Control Strategy and Simulation of a Modular Multilevel Converter (MMC) Based Pump-Back System for Variable Speed Drive Application

A standard pump-back system consisting of two Modular Multilevel Converters (MMCs) has been proposed. Compared with traditional two-level inverter based pump-back system, the MMC based topology is able to produce a virtual grid with multilevel voltage output. In addition, energy control loops are unique for MMC to ensure the energy balance between its DC and AC side. The associating challenge is to retrieve the moving average of the capacitor energy under variable frequency condition. The basic operation principle and control strategies including all the energy and current loops have been discussed in detail. Moreover, an ABC to Alpha-Beta transformation based method has been implemented in order to solve the moving average problem. A PLECS simulation with equation based switching mode has been conducted to test the performance of the control algorithms and to improve the simulation speed. The simulation results and analysis have been provided and future work has been mentioned.