follow the index law. Residual DC currents also have nonlinear relationship. It is considered that voltage distribution relations of two components in the oil-paper insulation are independent of each other. The voltage amplitude and waveform on the oil-gap varies with the Pr component, and the breakdown voltage decreases along with the increase of the voltage applied time, these characteristics determine the law of breakdown.