EFFICIENT CURRENT STRESS REDUCTION FOR AC-MMCS IN MACHINE DRIVE SYSTEMS

Abstract

This paper proposes a new approach for defining the average voltage of submodule (SM) capacitors in Active Cross-Connected Modular Multilevel Converters (AC-MMCs) intended for medium-voltage motor drive applications. The concept relies on allowing a larger low-frequency voltage ripple to appear on the SM capacitors by intentionally limiting the compensation of low-frequency power variations. The resulting ripple information is then used to regulate the average capacitor voltage so that its deviations remain within the required operating bounds. By doing so, the magnitude of the AC circulating current is lowered and part of the low-frequency power oscillation is suppressed. This leads to reduced current stress on the power semiconductors, inductors, and flying capacitors. Simulation has been done to confirm the effectiveness of the proposed control method.