Study On Surface Charge Accumulation Characteristics Of Epoxy Resin Insulator Under DC Voltage Based On Electro-thermal Coupling Theory

High voltage direct current transmission technology（HVDC）has developed rapidly due to its unique advantages of large transmission capacity and small loss in long distance transmission.However,the insulation problem of DC power equipment is the bottleneck of DC power transmission technology.At present,epoxy resin is widely used as the insulation material of HVDC transmission lines,circuit breakers,HVDC bushing and so on.The insulation strength of epoxy resin has attracted extensive attention.The boundary of gas-solid is the weak point of insulation system,because the uneven electric field often leads to surface discharge and even flashover.Flashover characteristics are affected by surface charge polarity and electric field distribution on insulating surface,but the correlation between surface charge and flashover characteristics still needs to be further studied and clarified.Firstly,the surface charge accumulation and dissipation characteristics of epoxy resin base insulation material in SF₆ gas under DC voltage are studied.Based on the surface charge distribution law,the surface charge accumulation and dissipation mechanism is revealed,and the correlation between surface charge accumulation and flashover characteristics is clarified.It is found that homopolar charges mainly accumulate on the surface of the planar electrode,and they are mainly distributed near the high voltage electrode.The higher the DC voltage amplitude is,the higher the surface charge density is.The surface charge distribution pattern has strong polarity effect.The surface charge is mainly concentrated near the high voltage electrode under the positive polarity voltage,while the surface charge is relatively dispersed under the negative polarity voltage.The low density charge region near the high voltage electrode is the result of the joint action of electrode injection,reverse discharge and gas ionization.The surface charge dissipation includes two stages,which satisfy the double exponential function law.The rapid dissipation stage corresponds to the charge detrapping from the shallow trap.The slow dissipation stage corresponds to the charge falling out of the deeper trap.The pre-deposited charge on the insulation surface has a great influence on the flashover characteristics.The pre-deposited positive charge can increase the positive polarity flashover voltage,but decrease the negative polarity flashover voltage.The difference of the positive and negative flashover voltage is caused by the difference of the surface charge distribution under different polar DC voltages.The effects of SiC/epoxy resin and TiO₂/epoxy resin on the surface charge accumulation、dissipation and flashover characteristics of epoxy resin substrate under multiple factors were studied.It was found that with the increase of the content of SiC/TiO₂composite coating,the trap energy level of epoxy resin substrate decreased and the number of traps increased.The electrical conductivity of insulating medium could be improved by increasing the test temperature.Increases the rate of charge dissipation.The surface charge attenuation rate of SiC and TiO₂ composite coating is slow at low content（5%wt,10%wt,20%wt）and low test temperature（20℃,40℃）,while at high content（30%wt,40%wt,50%wt）and high test temperature（60℃,80℃）,the surface charge attenuation rate of SiC and TiO₂ composite coating is slow at low content（5%wt,10%wt,20%wt）and low test temperature（20℃,40℃）.The surface charge decay rate is fast.The flashover voltage of SiC/epoxy resin and TiO₂/epoxy resin decreases firstly,then increases and then decreases with the increase of SiC and TiO₂ content.When the SiC content is 30%wt,the maximum positive polarity flashover voltage of SiC epoxy resin composite coating is 23.68 k V,and the maximum negative polarity flashover voltage is-25.51 k V.When TiO₂ content is 40%wt,the maximum positive polarity flashover voltage of TiO₂/epoxy composite coating is18.2 k V,and the maximum negative polarity flashover voltage is-18.34 k V,which is 15.1 and 10.9%higher than that of pure epoxy resin,respectively.Finally,a charge distribution simulation model with electric-thermal multi-physical coupling was established,and the correctness of the simulation model was verified by comparing the simulation results of slice epoxy resin samples with the experimental studies in chapter 3 and chapter 4.Then,the simulation model is applied to true basin insulator to study the relationship between surface charge,surface electric field distribution and surface conductivity of true basin insulator in electrothermal coupling field.It is found that the optimal surface coating conductivity of 2.15×10^-15s/m can minimize the accumulation of surface charge on insulator.A coating design scheme is proposed to improve the electric field and charge distribution of basin insulator by adjusting coating parameters.