基于改进阶梯波调制的模块化多电平换流器环流抑制策略

A Modified Ladder Wave Modulation-Based Circulating Current Suppressing Strategy for Modular Multilevel Converter

将模块化多电平换流器(modular multilevel converter,MMC)的桥臂电流分为不可控电流和可控电流:不可控电流由负载决定,不能通过所采取的控制策略进行控制;可控电流为相间环流,是由三相桥臂投入电压总和不平衡造成的,可以通过特定的优化算法加以控制。在分析相间环流本质后,得到了抑制环流大小需要改变投入模块数的结论。提出了适用于逆变站的环流抑制控制策略,该策略结合改进后的阶梯波调制方法可在不增加外围硬件电路的情况下自适应负载变化,大大降低MMC的相间环流以及MMC交流侧输出相电压的畸变率。算例结果验证了所提算法的有效性。

Dividing the current flowing through bridge arms of modular multilevel converter （MMC） into two parts, namely the controllable and the uncontrollable current, the former depends on the load and cannot be controlled by the adopted control strategy and the latter is the interphase circulating current caused by the total unbalance quantity of voltages applied to three phase bridge arms and can be controlled by specific optimization control algorithm. Based on the analysis on the essence of the interphase circulating current, a conclusion that to suppress the circulating current the numbers of inserted modules are to be changed is obtained and a control strategy of suppressing circulating current, which is suitable for inverter station, is proposed. Combining with improved ladder wave modulation method, the proposed strategy is self-adaptive to load variation without adding peripheral hardware circuits, and reduces the interphase circulating current of MMC evidently as well as reduces the total harmonic distortion of three-phase voltage output at AC side of MMC. Based on Matlab, a simulation model containing 100 sub-modules in each phase is constructed and simulation results show that the proposed algorithm is effective.