The commutation failure immunity index (CFII) recommended by CIGRE is an useful indicator for quantifying the immunity of LCC-HVDC inverters to the CF. It should be calculated by the electromagnetic transient (EMT) simulations under the worst fault condition that doesn't cause the CF as per its definition. Moreover, the symmetrical three-phase line-to-ground fault has been empirically claimed by CIGRE as the worst one in the previous calculation method. However in this letter, it is firstly found that the unsymmetrical double-phase fault rather than the three-phase one is actually the worst as clearly observed from large numbers of the EMT simulations. This is achieved by the comprehensive analysis of the simulated CF characteristics when taking into account the diversified fault conditions such as fault type, property, severity, initiation time. Secondly, the above interesting finding motivates a corrected CFII calculation method to be proposed considering the double-phase fault. The proposed method is further shown to be more superior than the previous method by the case studies under various ac grid strength.

中文翻译：

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LCC-HVDC逆变器换相失败抗扰度指标计算方法的修正


CIGRE推荐的换相故障抗扰度指数（CFII）是量化LCC-HVDC逆变器对CF的抗扰度的有用指标。它应该通过在不会导致其定义的 CF 的最坏故障条件下的电磁瞬态 (EMT) 模拟来计算。而且，在之前的计算方法中，CIGRE根据经验认为对称三相线路接地故障是最严重的故障。然而，在这封信中，我们首先发现，从大量的EMT模拟中可以清楚地观察到，不对称双相故障而不是三相故障实际上是​​最严重的。这是通过综合分析模拟的 CF 特性，同时考虑故障类型、性质、严重程度、启动时间等多种故障条件来实现的。其次，上述有趣的发现促使提出考虑双相故障的修正CFII计算方法。通过在各种交流电网强度下的案例研究，进一步表明所提出的方法比以前的方法更优越。