Avoiding module capacitance over-dimensioning of Modular Multilevel Converters with constrained and unconstrained modulator-based Model Predictive Control

Abstract

The power density of Modular Multilevel Converters (MMCs) is often limited by the over-dimensioning of the module capacitance value. This is common because with conventional control schemes, large voltage and energy margins must be included in the MMC design to avoid overvoltages in the module capacitors and saturation effects due to undervoltages in the module capacitors. This paper presents two Model Predictive Control methods that can reduce the necessary voltage and energy margins to a minimum and therefore increase the power density of the MMC system compared to conventionally controlled MMCs. A trade-off between computational effort and module capacitance over-dimensioning is discovered for the proposed constrained MPC. The unconstrained MPC is compared to the constrained MPC and conventional PI control using simulations. Finally, the performance of the unconstrained MPC is experimentally verified with an MMC system prototype and compared to a traditional PI control system.