Improved equivalent circuit model of MMC and influence analysis of simulation time step

Modelling and simulation of a high voltage-level modular multilevel converter (MMC) is challenging due to large amount of semiconductor switches, various submodule (SM) circuits, and different operating conditions. Several equivalent models have been proposed for modelling and simulation in system level, which did not consider an industrial or digital controller. To accelerate MMC station design, a highly efficient model is needed with considering control parameters/strategies and accurate dynamics of voltage/current. In this study, an improved equivalent circuit model (ECM) is developed for large-scale system simulation and MMC station design. The proposed ECM considered the MMCs based on various SM circuits under different operating conditions. It can generate bipolar arm voltage and simulate static compensator under dc fault condition. Based on the proposed ECM, the influences of simulation time step on accuracy/efficiency are investigated by considering the sample period and voltage balancing strategies. This research is conducted based on an MMC-HVDC system in the PSCAD/EMTDC software. The study results demonstrate the effectiveness of the proposed ECM and show the relationship between simulation accuracy/efficiency and control/simulation parameters for determining simulation time step. Based on study results, the computational efficiency of proposed model is not significantly affected by the complexity of SM circuit.