The voltage equalization control method of MMC sub-modules based on packet sorting

摘要

When the converter works, the process of charging and discharging frequently can not be accurately controlled, this will cause the voltage imbalance between the capacitor of each sub-module, which leads to abnormal operation of the converter. only to maintain the balance of the sub-module capacitor voltage stability, the converter will operate normally. In the traditional MMC sub-modules voltage equalization method, the switching state of sub-modules is determined by the direction of the bridge arm current and the capacitance voltage size, this will lead to the original sub-modules have to switch again, the frequent changes of the switching status will make a higher switching frequency of IGBT, consequently, the switching losses of the converter are increased. A method of capacitance voltage equalization for MMC sub-modules with optimized switching frequency is proposed, first, the sub-modules of the bridge arms are grouped, and then sorted according to the size of each group voltage, then set the maximum permissible voltage difference between groups, finally, the retention factors are introduced in each group. The optimization method can reduce the switching frequency more effectively while the sub-modules are equalized, thus reducing the switching loss.