Reducing Common-Mode Current: A Modified Space Vector Pulsewidth Modulation Scheme

With the conventional space vector pulsewidth modulation (SVPWM) scheme, ac drives generate high-peak common-mode currents (CMCs), particularly at a very low modulation index. This occurs because of the low dwell times on the active voltage vectors, resulting in nearly simultaneous switching on all three phases. High-peak CMCs present several issues-high-voltage spikes from the dc bus to ground, a pump-up of the dc bus voltage, and increased conducted emissions-that can affect the reliability and performance of the ac drive. The solutions for addressing this phenomenon include placing capacitors to ground on the dc bus, which cannot be applied when the power system is not solidly grounded (e.g., high-resistance grounding), and filters at the output of the drive, which are bulky, expensive, and may not always be effective. In this article, a modified SVPWM scheme is presented that reduces the instantaneous peak and root-mean-square (RMS) CMC at the output of the drive. Experimental results are presented to demonstrate that the method works with all types of system grounding and is more effective than commonly used drive output filters at a very low modulation index.