Capacitor RMS Current Reduction for Back-to-Back Three-Level Converters Based on Zero-Sequence Voltage Injection

Thermal stresses induced by high root-mean-square (rms) currents in dc-link capacitors significantly affect their lifetimes and compromise the overall reliability of converter systems. This article proposes a novel zero-sequence voltage (ZSV) injection method to reduce the rms current in dc-link capacitors for back-to-back three-level converters. The proposed method adjusts the overlapping intervals of dc bus currents by injecting a differential ZSV on either the rectifier or inverter side, while a common ZSV is injected on both sides to control the neutral point voltage. Simulation and experimental results demonstrate that the proposed method reduces the capacitor rms current by up to 40%–50%, with minimal effect on the total harmonic distortion of the output currents. The proposed method improves system reliability by reducing power losses in the capacitors and extending their operational lifetimes, making it highly suitable for applications that demand both high efficiency and reliability.