Energy-Balancing of a Modular Multilevel Converter with Pulsed DC Load Using an Online Trajectory Planning Algorithm

Modular Multilevel Converter (MMC) energy balancing is a nontrivial open-loop control problem since the choice of a technically meaningful output leads to an internal dynamics. A large voltage ripple on the dc side of an MMC may cause permanent arm power imbalances. For example, the large voltage ripple at the smoothing capacitor of a pulsed dc load can affect the supplying MMC. This paper focuses on the energy balancing task of MMCs supplying the smoothing capacitor of a pulsed dc load. The approach introduced in this paper is to define feasible stationary operating regimes with circulating current injection and plan appropriate trajectories solved analytically for all variables. This lends itself to an online implementation and provides a feedforward energy control supporting the standard feedback based energy balancing. Test bench measurements confirm improved energy control performance compared to standard feedback balancing.