Efficient and accurate hybrid model of modular multilevel converters for large MTDC systems

Modular multilevel converters (MMCs) play a critical role in the future energy Internet pattern. Therefore, proper models for the simulation of complex systems with multiple MMCs are required. This study proposes a simplified equivalent model (SEM) based on the implicit trapezoidal method and Thevenin equivalent. The model can accurately simulate arm voltages of the MMC and is thus capable of simulating system-level transients after faults and disturbances, circulating currents and subsynchronous oscillation components. Subsequently, a hybrid model combining SEM and a detailed equivalent model (DEM) is built to improve the overall simulation speed in scenarios where submodule-level transients are mixed with system-level ones. Finally, this study notes that the major difference between DEM and SEM results only from the voltage-balancing cycle. An experiential formula is given to regulate SEM when the voltage-balancing cycle is large, where the difference may become obvious/insufferable.