A digital control algorithm for modular multilevel converters

We present a method for designing a stabilizing reference circulating current for modular multilevel converters. To this end, an optimization problem is formulated. This optimization problem takes into account conflicting control requirements such as reducing the oscillating component of the circulating current as well as the oscillating component of the arm voltages. Tracking of the obtained reference signal is ensured by a robust controller with gains chosen in order to attenuate the measurement noise. The control synthesis is carried out in the discrete-time domain. Such an approach yields simpler realtime implementation of the control algorithm. Performances of the proposed method are verified on a grid-connected prototype of modular multilevel converter.