模块化多电平换流器的子模块电容电压分层均压控制法

为了提高高电平模块化多电平换流器(MMC)均压控制的排序速率和降低换流器的开关损耗,提出了一种适用于高电平MMC的子模块电容电压分层均压控制法。首先,根据桥臂电容电压最值确定电压分层容器,并根据排序的计算复杂度和均压控制效果对分层数进行讨论;然后,根据电容电压大小将子模块放入对应的分层容器,根据桥臂需投子模块个数和桥臂电流方向进行优化排序,确定各子模块的投切;最后,引入容器重新划分判据以减少计算量,即如果任一一个投入子模块的电容电压变化值小于容器的电压间隔,则不重新分层。PSCAD/EMTDC中21电平MMC系统的仿真结果表明,提出的均压控制法能够提高均压控制排序的速率和减少不必要的容器内排序,同时降低绝缘栅双极型晶体管(IGBT)的开关损耗。

A Stratified Voltage Balancing Control Method of Sub-module Capacitor Voltage for Modular Multilevel Converter

To improve the sorting process and reduce the switch frequency of sub-modules in the capacitor voltage balancing control of modular multilevel converter (MMC), a stratified voltage balancing control method of sub-module capacitor voltage for high voltage level MMC system is proposed. Firstly, the capacitor voltage container is determined according to the maxi and min capacitor voltages. The number of containers is discussed according to the time complexity of the algorithm and the effect of balancing control. Secondly, all the sub-modules are arranged into the corresponding containers according to their capacitor voltages. In accordance with an arm current and the number of sub-modules needed to be inserted in the arm, the correct sub-modules are chosen for insertion via the stratified capacitor voltage balancing control and a proposed optimal process. Finally, a criterion for resetting the containers is presented to reduce the amount of calculation. If the voltage variation of any inserted sub-module is less than the voltage interval of container, then the voltage margins of containers remain unchanged. Otherwise, they should be reset. Simulation results of a 21-level MMC in PSCAD/EMTDC show that the proposed controller is able to guarantee excellent voltage balancing results and high efficiency of sorting rate. Furthermore, the proposed method will significantly decrease the switching losses in insulated gate bipolar transistor (IGBT) in comparison with the bubble sort method. This work is supported by National Natural Science Foundation of China (No. 51277068) and Program for New Century Excellent Talents in University (No. NCET-12-0846).