【作者】 吴变桃；

【导师】 肖登明；

【作者基本信息】 上海交通大学 ， 电气工程， 2007， 博士

【摘要】 由于SF₆气体高的温室效应,迫切需要找到一种新的低温室效应SF₆绝缘气体替代物,用于现有的或经过改造的电力设备。1997年美国国家标准和技术协会会议上已把c-C₄F₈混合气体列为最有潜力的SF6绝缘替代物,然而除了日本东京大学2001年研究了其在短电极间距下的交流绝缘性能外,国内外再无相关研究。本文将比较全面的研究c-C₄F₈及其和N₂,CO₂和CF₄混合气体在均匀和不均匀电场下的交流和雷电冲击绝缘强度。由于均匀电场下,绝缘强度可近似用仿真气体放电得到的临界击穿场强表示。本文将建立蒙特卡罗仿真模型,并针对其仿真速度慢的缺点,改进传统的空碰撞法,在改进方法中,对每种碰撞过程对应的频率都预先设定一个上限,先根据这些上限确定有可能发生的碰撞过程,再根据这种碰撞对应的截面积来计算发生真实碰撞的概率,这样的话不管气体存在几种碰撞过程,在电子每个自由飞行时间段,仅仅需要计算一个碰撞截面就可以继续仿真。尤其对于存在多种碰撞过程的混合气体来说,减少了每次处理所有碰撞截面的复杂过程,极大地提高了计算效率。并用研究比较成熟的SF₆和CO₂混合气体验证其正确性。通过这种改进的蒙特卡罗模型,引用仿真和纯净气体放电参数一致的各向同性截面积,仿真电子在混合气体中的运动,求出c-C₄F₈及其和N2,CO2和CF4混合气体的有效电离系数和临界击穿场强,从气体微观方面研究其均匀电场下的绝缘性能,同时计算其要达到SF6绝缘强度需要增加的压强比率和对应的温室效应GWP,并与相应的SF6混合气体进行了比较。结果表明,c-C₄F₈及其和N₂,CO₂和CF4混合气体均匀电场下的绝缘强度随c-C₄F₈含量几乎线性增长,尽管在低c-C₄F₈含量时稍低,当c-C₄F₈含量增加时,开始越来越大于相应的SF6混合气体。要达到纯净SF6气体的绝缘强度,c-C₄F₈及其和N2,CO2和CF4混合气体需要增加的压强率和相应的SF6混合气体差不多,但温室效应却大大降低,甚至为SF6气体的十分之一,大大降低了绝缘气体对环境的影响。对于不均匀电场,由于击穿过程复杂,用仿真法不容易求出气体的绝缘强度。本文用实验方法,在不同压强、不同电极间距及不同混合比下,测量c-C₄F₈及其和N2,CO2和CF4混合气体在不均匀电场下的工频交流和雷电冲击电压,进而分别与SF6/N2混合气体和纯净SF6气体的绝缘强度进行比较。为了用尽量少的c-C₄F₈气体,获得高绝缘性能,在交流击穿实验中,考虑在N2,CO2中加入1%c-C₄F₈,测试微量c-C₄F₈对N2,CO2绝缘强度的影响。同时也对三元10%c-C₄F₈/N2/CO2混合气体的交流绝缘强度进行研究,探索CO2气体对10%c-C₄F₈/N2混合气体绝缘强度的影响。寻找c-C₄F₈混合气体耐电强度更高的最佳混合比以及耐电强度与压力的关系等,为未来c-C₄F₈混合气体可能用作绝缘介质提供理论和实验基础。实验结果表明,对于工频交流绝缘强度来说,c-C₄F₈/N2和SF6/N2混合气体差不多;c-C₄F₈和CO2,CF4混合气体高压时大于SF6/N2很多;c-C₄F₈/N2和c-C₄F₈/CO2两种混合气体都存在最优混合比;在10%c-C₄F₈/N2混合气体中加入1%CO2,只要提高压强,就可使其绝缘强度最大值大大增加,极大的改善了10%c-C₄F₈/N2混合气体的绝缘特性。三种c-C₄F₈混合气体的负、正雷电冲击绝缘强度在低压时都低于SF6很多,但随着压强的增加,除c-C₄F₈/N2混合气体的正极性雷电冲击低外,都大于SF6气体,尤其是c-C₄F₈/N2混合气体负极性和c-C₄F₈/ CO2混合气体的正极性雷电冲击电压。更多还原

【Abstract】 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更多还原