Electrical breakdown characteristics of oil-paper insulation under pulsating voltage

With the improvement of voltage level of HVDC transmission, the insulation of converter transformer has become increasingly prominent. Different from the general power transformer, valve side windings on converter transformer withstand pulsating voltages, which makes insulation problem get more serious. In this paper, experiments were carried out under pulsating voltage over a wide range of the pulsating coefficients defined as p=Eac/Edc, which can obtain the relationship between p and breakdown voltage of oil-paper insulation. Experiment results showed that the breakdown voltages were regularly decreased by the increases of p, and the breakdown voltage value of pre-stressed DC-voltage is higher than that of pre-stressed AC-voltage, also the scatter of the pre-stressed DC-voltage is better. Moreover, the short-time breakdown tests of oil-paper insulation specimens were also carried out at different levels of temperature. The results showed that when the temperature exceeds a certain threshold, the temperature will show a significant impact on the breakdown strength. A great influence of temperature on the DC component of the breakdown voltage was observed, while the effect on AC component is relatively small. At last, this paper presents an expression which relates the breakdown strength to the temperature.