Novel Method for Instantaneous Neutral Point Voltage Balancing for a Stacked Multilevel Inverter-Fed Six-Phase Induction Motor Drives

This article proposes a novel method for instantaneous neutral point (NP) balancing for a six-phase stacked multilevel inverter (MLI)-fed induction motor (IM) drive. A stacked inverter requires as many dc sources as the number of stackings. A single dc source is ensured with instantaneous NP balancing although there are two stackings of MLIs. A single dc-link enables direct back-to-back power conversion and power can be fed back to mains at the desired power factor. Instantaneous NP balancing is achieved by ensuring that at any instant, no midpoint current is drawn from the NP (dc-link capacitor midpoint). This work shows that the midpoint is used only as a tapping point to generate the different pole voltage levels and no current needs to be drawn from it. V/f experiment is conducted on a six-phase IM with a nine-level stacked MLI, which is developed by stacking two five-level inverters where each of the five-level is developed by cascading a flying capacitor with a capacitor fed cascaded H-bridge. The results ensured that the proposed method for zero instantaneous NP current is valid for both steady-state and transient conditions of a six-phase IM drive using stacked MLI.