Model Predictive Control of MMC-based Medium Voltage Microgrid for Grid Connected and Islanded Operation

Abstract

This study presents a model predictive control (MPC) of modular multilevel converter (MMC)-based microgrid for grid connected mode (GCM) and islanded mode (ISM) operations with seamless transition capabilities. Submodule (SM) capacitor voltage control of the MMC is the major challenge in improving the efficiency and reliability of the MMC system. Therefore, a model predictive current control (MPCC) scheme is developed to balance the SMs' capacitor voltage and supply the power generated by the renewable energy sources to load and the utility grid. In the ISM, model predictive voltage control (MPVC) is developed to achieve constant voltage across the load terminals and control the SMs' capacitor voltage. The developed MPC techniques estimate the future behavior of control variables and uses a cost function to produce optimal switching sequence for the MMC. Thus, in comparison to the classical d-q frame control, the developed MPC techniques avoid the use of pulse width modulation schemes, proportional integral controllers and rigorous tuning efforts. Moreover, for seamless transition between ISM to GCM and vice-versa, a phase-locked loop (PLL)-based seamless transition sequence is devised. The performances of the MPCC and MPVC are validated against the conventional d-q control via simulation results under different operating conditions.