Control and Modulation Methods of Single-/Three-Phase PWM Converter for Suppressing Leakage Current

Transformerless power converters are increasingly getting attention in the ac–dc power conversion due to their advantages in improving the efficiency and the power density. However, the removal of the isolation barrier with high common-mode (CM) impedance generates a leakage current issue due to the common-mode voltage (CMV), posing a human safety risk. This article comprehensively analyzes the influence of the CMV sources on the leakage currents, dividing into the low- and high-frequency (HF) components. To this end, a CM equivalent circuit is developed, considering the filter and grounding system. In order to mitigate the low-frequency (LF) leakage currents, a CMV control method is proposed, which generates the LF CMV through the pulsewidth modulation (PWM) converters. The leakage current flowing into the ground wire is directly measured and utilized as a feedback for the controller. Subsequently, a high-dimensional space vector PWM (HDSVPWM) is introduced to synthesize both differential mode voltage and CMV references for the single-/three-phase PWM converter. The proposed HDSVPWM minimizes the HF CMV ripple, reducing the HF leakage currents. The effectiveness of the proposed control and modulation methods is verified with an experimental setup of grid-connected transformerless PWM converter, ensuring a compliance with the safety regulations.