Decoupled control of capacitor voltages in a PWM cascade StatCom

This paper proposes a novel control strategy to maintain the balance of the capacitor voltages in a cascade PWM StatCom. The method is based on injection of a voltage/current component (the balance component) which is orthogonal to the fundamental one, and it allows independent control of the power exchanged by each particular module of the string. Overall capacitor voltage is controlled adjusting a current component which is in phase with the line voltage. Deviations from the balanced condition are controlled adjusting, in each H-bridge module, the magnitude of the balance voltage component and forcing a small balance current component, which is furthermore kept constant. Unlike standard switch redundancy-based methods, performance of the proposed scheme is independent of the fundamental reactive current drawn by the StatCom, but it injects a small distortion into the line. However, this can be avoided, as in the case of a delta- connected StatCom, if balance current component is supplied by means other than directly from the line. Experimental results confirm the fundamentals underlying the strategy and demonstrate its superior controllability when compared to a method which controls only the fundamental components. Simulation results for a three- phase delta-connected StatCom confirms elimination of the distorted balance component from the line.