Study On Insulation And Arc-quenching Characteristics Of Eco-friendly C₄F₇N Gas Mixture In High Voltage Circuit Breaker

Perfluorinatedisobutyronitrile（C₄F₇N）is a synthetic gas,which has become a very promising SF₆ replacement gas due to its good environmental characteristics and excellent insulation performance and has attracted the attention of the power industry and researchers.As one of the most important carrying,protection and control units in the power system,the high-voltage circuit breaker’s（HVCB’s）breaking and insulation are still heavily dependent on SF₆ gas.Whether the electrical performance of C₄F₇N gas mixture can meet the functional requirements of HVCB needs the in-depth research urgently.In this article,a 40.5k V SF₆circuit breaker prototype was used as the carrier,and the C₄F₇N gas mixture replaced the SF₆in the circuit breaker arc extinguishing chamber directly.We used the combination of simulation and experiment as our research means to obtain the dynamic and static insulation characteristics of C₄F₇N gas mixture,revealed the discharge decomposition and recombination mechanism and reaction kinetic behavior of C₄F₇N,and evaluated the arc-quenching performance of C₄F₇N gas mixture.The main research work is as follows:（1）The dynamic and static breakdown voltage measurements of C₄F₇N/CO₂ mixed gas and C₄F₇N/N₂ mixed gas of the HVCB contact gap were carried out to study the dynamic and static insulation characteristics of C₄F₇N gas mixture.It was found in the experiments that the dynamic breakdown voltage dispersion of the two C₄F₇N mixed gas was large,which has obvious“undervoltage breakdown”phenomenon and“antipolarity”effect.Increasing the C₄F₇N content in the gas mixture and arc extinguishing chamber fill pressure couldn’t make much difference to the“undervoltage breakdown”and large dispersion of the gas mixture.By comparing the dynamic and static breakdown voltages,we found that the dynamic breakdown voltage of pure SF₆,C₄F₇N/CO₂,C₄F₇N/N₂ and pure CO₂ were lower than the static ones within a period of open distance just before or just after the contacts are closed.Thus,when designing the break insulation of the arc extinguishing chamber,especially for the working condition of very short arc burning or small current arc breaking,the design of fracture insulation should be based on the dynamic insulation characteristics of the gas medium.（2）We studied the decomposition and recombination process of C₄F₇N and its mixed gas to obtain the optimal molecular configuration of the reactants,transition states,intermediates,and products in the C₄F₇N mixed medium system at high temperature,which showed the information such as the reaction energy barrier and the resonant frequency of the breaking of chemical bonds of particles.Also,the free radical recombination during decomposition and recombination reactions and the lowest energy reaction path was given.The decomposition and recombination mechanism of C₄F₇N mixed medium system was revealed,the quantitative relationship between reaction rate constant and temperature was obtained,and the dynamic change law of the molar fraction of each particle in C₄F₇N mixed medium system with reaction time at different temperatures was obtained as well.We constructed the correlation between the ionization energy,electron affinity energy and electron polarizability with the macroscopic insulating strength,establishing the prediction model of the insulation strength of the gas medium.The relative SF₆ insulation strength of each particle in the C₄F₇N mixed medium system was obtained,and the model correlation could reach 0.8 or more.（3）We established the arc mathematical model of short circuit current of HVCB with the multi-physical field coupling to analyze the effects of two buffer gases,CO₂ and N₂,on the arc properties of the C₄F₇N mixture gas,study the distribution characteristics and variation law of the macroscopic parameters of mixed gas in the arc extinguishing chamber and analyze the decomposition and recombination mechanisms of the C₄F₇N mixed gas in the whole process closing and opening.Thus,the thermal and dielectric recovery strengths of the gas mixture were obtained.The research proves that,compared with SF₆,C₄F₇N gas mixture in the arc burning stage has stronger convective heat transfer ability and lower arc temperature,but the pressure gas effect is poor and the post-arc temperature decay rate is slow.When the 40k A short-circuit current is broken,the 9%C₄F₇N/91%CO₂ and9%C₄F₇N/91%N₂ mixture’s thermal breaking ability of 0.6MPa were only 38.7%and 34.1%of SF₆ under the same conditions,the dielectric recovery strengths are 56.9%and 61.8%of the SF₆ under the same conditions,post-arc C₄F₇N cannot recover.When CO₂ serves as a buffer gas,the molar fraction of CO in the recovered product can be up to more than 40%.（4）The arc ignition experiment of C₄F₇N/CO₂ gas mixture was conducted to study the arc ignition characteristics of C₄F₇N/CO₂ gas mixture in circuit breaker under different conditions of C₄F₇N content,filling pressure,arc ignition time and switching times.Based on this,the arc mathematical model was validated,to further study the influence of different factors on the arc quenching performance of the mixed gas.The results show that the arc voltage and arc quenching peak of C₄F₇N/CO₂ mixture are lower than those of SF₆,which is unfavorable to arc quenching.Increasing the fill pressure of the arc extinguishing chamber can improve the arc extinguishing performance of the mixed gas more effectively than increasing the content of C₄F₇N.Considering the for the requirements operating temperature range of high pressure circuit breaker,it is recommended to adopt a gas configuration scheme with low C₄F₇N proportion and high inflation pressure,such as 0.8MPa,5%C₄F₇N/95%CO₂,which can be applied to the ambient temperature of-26℃at minimum,whose thermal breaking capacity and dielectric recovery strength are 41.1%and 51.8%of SF₆ at 0.6MPa,respectively.The research results of this article lay a theoretical foundation for the design,development and application of the new generation of green high-voltage circuit breakers,and provide experimental data and research methods,which plays a promoting role in pushing forward the green and low-carbon development process of power and energy equipment and driving the early realization of the“double-carbon”goal.