中文摘要：

分析故障后不同电压矢量调制算法，并指出各种矢量调制算法之间的本质区别。利用几何相似原理，计算各个扇区零矢量作用时间。推导中性点电压数学表达式，电容中性点电压波动是呈电网频率波动；中性点电压偏置是由畸变电流或电容初始电压不均衡造成的。进一步分析中性点电压不平衡对系统线性调制的影响。分析发现，传统调制度M无法评估故障后变换器的调制性能，变换器并网电流存在低次谐波及三相不平衡电流。为此，定义新的线性调制函数，该函数可准确描述线性调制与电压偏置和负载电流之间的数学关系。利用该函数可进一步计算线性调制所需直流电压。实验结果证实，所提线性调制函数能确保容错运行的变换器完全运行于线性调制区域，大大降低并网电流中的低次谐波电流及不平衡电流，避免发生二次故障。

英文摘要：

The essential difference among the various space vector modulation （SVM） approaches is investigated. Using the geometric similarity principle, the time allocated for the zero voltage vectors is obtained. The dynamic equation of the capacitor voltage is derived, and the equation indicates that the capacitor voltage imbalance composed of capacitor voltage oscillation and deviation. The analytical results in this paper demonstrate that the conventional modulation index cannot guarantee the linear modulation of the fault-tolerant converter. A rich content of the low frequency current is included in the three-phase current of the converters using the DC-link voltage defined by the conventional index M, deteriorating the grid stability and causing a secondary fault. The effect of the capacitor voltage deviation and capacitor voltage oscillation on the linear modulation is investigated in detail, and a linear modulation index function is developed. Based on the linear modulation index function, an equation of the minimal DC-link voltage for the linear modulation of post-fault converter is proposed. Experimental results demonstrate that effectiveness of the proposed linear modulation index function.