Study On The Effects And Differences On Oil-paper Insulation Flashover And Partial Discharge Under AC,DC And AC/DC Composite Voltage

At present,insulation failures of UHV converter transformers still occur sometime.Among them,the flashover and partial discharge are the common insulation faults in converter transformers.The discharge characteristics of oil-paper insulation under AC,DC and AC/DC composite voltages have attracted attention from scholars at home and abroad.Many achievements on flashover and partial discharge characteristics have been made in the study of low-voltage small-scale oil-paper model.However,there is still a lack of research on the development process of flashover and partial discharge of typical oil-paper insulation structures under high field strength.The mechanism of flashover and partial discharge of oil-paper insulation under AC/DC composite voltage still needs analysis in-depth,in order to further support the insulation performance improvement and defect detection of the oil-paper system of the converter transformer.In this paper,the different effects of AC,DC and AC/DC composite voltages on flashover and partial discharge are studied,and an experiment platform for oil-paper insulation flashover and partial discharge is built.First,the flashover characteristics along the AC,DC,and AC/DC composite voltages were studied based on the ball-ball electrode model.Then,the characteristics of PD were studied.Finally,the effect of AC/DC composite voltage on the flashover and partial discharge have been analysised.The main results in this article are as follows:(1)The flashover characteristics of oil-paper insulation under AC,DC and AC/DC composite voltages were obtained,and the differences in the flashover behavior under different voltage types were analyzed.The flashover voltage shows that under the same experimental conditions,the AC/DC composite flashover voltage is the largest,followed by the DC flashover voltage,the AC flashover voltage is the smallest;The insulation failure field strength under different voltage types based on the finite element method shows that the increase in the electrode distance makes the insulation failure field strength decrease rapidly and then slowly.The location of the gap has little effect on the field strength;Based on the analysis of the flashover discharge process and the difference in the surface morphology of the cardboard,it indicates that the AC discharge starts at the "three junctions" at both ends of the electrode,and the DC discharge start at the "three junctions on the high-voltage side",the AC/DC composite voltage flashover causes the most damage to the cardboard and DC voltage.causes the least damage.(2)The partial discharge characteristics of oil-paper insulation under AC,DC,AC/DC composite voltage were obtained,and the differences in partial discharge behavior under different voltage types were analyzed.Based on the experimental analysis of the PDIV,waveform pattern,statistical parameters,and surface morphology under different voltage types,it shows that AC voltage has the largest discharge amount and the number of discharges,DC voltage discharges number is the smallest,and When the AC / DC ratio is 1: 5,the discharge amount is the smallest;with the increase of the positive DC component in AC/DC composite voltage,the discharges number and the discharge amount are reduced,the initial discharge phase gradually moves to 90°,and the phenomenon of greasy accumulates is becoming more and more obvious.(3)Revealed the influence mechanism of AC/DC composite voltage on the flashover and partial discharge of oil-paper insulation.The analysis of the electric field distribution of the oil-paper system and the development process of the surface discharge of the AC/DC composite voltage based on the WZ fractal theory show that the critical discharge field strength,discharge probability,and channel maintenance field strength are the key factor of the number of branches along the development of flashover.The larger the DC component in the AC/DC composite voltage,the lower the critical discharge field strength and the more the number of discharge branches;The simulation analysis of partial discharge based on electrohydrodynamic theory shows that the width of the single partial discharge of the AC component is smaller than that of the DC component,and the interface charge accumulation process of the DC component is more obvious than that of the AC component.the greater the DC component,the faster interface charge gathers,and the streamer is wider.The simulation results are consistent with the experimental results.The partial discharge physical process analysis of the AC/DC composite voltage shows that the greater the DC component,the larger size and number of "bubble",the "carbonization pit" and "bubble" caused in the field is the main reason for maintaining partial discharge.The research results obtained in this article will help to deepen the understanding of the development process and mechanism of discharge under different voltage types.It will give scientific basis for monitoring the insulation status of converter transformers and stable operation,instructing the design of insulation margins optimizing.