Calculation And Analysis Of Dielectric Recovery Characteristics Of SF₆/CF₄ High Voltage Circuit Breaker

With the rapid development of the national economy,the power industry continued to develop in the direction of large unit,long-distance,EHV/UHV,the improvement of the transmission voltage level put forward higher requirements for the insulation of power equipment,SF6 gas is widely used in gas insulated equipment because of high dielectric strength,strong arcing ability.However,SF6 gas has a serious greenhouse effect,so it will be significantly reduced or even be disabled in the power industry before 2020.In order to eliminate the influence of SF6 gas on the environment,a lot of researches have been done to find a new type of environmentally friendly gas.However,it is not possible to find a single gas which can effectively replace SF6 gas yet.The mixed gas which can achieve the insulation and arc quenching performance similar to SF6 can not only reduce the amount of SF6,but also meet in reliability and miniaturization requirements of high voltage electrical equipment,which has become a very important research content,and has high industrial application value.CF4 gas has well insulation and arc extinguishing performance,low liquefaction temperature,stable chemical and physical properties and low GWP,which has high research value.In this paper,the thermodynamic parameters and electrical characteristics of SF₆/CF₄ were researched,the calculation results of specific heat,viscosity,thermal conductivity and other thermodynamic parameters of the different mixing ratio of mixed gas under different pressure and different temperature were given,under the plate electrode and needle plate electrode,the SF₆/CF₄ mixed gas in the gas breakdown experiment were studyed,the insulation characteristics were obtained.The thermodynamic parameters of SF₆/CF₄ mixed gas were applied to the numerical calculation of the arc quenching chamber in gas flow filed,SF₆/CF₄ mixed gas circuit breaker in different charging scheme during the breaking process were analyzed.Combined with the model of streamer discharge in SF₆/CF₄ gas,through the coupling flow field and electric field calculation,the dynamic characteristics of arc pressure gas arc chamber mixture calculated and the breakdown field strength is obtained,and the dielectric recovery characteristics of the circuit breaker in the process of breaking off are attained.The results of the study research show that the specific heat increases with the temperature and pressure of the SF₆/CF₄ gas mixture increase,the viscosity decreases with the increase of temperature,and increases with the increase of pressure and temperature,weakening the influence of gas pressure on viscosity.The thermal conductivity of SF₆/CF₄ mixed gas increases with the increase of temperature and pressure.SF6 gas content in SF₆/CF₄ gas mixtures is low pressure cylinder during the compression process,the pressure rise more quickly,the gas pressure is high,the required circuit breaker operation power is greater.With the increase of the content of CF4 gas in the mixed gas can greatly improve the pressure difference between the upstream and downstream of the nozzle and improve the arc extinguishing performance of the circuit breaker.The increase of the content of CF4 gas in the mixed gas can reduce the insulation performance of the circuit breaker.The content of SF6 gas is higher,the circuit breaker is higher at the same distance under the critical breakdown voltage,high breaking speed can increase the size of the nozzle opens before the recovery speed of medium,but the fast breaking speed also increases the pressure strength of circuit breaker,and reduces the nozzle opens after the dielectric recovery strength.