Experimental verification of capacitance reduction in MMC-based STATCOM

This paper proposes capacitance reduction in modular multilevel converter (MMC) based static synchronous compensator (STATCOM). The MMC-based STATCOM consists of several series connected single-phase STATCOM; therefore, the required capacitance to achieve a constant dc voltage is comparatively high. The basic concept of the capacitance reduction and control principle have been proposed in single-stage STATCOM. This paper extents them to MMC configuration. A control method including capacitor peak voltage based control and voltage balancing control is proposed. The control was demonstrated in a fabricated small-scale setup of 3-cascaded single-phase STATCOM, and waveforms in stead-state and transient are shown. By applying the proposed control technique, the peak voltage was confirmed to be almost constant while the current set-point is changed in step, and a good voltage balancing in dc-side capacitors of each cell was also confirmed.