一种适用于MMC的闭环均压控制策略

A Voltage Balancing Strategy With Closed-loop Control for MMC

子模块电压不平衡度和绝缘栅双极型晶体管(IGBT)开关频率是模块化多电平换流器(MMC)均压算法的两个重要指标,因两者存在矛盾性一般需要根据系统实际运行情况权衡两者。这里提出了一种在闭环控制子模块均压效果的基础上自动优化开关频率的均压策略。通过分析MMC运行的基本原理,提出对同一桥臂上的子模块根据投入/切除状态进行分组,组内按照电压大小进行排序,然后将预设均压效果阈值与实际均压效果的差作为比例积分(PI)控制器的输入,PI控制器的输出为均压效果阈值,最后基于桥臂电流方向选择由子模块投入差值、分组排序结果和均压效果阈值构成的条件关系式,计算得到最小交换值。最小交换值是为保证预设均压效果需要进行投切交换的最少子模块数,以此刷新的触发脉冲可有效降低开关频率。最后通过构建半实物仿真平台验证了所提策略的有效性。

The unbalanced degree of sub-module voltage and switching frequency of insulated gate bipolar transistor(IGBT)are two main indicators of voltage balancing algorithm in modular multilevel converter(MMC),and because of the contradiction between these two indicators,they needs to be balanced according to the actual operation of the system.A voltage balancing strategy is presented to automatically optimize the switching frequency on the basis of closed-loop control for voltage balancing effect.By analyzing the basic operating principle of MMC,it is proposed that sub-module on the same bridge arm are grouped according to the switch on/off state,and are sorted based on voltage within their grouping,then the difference between preset threshold of voltage balancing effect and actual voltage balancing effect is taken as the input of proportional integral(PI)controller,and the output of PI controller is threshold of voltage balancing effect,finally the minimum exchange value is calculated,from conditional relationships of choice based on the current direction of the bridge arm which is composed of the difference between the number of sub-module shall switch on and already switched on,the sorted result within the group,and the threshold of voltage balancing effect.The minimum exchange value is the minimum number of sub-module that need to be switched between on/off state to ensure the preset voltage balancing effect,by which the firing pulses generated can effectively reduce the switching frequency.Finally,the effectiveness of the proposed strategy is verified through constructing a simulation platform.