Monte Carlo Simulation And Experiment Study On The Alternative Gases Of SF₆

SF₆ is widely used in power system and electrical equipment as an excellent insulation and arc extinguishing medium. 1997, SF₆ (GWP=23900) was one of six greenhouse gases that needed to reduce the use amount. Therefore, it is an urgent case to reduce and control the emissions of greenhouse gas. With the influence of non-uniform electric field, conducting particle and roughness of the electrode surface, the dielectric strength of SF₆ decreases. Therefore, it is imperative to seek the substitution gas of SF₆. The short task on solving the GWP of SF₆ is to reduce the used amount in power system, such as using SF₆ gas mixtures. The long task is to search new alternative gases to SF₆, but it is a long task.In 1997 the National Institute of Standards and Technology (NIST) had led the c-C₄F₈ mixtures as the most possible alternative to pure SF₆. From the 80s of 20th century the foreign researchers researched the direct current and pulse breakdown characteristic, afterward researches were few. Recent years the c-C₄F₈/N2 gas mixture was done more studies on the discharge parameters of electron swarm. In 2001 the alternating current insulation characteristics of c-C₄F₈ in short gap distance was studied by Kyoto University.This paper uses the Monte Carlo simulation method to describe the relation of all kinds of collision cross sections and electron energy distribution, namely using a microscopic model to calculate the development process of electron swarm. Its correction of calculating results is nearly correlative to the choice of collision cross sections and its correction. And those collision cross sections may be determined by fitting the simulation results from experiment data. At first, we calculate the effective ionization coefficients (α-η)/N and drift velocity Ve of pure gas to determine collision cross sections in this paper, and the calculated results are agreement with the experiment data. It is confirmed the chosen cross sections is valid.The data of c-C₄F₈ collision cross section is scarce; such as no determination of the momentum transfer cross section. The data from different researchers is widely discrepant, and vibrational and electronic excitation cross sections are also needed. This paper uses the Monte Carlo simulation method to determine a set of more exact and complete collision cross sections for c-C₄F₈ by fitting the transport coefficients calculated from MCs method to the experimental data. The electron swarm parameters of c-C₄F₈, c-C₄F₈/N2, c-C₄F₈/CO2 and c-C₄F₈/CF4 are calculated by the model of swarm development for gas mixture in this paper, which are agreement with experiment data. This paper improved previous researchers proposed the method of judging which kind of gases electron will collide with. The more exact the limiting field (E/N)lim, (α-η)/N=0, the value of (E/N)lim is more agreeable the experiment data.This paper has obtained the (E/N)lim, according to the curve of c-C₄F₈/N2, c-C₄F₈/CO2 and c-C₄F₈/CF4 by MCs method, which all are higher than those of SF₆. It is presumed that the insulation characteristics are proportional to gas pressure, the required gas pressure ratios of three c-C₄F₈ gas mixtures comparable with the insulation property of SF₆ and GWP at this gas pressure have also been investigated. Considered from insulaton strength and GWP, three mixture gases may displace SF₆, whose order of superiority is c-C₄F₈/N2>c-C₄F₈/CO2>c-C₄F₈/CF4. This paper used the pulse Townsend (PT) discharge method to research the mechanism of swarm. PT method uses the single pulse of high energy laser to release the initial electron from the cathode. According to the measured curve of electron current, the process of collision and transport is determined. The choice of experiment setup material and the construction of anode in this paper were carried on the comparison, which achieved the requirement of experiment. In the present experiment, the pulse width (40 ps), wave length (266 nm) and energy of per photon (4.66 eV) of the picosecond mode-locked Nd-YAG laser guaranteed the correctness of the experiment, which is the progress in the experiment instrument.This paper used the pulse Townsend discharge method and measured the swarm current waveform of N2, CO2, CF4, c-C₄F₈, N2O and CHF3, according to the nature of gases, we analyzed the possible processes in the swarm, including diffusion, ionization, attachment, detachment and conversion, and we obtained the density-normalized effective ionization coefficient (α-η)/N and drift velocity Ve. Considered from insulaton strength, GWP and the development of swarm, N2, CO2, CF4, N2O, CHF3 and c-C₄F₈ were not suitable to displace SF₆ insulation gas.