Conducted EMI Reduction by Active Power Filter Embedded in Neutral-Point-Clamped Converters

This article proposes a novel modulation strategy with an active power filter (APF) for reducing conducted electromagnetic interference (EMI) in three-level neutral-point-clamped ac/dc pulsewidth modulation (PWM) converters. Common-mode (CM) voltage is identified as one of the main causes of EMI in power electronics systems driven by PWM converters. To address this issue, the article proposes a CM voltage reduction strategy using a fourth-leg APF with shunt capacitors, which overcomes the limitations of existing methods. The proposed strategy allows for the complete elimination of CM voltage, and compensation is proposed to eliminate residual CM voltages due to the dead time effect. In addition, the article presents a design procedure for the APF circuit to obtain a proper reduction performance. Simulation and experimental results demonstrate the effectiveness of the proposed strategy in reducing both CM voltage and current, as well as conducted EMI.