Research On Accumulative Breakdown And Partial Discharge Characteristics Of Oil-pressboard Insulation Under Repeated Switching Impulse Voltage

As the most important part of UHVDC,converter transformer has complex structure and high insulation requirements.Its safe operation largely determines the reliability of HVDC system.Converter transformer will be subjected to the impact of multiple switching overvoltage during its service life,which easily induces the deterioration of oil-pressboard insulation of valve side windings and poses a serious threat to the safe operation of converter transformer.At present,the cumulative damage characteristics of oil-pressboard insulation under switching impulse overvoltage have not been studied completely,especially its cumulative breakdown mechanism and the partial discharge(PD)characteristics after cumulative.In order to solve the problem of oil-paper insulation deterioration and complicated partial discharge caused by cumulative switching overvoltage on the valve-side winding of the converter transformer,repetitive switching impulse voltage generation platform and PD test platform based on impulse current method are built,and electrode models of column-plate and needle-plate are designed,the cumulative breakdown characteristics and mechanism of oil-pressboard insulation under different factors with the action of switching impulse overvoltage,and the PD characteristics under AC/DC composite voltage after cumulative damage are studied in this paper.This paper provides theoretical support for the oil-pressboard insulation under multiple operating impulse voltage.Firstly,the cumulative breakdown characteristics and mechanism of oil-pressboard insulation under switching impulse voltage are studied.The Weibull distribution based on SVR was used to analyze the cumulative breakdown times under different voltages.The results showed that its cumulative failure probability conformed to the Weibull distribution,and the characteristic parameters were calculated,then the U-N characteristics are obtained.The residual charge after accumulation at different times was measured,it was found that the residual charge density and injection depth increased with the increase of the impact accumulation times,and the cumulative number for breakdown increased significantly after adding the charge dissipation process,which shows that space charge injection is an important factor to promote the occurrence of cumulative breakdown,and the root cause of breakdown is the electrical damage to the insulating paper caused by switching overvoltage,which is more obvious under needle-plate electrodes with more concentrated electric field.Secondly,the influence of different factors on the cumulative breakdown characteristics is studied.The results show that with the increase of the thickness of insulation paper,the minimum single breakdown electric field strength E0 and the cumulative coefficient m both decrease,the E-N moves downward,and the cumulative tolerance to the switching impulse voltage decreases.With the increase of thermal aging time,the minimum single breakdown voltage U0 and the cumulative coefficient m both decrease.The cumulative tolerance becomes weaker,and the parameter decreases with a certain saturation trend.The reason is that the thermal aging process mainly damages the amorphous phase region.With the increase of aging time,the deterioration trend also becomes slower.Wave-head time has a significant effect on the operational shock tolerance.With the increase of wave-head time,the Uo and m both become larger.The cumulative shock tolerance to steep wave front operational shock voltage is smaller.The shorter the wave-head time,the greater the current density flowing through the paper when the switching impulse voltage acts on oil-pressboard insulation.Finally,the PD characteristics under AC/DC composite voltage after the accumulation of switching impulse voltage are studied.The results show that the PD under AC/DC composite voltage is concentrated near the rising edge and peak value of positive and negative half cycle.As the negative DC component raises the voltage of negative half cycle,Qave and Fnc of negative half cycle are larger,while the insulation without impulse action is mainly discharged along the lateral surface,and Qave and Fnc are smaller.After 200 times of switching impulse,the paper began to deteriorate,cellulose impurities and bubbles appeared,a small number of pulses with large discharge amount appeared,and the PD type included the small bridge discharge;after 400 times of impulse,the surface appeared small depression,the interface between the tip of the needle and the oil paper produced a section of oil gap,the PD type changed to streamer discharge,the discharge phase was obviously widened,Qave and Fnc were significant;After 600 times of impulse,the tip of the insulating paper deteriorated seriously,and there was a deep depression,Qave and Fnc began to decline,and the PD type was mainly along the surface discharge.