Voltage droop control at point of common coupling with arm current and capacitor voltage analysis for distribution static synchronous compensator based on modular multilevel converter

This study presents a kind of three-phase three-wire double-star-type transformer-less modular multilevel converters (MMCs) in medium-voltage distribution static synchronous compensators (DSTATCOMs) application based on pulse-width-modulated cascaded half-bridge modules. An advanced steady-state analysis for arm current and capacitor voltage is presented to give the explicit converter performance evaluation in project installation. Two key equations based on first and second component equivalent circuits are formulated to obtain the undetermined circulating second harmonic current and actual modulation index. Based on the proposed analysis, the internal active and reactive power control and the external droop control for the voltage at the point of common coupling (PCC) are introduced for MMC-DSTATCOM, respectively. The droop control enlarges the operation area of the converter and the PCC voltage is allowed to vary in a larger acceptable range. Experiment tests acquired from a 60 V/2 kVA prototype in a simplified downscaled power distribution system verify the accuracy of the analysis and the feasibility of the PCC voltage droop control simultaneously.