Long-horizon predictive current control of modular-multilevel converter HVDC systems

High voltage direct current (HVDC) transmission systems play an increasingly important role in offshore wind energy and long distance energy transmission systems. Modular multilevel converter (MMC) is an attractive topology for HVDC systems, due to its good modularity, scalability and inherent fault tolerant capabilities. For such topology, model predictive control (MPC) is a promising alternative. In particular, the long-horizon MPC provides better performances in terms of smaller THDs at very low switching frequency. However, its computational load is seen as a big challenge. In this paper we apply a long-horizon model predictive current control (MPCC) to a seven-level MMC-HVDC system. A switch and extrapolation and capacitor voltage sorting techniques are developed and combined to reduce the total computational burden. Performances of the proposed control strategy are evaluated with simulation results at a 65 MVA back-to-back 7L-MMC-HVDC configuration.