A simplified dual-stage model predictive controller for modular multilevel converters

The Modular Multi-Level Converter (MMC) topology is suitable for use in medium and high voltage applications. To cater to the multi-objective control requirements of the MMC, several Model-Predictive Control techniques have been developed. However, most of the available MPCs do not sufficiently address the simplification of the apparently complex switching logic of the MMC in regard to its practical implementation. To fill this gap, a modulated dual stage MPC is proposed here that controls the ac output current using a few simple steps, while maintaining sufficient control over the circulating current. The two MPC controller stages are used in a cascaded manner. The first stage calculates the voltage reference for grid current control and the second stage MPC selects the appropriate switching state to ensure circulating current control. The proposed technique uses carrier-based PD-PWM to retain its superior harmonic performance and ensures 2N + 1 output pole voltage levels. While avoiding use of any cost function, a generalized fixed step approach is also presented that is independent of N value. The dual stage MPC presented in this paper is executed once in every switching time period to maintain a fixed switching frequency operation. Simulation and experimental studies show satisfactory performance of 25level and 7-level MMC respectively, in a grid connected mode. Transient case study is also shown to demonstrate fast dynamic performance of the proposed controller.