PWM scheme to reduce the common-mode current generated by an AC drive at low modulation index

With the conventional space vector PWM (SVPWM) modulation scheme, AC drives generate high peak common-mode currents, particularly at 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 common-mode currents present several issues - high voltage spikes from DC bus to ground, pump-up of the DC bus voltage, increased conducted emissions - that can affect the reliability and performance of the AC drive. Solutions to address 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 (HRG)), and filters at the output of the drive, which are bulky, expensive and may not always be effective. In this paper, a modified SVPWM scheme is presented, which reduces the instantaneous peak and RMS common-mode current 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 very low modulation index.