中文摘要：

中点不平衡问题是三相四线制三电平三桥臂变换器控制的难点问题,详细分析该拓扑变换器直流中点偏移的内在影响因素,提出中点平衡因子的概念,并导出中点平衡因子的计算公式。基于中点平衡因子,在a-b-c正交坐标系下提出一种新型三维调制策略,其可将多种复杂情况统一成3种基本类型,大大简化了因解决中点平衡问题带来的三维调制策略复杂性大幅增加的难题,为调制策略的数字化实现提供了便利。仿真分析和实验结果证实了新型三维调制策略的正确性及其对中点控制的有效性。

英文摘要：

The neutral point potential unbalance problem is a key issue for 3-phase 4-wire tri-level 3-leg converter. A detailed discussion on this issue was carried out. A new concept named Factor （F） of neutral-point balance was proposed and the calculation formula was derived. Based on F, a novel 3-D modulation scheme was proposed in a-b-c coordinates. By classifying all the different complex conditions as three basic types, the complex process of vectors selection and calculation for neutral point control becomes much easier for digital realization. The simulation and experiment result verifies the correctness and validity of this proposed 3DSVM scheme.