Correlation Characteristcs Between Negative DC PD Quantities And SF₆ Decomposition Under The Needle-Plate Electrode

With the successful development of key components such as ultra-high voltage DC SF6 gas insulated bushing,the application of DC SF6 gas insulated equipment in the power transmission and transformation field is not far away.However,according to the decades of operating experience of AC SF6 gas insulated equipment,there will be inevitably some insulation defects inside the DC SF6 gas insulated equipment.These insulation defects will lead to Partial Discharge(PD).Under DC PD SF6 will decompose to generate many stable products such as SOF2,SO2F2,SOF4,SO2,CO2,H2S,CS2,CH4 and so on.And the decomposition characteristics of SF6 has a close relationship with the insulation fault inside the equipment so that the fault can be diagnosed via its decomposition characteristics.At present,many encouraging achievements have been achieved which are about the research on the fault diagnosis of AC gas insulated equipment by the means of SF6 decomposition component analysis(DCA).However,it is an unsolved problem to diagnose the insulation fault of DC gas insulation equipment using the decomposition characteristics of SF6.The discharge quantity is an important parameter that characterizes the DC PD,which can effectively reflect the seriousness of the defects in the DC SF6 gas insulated electrical equipment.Therefore,obtaining the correlation between DC partial discharge quantity and SF6 decomposition process components is one of the basis to establish the SF6 decomposition component analysis(DCA)fault diagnosis method.In order to obtain the correlation characteristics between the DC PD quantity and the SF6 decomposition components,this paper used the needle-plate electrode to simulate the metal protrusion defect.Applying different levels of DC voltage to the electrode made SF6 gas insulation medium decompose under the different PD discharge quantites so as to obtain the SF6 decomposition characteristics under different discharge quantites.On this basis,the relationship between PD discharge quantities and SF6 decomposition was established by analyzing the DC PD pulse and SF6 decomposition process.Through the above research work,the innovative conclusions obtained in this paper are as follows:(1)The n-q statistical histogram of DC PD pulse proves that the type and production distribution of SF6 decomposition components are closely related to the amplitude and frequency of PD pulse.So this paper selected the expected value of discharge quantity per second Qsec as the PD characteristic patameter.The results show that the concentration of CO2,SO2F2,SOF2 and(SO2F2+SOF2+SO2)is well related with Qsec,the generation of which is appproximatly "linear-saturated" with Qsec.The mean square production rate RRMS of SO2,SO2F2,SOF2 and(SO2F2+SOF2+SO2)increases linearly with Qsec and can be used as the component characteristic parameter for judging the DC PD quantities.The effective concentration ratio of characteristic components CRRMS(CF4/CO2)can be used as a supplementary criterion for larger discharge strength.The value of CRRMS(SO2F2/SOF2)has a positive correlation with Qsec.(2)Compared with the correlation between the AC PD quantities and the SF6 decomposition components,CRRMS(CF4/CO2)、CRRMS(CF4+CO2)/(SO2F2+SOF2)and CRRMS(SO2F2/SOF2)have the similar trend with Qsec.Whether under DC or AC PD,the characteristic ratio of the composition has a very obvious mapping relationship with the discharge quantity and can reflect the law of SF6 PD decomposition to some extent.(3)The paper defined CRRMS(SO2F2/SOF2)as effective energy eigenvalue ERMS representing the charge quantity and severity of PD.The effective energy eigenvalue ERMS of DC and AC PD increases rapidly with the increase of discharge quantity,then gradually slows down and tends to be gentle,finally reaching a critical value.So,the ERMS defined herein can be used as a feature parameter that represents PD energy strength and fault severity regardless of AC or DC SF6 gas insulation equipment.The greater the value of ERMS is,the greater the PD energy is,and the more severe the fault is.