电网不对称故障下MMC自适应相功率均衡控制策略

交流电网不对称故障工况下模块化多电平换流器(modular multilevel converter, MMC)存在相功率不均衡的问题，常规的基于直流环流注入的调控方法会引起桥臂电流不对称，导致各相电流应力不相等；而基于零序电压注入的方法可能导致系统过调制，危害系统安全稳定运行。针对传统相功率均衡控制策略的局限性，提出一种交流电网不对称故障下MMC自适应相功率均衡控制策略。首先，分析基于零序电压和直流环流注入的MMC相间功率调控原理，指出不同方法单独进行相间功率均衡的局限性。其次，研究零序电压注入方法的过调制边界，引入相功率分配系数，给出不同故障下相功率系数优化方法，提出基于零序电压和直流环流注入协调的MMC自适应相功率均衡控制策略。最后，通过仿真验证了机理分析与所提控制策略的正确性和有效性。

Adaptive leg-balancing control strategy of an MMC in an asymmetric AC grid fault

A modular multilevel converter (MMC) has the problem of unbalanced power in asymmetric fault conditions. The conventional control method based on DC circulating current injection will cause asymmetry of arm current, resulting in unequal current of each phase. The method based on zero-sequence voltage injection may lead to over modulation and endanger the safe and stable operation of the system. Given the limitations of traditional leg-balancing control strategies, an adaptive leg-balancing control strategy of an MMC with asymmetric faults is proposed. First, the principle of leg-balancing based on zero-sequence voltage and DC circulating current injection is analyzed, and the limitations of different methods are pointed out. Second, the overmodulation boundary of the zero-sequence voltage injection method is studied, and the phase power distribution coefficient is introduced. Then the optimization method for a leg-power distribution coefficient with different faults is given and an adaptive leg-balancing control strategy of the MMC based on the coordination of zero-sequence voltage and DC circulating current is proposed. Finally, the correctness and effectiveness of the mechanical analysis and the proposed control strategy are verified by simulation.