中文摘要：

中点平衡问题是三相四线制三电平三桥臂有源电力滤波器（active power filter,APF）的一个固有问题，目前在矢量调制中尚无完善的控制理论完全解决此问题。该文通过对各种电流电压状态下每个三维空间矢量对直流中点电位的影响进行分析，提出了“中点控制度”（neutral-point control grades，NPCG）的概念，并给出了NPCG的计算公式；基于NPCG，在α-β-0坐标系下提出一种基于坐标平移原理的新型“三维矢量优化选择调制策略”（optimized 3D space vector modulation，O3DSVM），该调制策略能优化选择NPCG值较高的矢量以合成目标矢量，从而达到最大程度控制直流中点电位偏移的目的。仿真分析验证了中点控制度理论及O3DSVM调制策略的正确性和可行性，采用该策略的APF不但中点电位得到了有效控制，且补偿效果良好。

英文摘要：

For tri-level 3-leg converter based 3-phase 4-wire active power filter （APF）, the direct current （DC） bus neutral-point potential unbalance problem is an inherent problem. There has not been any perfect theory to solve this problem completely in 3-D space vector modulation scheme. Based on the analysis of the affected extent of the neutral-point potential by every space vector on all kinds of voltage and current conditions the concept named neutral point control grade （NPCG） was proposed for the first time in this paper. The calculation formulas of NPCG were presented as well. Based on NPCG, a novel modulation scheme called optimized 3D space vector modulation （O3DSVM） scheme was proposed in α-β-0 coordinates based on the translation principle of axes, by which the space vectors with higher NPCG values are selected to synthesize the object vector to suppress the unbalance of the neutral-point potential. The simulation analysis verifies the correctness and feasibility of the proposed NPCG theory and the O3DSVM scheme. The neutral point potential was effectively controlled and the harmonic current was perfectly compensated by the APF with this proposed scheme.