SF₆ Partial Discharge Decomposition Characteristics And Influence Factors Under Dc

With the successful development and operation of ±1100kV DC wall bushing and other Ultra High voltage Direct Current(UHVDC)gas insulated equipment,gas insulated equipment will be bound to be more and more widely used in UHVDC transmission engineering.However,the latent insulation defects of gas insulated equipment would be produced inevitably in the process of production,transportation,installation and operation.If not handled in time,they would even further develop into equipment insulation failure,even threatening the safety of power supply.Therefore,realizing on-line condition monitoring and defects diagnosis of DC gas insulation equipment and formulating specific maintenance plans are very important to ensure the safety and reliable operation of DC gas insulated equipment.The method of SF6 gas insulated equipment condition monitoring and defects diagnosis based on decomposition components analysis(DCA)have such advantages as immunity from the effects of electromagnetic interference,high sensitivity and the capacity to identify the defect type,so it become a focus of research.At present,the defects diagnosis on typical insulation defects partial discharge(PD)in AC gas insulated equipment has been basically realized by using SF6 characteristic decomposition components.However,the SF6 PD decomposition characteristics of DC gas insulated equipment still have not been clarified.Many problems,such as the similarities and differences between SF6 DC PD decomposition and SF6 AC PD decomposition,Can the AC PD decomposition characteristic feature be used for defects diagnosis of DC gas insulated equipment and Whether the influence characteristics of related factors are consistent with AC PD,have not yet been solved.Therefore,in this paper,the SF6 decomposition characteristics and influence factors under positive and negative DC PD are studied through systematic experiments.The main work and innovative achievements are as follows:?Through a lot of experiments,the SF6 positive/negative DC PD decomposition characteristics under typical insulation defects are obtained.The experimental results showed that:the generation laws of five selected SF6 positive/negative DC PD characteristic decomposition components(CF4,CO2,SOF2,SO2F2 and SO2)under different insulation defects are significantly different.Under positive DC PD,the amounts and generation rates of SOF2 are the largest,followed by SO2F2 and SO2,and the amounts of CF4 is the smallest.If the defects involve organic solid insulation materials,the amounts of CO2 would increase significantly.Under negative DC PD,the generation laws of CF4,SO2F2,SOF2 and SO2 are similar to that under positive DC PD,while CO2 would increase significantly except by free metal particles defect.?The differences of SF6 decomposition characteristics of positive DC PD,negative DC PD and AC PD under typical insulation defects were compared and studied.It was found that:under the conditions of metal protrusions defects,contamination defect and gap defects,due to the higher total PD quantities per unit time,larger SF6 decomposition region and Trichel pulse shift,the amounts and generation rates of SF6 decomposition characteristic components under these three PD conditions are presented:negative DC PD>positive DC PD>AC PD;Because of the more intense movement of free metal particles and the higher energy density of single discharge under DC PD,the generation rates of SF6 characteristic decomposition components of AC PD is lower than that of DC PD,while that of positive DC PC is closer to that of negative DC PC.However,due to the more intense thermal effect of positive DC PD,the PD intensity was gradually weakened with the deformation of metal particles,resulting in a gradual decrease in the generation rates of the SF6 characteristic decomposition components directly generated by SF6 PD primary products.In addition,the characteristic of five SF6 decomposition components were calculated and analyzed by molecular dynamics simulation based on density functional theory(DFT).The structures and energies of the reactants and generators of seven main or typical reaction were determined,and then the energy changes of each chemical reaction were obtained.The results show that the simulation model could well explain the macro-experimental results of this study from the micro-level.?Three type of five characteristic feature,namely,the total amounts of characteristic decomposition components,the effective gas generation rates and the ratios of characteristic decomposition components,were constructed.In addition,combining with C4.5 algorithm,three decision trees of positive/negative DC and AC PD were generated respectively.The results showed that:the branching and classification methods of the three PD decision trees is the same.However,due to the difference of SF6 decomposition laws and mechanisms under different PD conditions,the characteristic decomposition component ratios were obviously different under different PD condition.As a consequence,when analyzing and diagnosing the defects in positive/negative DC SF6 gas insulation equipment,the same technical ideas with AC conditions can be adopted,however,the corresponding defects threshold values need to be redefined according to the actual situation.?The influence and internal mechanism of gas pressure on SF6 positive/negative DC PD were studied.The SF6 positive/negative DC PD decomposition characteristics under different pressures(0.30MPa,0.35MPa,0.40MPa,0.45MPa and 0.50MPa)were studied experimentally.It was found that:the concentration C of SF6 primary decomposition products is only related to the electric field strength E and gas pressure P,c ? Dd((?)/P1/2)exp(-Bd P/E).Under constant electric field strength,the larger the gas pressure is,the larger the amounts of SF6 positive/negative DC PD characteristic decomposition components are,but gas pressure has no significant impact on the accuracy of the DC PD ratio diagnostic decision trees.?The influence and internal mechanism of copper,aluminum and 18/8 stainless steel on the generation of SF6 DC PD decomposition components were studied.It was found that,the electron current density is related to metal material escape work gamma and electric field E,j(O,E)=BE2/?exp(-D?3/2/E).In addition,combining the difference among the existence of oxygen passivation film on the plate electrode and the halogenation resistance of metal,three aspects work together to lead to the difference of SF6 decomposition under different metal materials.However,the types of metal materials has little effect on the diagnostic accuracy of the decision trees based on the ratio of characteristic decomposition components under the positive/negative DC PD as well.