SVM Strategies for Common-Mode Current Reduction in Transformerless Current-Source Drives at Low Modulation Index

Abstract

In transformerless high-power pulse width modulation (PWM) current-source drives, common-mode inductor is implemented to attenuate the common-mode voltage (CMV). However, the common-mode inductor is bulky and costly, so that scaling down the size of the common-mode inductor can achieve the system size reduction and efficiency enhancement. The size of the common-mode inductor is mainly related to the maximum common-mode current, which occurs at the resonant frequency of the common-mode circuit when motor speed is low. Previous research demonstrates that 3-segment average-value-reduction (AVR) space vector modulation (SVM) has good common-mode resonance suppression performance by attenuating the third-order CMV from the inverter. However, as studied in this paper, when modulation index of the inverter is lower than 0.4, the performance of 3-segment AVR SVM is affected as the dwell time of zero vector becomes too long to effectively compensate the CMV introduced by the active vectors. To overcome this problem, three modified AVR SVM schemes are proposed in this paper, where the single long zero vector is separated into two segments in the PWM sequence. The working principles, CMV reduction performance, harmonic performance, and switching frequency analysis of the proposed SVM methods are presented in detailed. Experiments are conducted to verify the effectiveness of the proposed methods.