Influence Of Free Metal Particles On The Power Frequency Breakdown Performance Of SF₆ And SF₆/CO₂

Sulfur hexafluoride（SF₆） has been widely used as a medium of gas insulation due to its excellent insulation performance and ability of arc extinguish. However, the existence of metal particles will cause an adverse impact on SF₆ insulation. Nowadays, most of domestic and oversea studies have paid their main attention to fixed metal particles. But there are rare theoretical analysis and experimental researches on free metal particles which are dominant inside equipment. Meanwhile, the researches about particle variety are far from abundant.According to the characteristic of bounce for free metal particles in the process of increasing voltage, in the paper, a ball-bowl electrode model is designed to avoid particles bouncing out of electrodes range. What’s more, the structure of uniform electric field between ball-bowl electrodes can be used to simulate the real electric field existing in gas insulation equipment. Through gradual voltage-rising method under power frequency, the paper investigates the impact of metal particle number, electrode distance, particle variety and pressure on SF₆ breakdown performance under power frequency. The results show that, breakdown voltage of SF₆ will decline obviously with the increase of particles number until the number reaches a certain level and then the voltage value is stable. Besides, when the electrode distance is shorter, the impact of free metal particles on the gap is greater. Obviously, if the length of a particle is closer to electrode distance, the size of the particle will have a greater effect on the distribution of the electric field between electrodes. The impacts of copper and aluminum particles on breakdown performance are approximate and both evidently greater than iron particles. This can be achieved by theoretical analysis and simulation calculation. The starting jumping voltage of copper and aluminum particles is lower than iron particles. Copper and aluminum atoms will cause the decomposition reaction barrier of SF₆ to decline obviously. Apart from that, with the growth of pressure, the nonlinear influence of metal particles on SF₆ breakdown performance is more and more obvious. To control the global warming, cutting down the usage amount of SF₆ is urgent because of its strong greenhouse effect. Recently, adopting SF₆ gas mixtures has been a most effective method to cut down the usage amount of SF₆ as well as maintain the insulation strength.The paper also investigates the impact of pressure and mixture ratio on power frequency breakdown performance of SF₆/CO₂ under a free metal particle. The results show that, when only one copper particle exists between ball-bowl electrodes, breakdown voltage of SF₆/CO₂ gas mixtures will rise nonlinearly with the growth of pressure（or mixture ratio） under the same mixture ratio（or pressure）. It is found that, the synergistic effect C is a value to decide synergistic effect types among various gas mixtures. According to the synergistic effect C, SF₆ has a synergistic effect relationship with CO₂. SF₆/CO₂ gas mixtures whose mixture ratio is 0, 10%, 30%, 50%, 70%, 100% are also studied. When mixture ratio is between 30% and 50%, C may be a minimum under the pressure of 0.10、0.15、0.20、0.25、0.30 MPa, which represents the best synergistic effect. When mixture ratio is 50%, relative breakdown intensity P under power frequency and different pressure of SF₆/CO₂ gas mixtures all can reach 80% or more of pure SF₆ under the same pressure. Further, after taking the liquefaction temperature into consideration, SF₆/CO₂ in mixture ratio 50% is regarded to has a prospect in engineering application to substitute pure SF₆.The study of SF₆ and SF₆/CO₂ breakdown performance under free metal particles and power frequency has shown that free metal particles will also strongly impair the insulation performance of SF₆ and SF₆/CO₂. In engineering application, copper and aluminum particles should be particularly avoided. At the same time, SF₆/CO₂ in mixture ratio 50% is potential to substitute pure SF₆ in specific conditions.