Predictive Control of a Modular Multilevel Converter for a Back-to-Back HVDC System

The modular multilevel converter (MMC) is one of the most potential converter topologies for high-power/voltage systems, specifically for high-voltage direct current (HVDC). One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents of converter arms while the capacitor voltages are maintained balanced. This paper proposes a model predictive control (MPC) strategy that takes the advantage of a cost function minimization technique to eliminate the circulating currents and carry out the voltage balancing task of an MMC-based back-to-back HVDC system. A discrete-time mathematical model of the system is derived and a predictive model corresponding to the discrete-time model is developed. The predictive model is used to select the best switching states of each MMC unit based on evaluation and minimization a defined cost function associated with the control objectives of MMC units and the overall HVDC system. The proposed predictive control strategy: 1) enables control of real and reactive power of the HVDC system; 2) achieves capacitor voltage balancing of the MMC units; and 3) mitigates the circulating currents of the MMC units. Performance of the proposed MPC-based strategy for a five-level back-to-back MMC-HVDC is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate a satisfactory response of the MMC-HVDC station operating based on the proposed MPC strategy, under various conditions.