A Study on DC Voltage Control for Chopper-Cell-Based Modular Multilevel Converters in D-STATCOM Application

This paper presents a three-phase transformerless structure of modular multilevel converters based on cascaded pulsewidth-modulated chopper cells in medium-voltage D-STATCOM application. It is suggested for installation on unbalanced medium-voltage power-supply systems to achieve higher performance in improving power quality. This paper proposes a new control strategy based on power-flow analysis with a focus on dc capacitor voltage control that is insulated from the source voltage fault. This control strategy enables the D-STATCOM system to compensate unbalanced nonlinear load even under asymmetrical grid fault conditions and it has no limitations on the cascaded number. Experimental results obtained from a 60-V 3-kVA downscaled prototype verify that this D-STATCOM system performs satisfactory, not only compensating for unbalanced nonlinear load but also keeping the capacitor voltages maintained at the given value even in unbalanced power systems.