A circuit-oriented average-value model of modular multilevel converter based on sub-module modelling method

The modular multilevel converters (MMCs) are attractive for high-voltage direct current (HVDC) transmission system. However, due to the complicated structure of MMC, which normally consist of thousands of sub-modules, how to accurately and efficiently model the MMCs is one of the key technical challenges. In this paper, based on sub-module modelling method, a circuit-oriented average-value model of MMC involving switching-cycle averaging is proposed. Different from traditional MMC model, which is based on the modelling of converter bridge and the dynamic performance of the sub-modules in which is ignored, for the proposed model in this paper the chopper-cells are replaced by controlled voltage and current sources. As a result, the proposed model can not only accelerate simulation speed but also describe the characteristics of sub-modules, such as the dynamic voltage-balancing process of sub-module capacitors. Then static synchronous compensator based on the modular multilevel converters (MMC-STATCOM) is simulated in PSIM simulations. Both the circuit-oriented average-value model and switching model are established to simulate the steady state of the system, as well as the procedures of one sub-module failure and transient of AC current command. The validity, rapidity, accuracy, intuitiveness and compatible reconfiguration of the presented model are well verified by simulations.