Influence Of Metal Materials On Insulation And Discharge Decomposition Characteristics Of C₄F₇N/CO₂ Gas Mixture

C4F7N/CO2 mixture is one of the most potential alternative gases to SF6.However,it is not known whether the material of the existing gas insulation equipment is suitable for C4F7N/CO2 mixture.The research shows that different metal materials will have an impact on the insulation and decomposition characteristics of gas.Therefore,the research on the insulation and discharge decomposition characteristics of C4F7N/CO2 mixed gas of conductor metal materials has a strong engineering value for the design and selection of environmentally friendly gas insulated switchgear.Firstly,a gas power frequency breakdown experimental platform was established to study the power frequency breakdown characteristics of C4F7N/CO2 mixed gas under different air pressure,different metal materials and different electric fields.The results show that the breakdown voltage of gas increases linearly with the increase of gas pressure when the electric field non-uniformity coefficient is in the range of 1.04～7.57.When the electric field inhomogeneity coefficient is 7.57～19.36,the gas breakdown voltage increases with the increase of air pressure and gradually tends to saturation.When the electrode material is copper,the gas breakdown voltage is the highest,followed by stainless steel,tungsten and copper,and when the electrode material is aluminum alloy,the gas breakdown voltage is the lowest.Secondly,a gas lightning impact breakdown experimental platform was built to study the lightning impact characteristics of C4F7N/CO2 mixed gas under different air pressure,different metal materials and different electric fields.The results show that the 50%breakdown voltage of gas lightning impact shows significant polarity effect under quasi-uniform field and slightly non-uniform field.In a non-uniform field,with the rise of air pressure,the lightning impulse voltage will change from positive polarity higher than negative polarity to negative polarity higher than positive polarity,namely"polarity reversal".When the electrode material is stainless steel,the 50%breakdown voltage of gas is the highest,and when the electrode material is aluminum alloy,the 50%breakdown voltage of gas is the lowest.Under the impact voltage of positive or negative polarity,the average breakdown field decreases with the increase of nonuniformity coefficient.Finally,an experimental platform for partial discharge decomposition of gas was built to study the discharge decomposition characteristics of C4F7N/CO2 mixed gas under different metal materials and pressure conditions.The results show that when the electrode materials are aluminum alloy and copper,the decomposition product of C4F7N/CO2 is the least.When the electrode material is stainless steel,the decomposition product of mixed gas produces the most.The main decomposition products of C4F7N/CO2 mixed gas suspension discharge are CF4,C4F8,C3F8,C3F6,CO,CF3CN and C2F4.The content of decomposition products decreased gradually with the increase of air pressure.Based on density functional theory,the effects of different metal materials on the decomposition process of C4F7N/CO2 mixture were analyzed.