Control of Modular Multilevel Converters under Loading Variations in Distributed Generation Applications

In this paper, a function-based modulation control strategy for modular multilevel converters (MMCs) in a distributed generation (DG) system is proposed. Two novel modulation functions are introduced in this paper for the switching state functions of the lower and upper sub-modules of the interfaced MMC, which is considered as the main contribution of this control technique over other control methods. The amplitude and phase angle of the output current of the interfaced MMC can be easily applied to the proposed modulation functions to prepare a specific active and reactive power injection into the demand side. In addition, the equivalent capacitors of the lower and upper sub-modules are defined by taking into account the introduced modulation functions to guarantee an appropriate operation for the interfaced MMC in DG systems. Simulation results validate the capability of the proposed control method for MMCs under load variations.