Research On Insulation Performance And Breakdown Mechanism Of Environmentally Friendly Gas For Medium Voltage Switchgear

SF₆ is highly electronegative gas,which is widely used in power equipment for its excellent insulation characteristic and arc extinguishing ability.But SF₆ is also recognized as the strongest greenhouse gas by international society,and is one of six greenhouse gases which is banned being emitted under the Kyoto Protocol.Therefore,to explore the environmental alternative gas of SF₆ becomes one of important technological problems in electrical equipment field.As a kind of environmental alternative gas of SF₆,C₄F₇N（perfluoro-isobutyronitrile）has become the most concerned synthetic gas among various environmental protection alternative gases of SF₆ because of its excellent insulation characteristic and high applicable economic value.In this paper,the insulating gases used in medium voltage switchgears are selected as the research object,and the physical characteristics of C₄F₇N gas are simulated and analyzed by Antoine equation.C₄F₇N gas has a high liquefaction temperature（liquefaction occurs at-4.7℃）.In order to meet the actual working conditions of medium voltage distribution equipment with environmental temperature（-25℃to+40℃）and charging pressure（0.1MPa to 0.2MPa）,the Antoine equation was extended to study the physical characteristics of C₄F₇N mixed with CO₂ and N₂.The relationship between saturated vapor pressure and temperature,liquefaction temperature and pressure of C₄F₇N mixed with CO₂ and N₂ was analyzed,which provided theoretical basis for the practical application of C₄F₇N mixed gas in medium voltage switchgears.Based on the stream discharge theory,the insulation characteristics of C₄F₇N and CO₂ and N₂ mixed gases were demonstrated.Through the derivation of Boltzmann equation,the effective ionization coefficient,electron diffusion coefficient and electron drift coefficient of C₄F₇N and CO₂ and N₂ mixed gases in the discharge process were obtained.Based on this,the relationship between the micro parameters of C₄F₇N and CO₂ and N₂ mixed gases and insulating strength of gases were verified,and the recommended mixing ratio of C₄F₇N and CO₂ and N₂ which meets the requirements of insulating performance of medium voltage switchgears was obtained.The arc extinguishing performance of environmental alternative gas of SF₆ in switch cubicle was studied.A theory of electron radiation based on the local thermodynamics of arc（conservation equations of mass,energy and momentum）was proposed.The dynamic equations of thermodynamic describing the mixture of C₄F₇N and CO₂ in breaking arc was constructed,the thermodynamic parameters of the mixture of C₄F₇N and CO₂ were calculated,and the characteristics of dynamic breaking arc and the characteristics of energy were analyzed,which provided theoretical support for the application of C₄F₇N and CO₂ mixture in arc-extinguishing of switchgears.A test device for gas insulating characteristics of medium voltage switchgear was designed.It was used in tests of power frequency partial discharge,power frequency breakdown,50%lightning impulse breakdown and other tests for environmental alternative gas of SF₆.According to the actual working conditions of medium voltage switchgear,different inflation pressure and different gas mixing ratios were set for C₄F₇N with CO₂ and N₂ mixture.The insulating characteristics of C₄F₇N CO₂ and N₂mixture,such as partial discharge,power frequency breakdown and lightning impulse,were tested.12k V/24k V ring main unit was selected as an example,the application of C₄F₇N and CO₂ mixture gas in medium voltage switchgear was studied.The difficulties in internal insulation design of ring main unit were analyzed.The arc extinguishing principle of load switch in arc chute was discussed.The electric field distribution of ring main unit was optimized.The insulating and arc extinguishing performance of the C₄F₇N and CO₂ and SF₆ mixture in 12k V/24k V ring main unit was verified by insulation test and rated current making/breaking test.The research results can provide a reference for the engineering application of the environmental alternative gas of SF₆.