Research On Dielectric Recovery Characteristics Of SF₆/N₂ And SF₆/CF₄ Mixed Gas Circuit Breaker

With the rapid growth of the national economy,the power industry has been gradually developing towards large units,long distance and ultra high voltage.The improvement of the voltage level of the power equipment will have higher insulation performance.SF₆ is widely used in high voltage circuit breakers and other insulation equipments because of its excellent dielectric strength and extinguishing capacity.However,SF₆ gas has the problems of greenhouse effect and low liquefaction temperature.It is imminent to find an alternative with excellent performance.Although a large number of studies have been conducted on the search for new environmental gases,scholars has not been to find a single gas or a mixture of gases that can effectively and completely replace SF₆ gas.In the short term the most effective method is to use SF₆ mixed gas to reduce the consumption of SF₆.Therefore,in this paper,two gas mixtures with high potential applications,SF₆/N₂ and SF₆/CF₄,are studied.The key factors such as the gas pressure in the interrupter are calculated by calculating the basic physical parameters.Based on the discharge theory,the research methods of breakdown criterion for mixed gas are put forward.The breaking performance of SF₆/N₂ and SF₆/CF₄ circuit breakers is explored,which provides the basis for the design of environmentally friendly circuit breakers.Mainly in the following areas:The basic physical parameters of SF₆/N₂,SF₆/CF₄ mixed gas.According to the thermal physical properties of gases,the calculation methods of thermal conductivity,viscosity coefficient and specific heat capacity of SF₆/N₂ and SF₆/CF₄ mixed gases were studied.The thermodynamic parameters of mixed gas at different mixing ratios were calculated,and the relationship curves of thermal conductivity,viscosity coefficient and specific heat capacity and temperature and pressure were established according to the mathematical model.The numerical calculation of the electric field and the airflow field during the breaking process of SF₆/N₂,SF₆/CF₄ mixed gas circuit breakers.Establish the simultaneous solving model of electric field and airflow field,and solve the state of the actual gas equation and the conservation equation.According to the thermodynamic properties of the gas mixture,develop different mixing ratios,different arc-chamber pressure,and different operating speeds of SF₆/N₂,SF₆/CF₄ mixed gas circuit breaker.The gas pressure characteristics,airflow characteristics and insulation characteristics of the mixed gas circuit breake during the breaking process were studied to obtain the temperature,density,and electric field distribution in the arc extinguishing chamber.Study on the breakdown criterion of SF₆/N₂,SF₆/CF₄ mixed gas circuit breakers.Firstly,the discharge coefficient of the mixed gas is calculated and substituted into the discharge model to calculate the breakdown field considering the nonuniform electric field.The Pedersen model of the original breakdown criterion is modified,and the breakdown judgment of the mixed gas circuit breaker is obtained.Compared the experimental value of the pure SF₆ to the calculated value of that one to,the reliability of the calculation can be determined.Then the critical breakdown field strengths of mixed gas are calculated and analyzed.Study on the dielectric recovery characteristics of SF₆/N₂,SF₆/CF₄ mixed gas circuit breakers during the breaking process.The dielectric breakdown characteristics of SF₆/N₂,SF₆/CF₄ mixed gas circuit breakers with different gas mixture ratios were calculated according to the critical breakdown field strength obtained from the breakdown criterion of mixed gas.Then the dielectric recovery curves of mixed gas in different interrupter chambers pressure,breaking speed are obtained.Based on the above study on the breakdown characteristics of circuit breakers,the best mixing ratio of SF₆/N₂ and SF₆/CF₄ mixed gas was determined.