Modular Multilevel Converter Sub-Module Fault Characteristics Analysis and Decoupling Control Strategy
Fan Zhihua1,2, Miao Shihong1,2, Liu Zhiwen1,2, Chao Kaiyun1,2, Kang Yilong1,2
1. State Key Laboratory of Advanced Electromagnetic Engineering and Technology School of Electrical and Electronic Engineering Huazhong University of Science and Technology Wuhan 430074 China; 2. Hubei Electric Power Security and High Efficiency Key Laboratory Huazhong University of Science and Technology Wuhan 430074 China
Abstract:Focusing on the situation that modular multilevel converter (MMC) with asymmetrical sub-module (SM) numbers of arms caused by sub-module fault, a mathematic model of arms under asymmetric operation based on the switch function is established. It is revealed that the inconsistent switching functions of upper and lower arm submodules are the cause of the asymmetric operation. By means of compensating impedance to counteract the asymmetry component caused by the unequal number of arm sub-modules, the unsymmetrical operating mode under the sub-module fault is transformed into the quasi symmetrical operating mode without sub-module fault, and then the coupling equivalent circuit among AC current, DC current and AC circulation is established under asymmetric conditions. The influence of coupling relationship among currents of the equivalent circuit on the internal operating characteristics of arms is studied. The analysis shows that the fundamental frequency current will appear in arm current, and the AC current will also introduce the DC current component and the double frequency current component. Finally, the mapping relationship between the current and voltage of arms at different frequencies is constructed, and the arms current control strategy with SM fault-tolerant ability is further proposed. Simulation verifies the effectiveness of proposed control strategy.
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