Characteristics Research On Surface Discharge Of Oil-Paper Insulation Structure Under Combined AC-DC Voltage

Converter transformer is the core equipment of the HVDC transmission system,It s normal operation is the necessary condition for the safe and stable production and tr ansmission of national grid.The work environment of Converter transformer obviously different from power transformer.valve winding withstand apart from AC and DC,it also withstand AC-DC,features and non-characteristic harmonics,transient impulse volt age.When there is the power flow reversal,the valve winding will also be affected b y the polarity reversal voltage.Therefore,the insulation structure is more prone to pro duce defects and induce surface discharge.At present,researches on the characteristics of oil-paper insulation along the surface discharge are focused on the characteristics re search and defect diagnosis.However,the effect of surface discharge characteristics an d aging degree on the development process and initial discharge characteristics of ACDC compound voltage is not well understood.Therefore,this paper mainly studies the above two aspects.In this paper,the electric field environment of the converter transformer is analyz ed,the RC equivalent circuit is equivalent to the ideal physical model of oil-paper ins ulation structure.The electric field distribution characteristics of oil-paper insulation str ucture under AC,DC and AC-DC compound voltage are analyzed.It was found that the voltage of oil-paper insulation was distributed according to capacitance under AC voltage,and it was distributed according to resistance under DC voltage.In engineerin g,it can be equivalent to the linear superposition of AC electric field and DC field.The ratio of AC-DC voltage to the oil-paper insulation structure of the valve side win ding is respectively 1:1,1:3,1:5 and 1:7 based on the number of the flow transforme r 6 pulsation valve bridge.Based on this design,an oil-paper insulation surface discharge model is designed,this paper established a set of PD test platform for oil-paper insulation under composit e voltage.The experiment was carried out on the surface discharge model board under the AC and AC-DC of the above four ratios.This paper also made the paper insulat ion test of different aging degree,carried out the experiment on the surface discharge of different aging test products under compound voltage.The results show:(1)The initial discharge voltage of the oil-paper insulation stru cture is the lowest,with the increase of DC component in the composite voltage,the initial discharge voltage level gradually increases,and the intensity of discharge also d ecreased significantly.The pulse current signal also decreases with the increase of DC component.The test products are characterized by stage characteristics with the increas e of voltage under different types of voltages.(2)The development process of discharg e can be divided into three stages: the initial stage of discharge,the development stag e of the discharge and the adjacent flashover state.(3)The initial discharge voltage of the oil-paper insulation structure under the same voltage type increases with the agin g degree,it shows the trend of rising first and then falling,The trend decreases with the increase of dc component in the composite voltage.