Improved Precision MMC Average Equivalent Model

Abstract

Modular multi-level converters (MMC) are increasingly used in DC transmission systems, and their fast equivalent models are the basis for related research on DC transmission systems. Aiming at the shortcomings of the simulation accuracy of the traditional average equivalent model, this paper proposes a high-precision average equivalent model. The AC side uses three pairs of controlled voltage sources to equivalent the output voltage of the bridge arm. The DC side uses a controlled current source and a pair of equivalent capacitors to equivalent the capacitance of the upper and lower bridge arms, while ensuring stable power exchange between the systems. The AC side establishes a mathematical model of the controlled voltage source by ignoring the difference in the capacitor voltage of the submodule and taking into account the on-state voltage drop of the semiconductor switching device. On the DC side, the mathematical model of the equivalent capacitance of the bridge arm and the controlled current source is established by the law of conservation of energy and the introduction of the calculation method of equivalent capacitance considering the high-frequency switching of the submodule. Finally, by building a detailed model and equivalent model in PSCAD, the superiority of the equivalent model in accuracy is verified.