| SF6 is widely used in circuit breakers whose voltage levels are more than 126kV for its excellent insulating and arc-extinguishing performances.However,using SF6 will exacerbate the greenhouse effect and the gas has a high liquefaction temperature.It is extremely urgent to find an alternative gas featuring satisfactory performances.So far,neither a kind of pure gas nor a mixed gas has been found that can completely replace SF6.One of the most effective solutions in short term is to reduce its use level in power system,for example using the mixture of SF6 and another gas.SF6/N2 and SF6/CF4 are selected as research objects in this paper for their great application potential.First the discharge mechanisms for the two gas mixtures are studied by the theoretical calculation and the experimental measurement methods,then the factors that affect the discharge properties are analyzed.The method for calculating the breakdown criterion in the mixed-gas circuit breaker is proposed,which is used to study the no-load breaking performances of the SF6/N2 and SF6/CF4 circuit breakers.The researches can provide theoretical basis for the design and development of the environmental friendly circuit breaker operating in the alpine region.The dissertation mainly conducts following several aspects:(1)The discharge mechanisms of SF6/N2 and SF6/CF4 are studied.The parameters such as the ionization coefficient and absorption coefficient of SF6/N2 and SF6/CF4 gas are obtained,which are used to reveal the mechanism of synergistic effect.Considering the synergistic relations between particles,a discharge model is established to simulate discharge processes.This model is used to analyze following three problems:the ionization between electrons and heavy particles,the adsorption between electrons and electronegative molecules,and the drift diffusion of microscopic particles.The changing rule of electron number density over time and space is obtained,and the influences of electrode structure and SF6 content on streamer developing processes are studied.Moreover,the streamer developing velocities and plasma channel radius are calculated.The distortion of space charge on electric field is analyzed to determine the electron number density in discharge gap when the distortion occurred.(2)The numerical calculations of corona inception voltages and breakdown voltages in SF6/N2 and SF6/CF4 are studied.According to discharge mechanism,a method is proposed considering electrode structure to compute corona inception voltages and breakdown voltages in SF6/N2 and SF6/CF4.Taking typical electrodes as examples,corona inception voltages,breakdown voltages,and electron number densities in plasma channel are obtained.The method is verified by corona discharge experiment,breakdown experiment and spectral experiment at both macroscopic and microscopic levels.Then synergistic effect coefficients are calculated based on computational and experimental values of breakdown voltage,respectively,which proves the feasibility of this method in analyzing synergistic effects.(3)The key factors affecting discharge properties in the SF6/N2 and SF6/CF4 gases are studied.By analyzing numerous experimental data,the influences of various factors on corona discharge and breakdown properties of SF6/N2 and SF6/CF4 are obtained,such as pressures,mixing ratios,voltage polarities,gap distances.The formation mechanism of polar effect is interpreted from the view of space charge affecting discharge process,and the changing rules of polar effect over pressure and mixing ratio are studied.Synergistic effects of SF6/N2 and SF6/CF4 are studied in different conditions to analyze the influencing factors for synergistic effects of gas mixtures.The sensitivity of breakdown voltage on electric field inhomogeneity are researched,and the reason for sensitivity difference in different gas mixtures is explored.(4)The dielectric recovery characteristics in SF6/N2 and SF6/CF4 circuit breakers are studied.Considering electric field inhomogeneity in arc extinguish chamber,the Pedersen model are improved through the computing method proposed in this dissertation,then a new breakdown criterion is proposed for mixed-gas circuit breaker.A mathematical model for flow field and electric field is set up.Using this model in combination with improved breakdown criterion,cold dielectric recovery characteristics of SF6/N2 and SF6/CF4 circuit breaker are calculated.Furthermore,the influences of mixing ratio,breaking velocity,and gas-charging pressure in arc extinguish chamber are analyzed.The re-strike problems in breaking process of circuit breaker are studied.Taking comprehensive account of the liquefaction problem and greenhouse effect,the optimal mixing ratio,gas-discharging pressure and breaking velocity for mixed-gas circuit breaker are proposed. |
