Researches On Insulation Characteristics Of C-C₄F₈ And Its Mixtures

It is urgent to find new alternative dielectric gases to pure SF6 for possible immediate or future use in existing or modified electrical equipment due to its high GWP. c-C₄F₈ gas mixtures have been listed as the most potential for use as a gaseous dielectric in NIST Technical Note 1425 in 1997. However, there has few researches on this gas mixture since AC insulation characteristics of c-C₄F₈ gas mixtures in short gap distance was investigated by Kyoto university in 2001. This paper will investigate comprehensively the AC and lighting impulse insulation characteristics of c-C₄F₈ gas mixtures with N2,CO2 and CF4 in uniform and non-uniform field.Because gas breakdown process in uniform filed is simple, the dielectric strength of gas can approximately be expressed as limiting field, which can be obtained by Monte Carlo simulation of gas discharge. In this paper, an improved method is advanced to accelerate the simulation of Monte Carlo in which an upper bound is set for each collision rate in advance. Whether the collision is real or null is decided after choosing a collision process that might have occurred according to these upper bounds. By this way, only one collision cross section needs to be computed at each time step to continue the simulation, which improves greatly the running speed, especially for simulation of gas mixture. At the same time, the improved method is verified by available data of SF6 and CO2 gas mixtures. After a set of isotropic collision cross sections is chosed which is consistent with gas discharges parameters measured in pure gas, the improved Monte Carlo is used to simulate the motion of electrons in c-C₄F₈ gas mixture molecule with N2, CO2 and CF4. Effective ionization coefficients and limiting field are obtained to analysis the insulation characteristics of these gas mixtures in uniform field. At the same time, required gas pressure ratio comparable with insulation property of SF6 and GWP of gas mixtures relative to pure SF6 are also calculated, and are then compared with those of corresponding SF6 gas mixture. The results show that insulation strength of c-C₄F₈ gas mixtures with N2,CO2 and CF4 in uniform field become more and more larger than that of corresponding SF6 gas mixtures when c-C₄F₈ gas content increase. Their required pressure ratio compared with insulation strength of SF6 gas are almost the same, but GWP of the former is far less than that of the latter.For non-uniform field, it is not easy to obtain dielectric strength only by simulation because process of gas breakdown is very complex. So we measured experimentally AC and lighting impulse breakdown voltage in different pressure, distance and mixing ratio to analysis insulation characteristics of c-C₄F₈ gas mixtures with N2,CO2 and CF4 in non-uniform field, which are then compared with that of SF6/N2 gas mixture and SF6 gas respectively. We also measured AC breakdown voltages of 1% c-C₄F₈ gas mixture and 10% c-C₄F₈/ N2/CO2 ternary gas mixture in order to obtain the best dielectric characteristics by as possible as low c-C₄F₈ gas content. Best mixing ration and the relationship of insulation characteristics with pressure are found, which will provide theoretical and experimental foundation for application of c-C₄F₈ gas mixture in high voltage apparatus.For AC breakdown, the insulation strength of c-C₄F₈/N2 is almost the same as that of SF6/N2, while that of c-C₄F₈ gas mixture with CO2, CF4 are larger than that of SF6/N2. There exists the best mixture ratio for c-C₄F₈/ N2 and c-C₄F₈/CO2 gas mixtures. The insulation strength of 10%c-C₄F₈/N2 gas mixtures will be increased greatly after mixed with 1% CO2 gas, which improve greatly the insulation characteristics of 10%c-C₄F₈/N2 gas mixtures.At low pressure, lighting impulse breakdown voltages of c-C₄F₈ gas mixtures with N2,CO2 and CF4 are all lower than that of SF6 gas. However, as pressure increase, insulation strength of all c-C₄F₈ gas mixtures are larger than that of SF6 gas except positive poliarity for c-C₄F₈/N2 gas mixtures, especially for negative poliarity of c-C₄F₈/N2 and positive poliarity of c-C₄F₈/ CO2 gas mixtures...