An optimization-based control strategy for modular multilevel converters: Design and implementation

In this paper we present an optimization-based procedure for designing a reference circulating current which stabilizes the internal dynamics of a modular multilevel converter. This procedure relies on unconstrained convex optimization and it takes into account conflicting performance requirements such as reducing the oscillating components of circulating current and arm voltages. Tracking of such a reference signal is ensured by a robust tracking controller with gains chosen in order to attenuate the measurement noise. Since we were interested in implementation of the control algorithm by using a digital simulator, the design procedure is carried out in the discrete-time domain. Effectiveness of the proposed strategy is confirmed on a prototype of three-phase modular multilevel converter with five sub-modules per arm and RL load.