Model predictive current control of modular multilevel converters

This paper proposes a model predictive controller for high-power modular multilevel converter operating at low switching frequencies. The controller regulates the load currents along their reference trajectories, controls the circulating currents and controls the sum of the capacitor voltages per branch. Upper limits on the branch currents and capacitor voltages are imposed in the controller formulation. The controller manipulates the voltage references of a carrier-based pulse width modulator. A subsequent balancing controller maintains the capacitor voltages within each branch around their average voltage. Unlike hierarchical schemes based on multiple PI control loops, the proposed controller achieves not only a very good performance at steady-state operation but also very fast current responses during load transients, while maintaining the branch currents and capacitor voltages within their safe operating limits.