考虑桥臂参数不对称和子模块故障的MMC协同控制策略

MMC Cooperative Control Strategy Considering Asymmetry of ArmParameters and Sub-Module Faults

近年来,模块化多电平换流器(modular multilevel converter,MMC)受到广泛关注并得到迅速发展,在实际工程中,MMC上、下桥臂参数不可避免地会存在不一致;另外,当MMC桥臂子模块因故障旁路时,会导致6个桥臂子模块数目的不对称,以上两种桥臂不对称现象,均会打破MMC内部原有的能量均衡,同时导致换流器阀侧电流出现直流和2倍频谐波,以及导致直流电流中出现基频和二倍频谐波等问题。为解决以上问题,该文考虑桥臂参数不对称和子模块数目不对称同时存在的复杂工况,提出了MMC协同控制策略,此策略在充分保留子模块电压保护裕度的前提下,即可抑制桥臂参数不对称导致的直流侧/交流侧谐波,又可抑制桥臂子模块数目不对称导致的谐波,有效的改善了MMC交直流输出外特性的性能。最后,搭建了双端MMC仿真模型,仿真结果验证了理论分析及所提策略的有效性。

The modular multilevel converter(MMC)has received wide attention and developed rapidly in recent years.In practical projects,the parameters of the upper and lower arms of the MMC are inevitably inconsistent.In addition,when the sub-modules of the MMC are bypassed due to faults,the sub-modules number of each arm could be different.The original energy balance in the MMC would be broken either of these two asymmetric conditions exists.As a result of the energy imbalance,the fundamental circulating current could flow into the DC side and the dc current flow into the AC side.To solve these problems,the appropriate MMC cooperative control strategy is proposed considering the asymmetric of arm parameters and asymmetric of sub-modules numbers.On the premise of fully reserving the voltage protection margin of sub-modules,this strategy can not only suppress the DC/AC harmonics caused by the asymmetry of arm parameters,but also suppress the harmonics caused by the asymmetry of arm sub-modules numbers,which effectively improves the performance of MMC’s AC/DC output characteristics.Finally,a 2-terminal MMC-HVDC system is established and the simulation results have validated the efficiency of the proposed strategy.