Performance Improved Multi-Level OEIM Drive With Voltage Boost Through Unequal Levels

This article proposes an energy efficient 5-level neutral point clamped (NPC) inverter based boost inverter topology for transformer-less medium voltage high power Open End Induction Motor (OEIM) drive. A conventional boost circuit cannot be used in such drives as single semiconductor device may not be able to block the total dc link voltage and simultaneously carry full load current. In the proposed scheme, two partly rated soft switched isolated dc-dc converters which generate only the required boost fraction of the voltage are connected in cascade with the two ends of the input dc bus. The input dc is divided into two equal parts using a capacitor voltage divider circuit. These four effective voltage sources in cascade serve as the four dc bus for the two five level NPC VSI of the OEIM drive. A substantial amount of power directly flows to the inverter through the capacitor voltage divider circuit and hence the size as well as power loss of the dc-dc converter of the proposed scheme is much smaller. The voltage generated from the dc-dc converter is lower than the voltage in the capacitor voltage divider circuit, creating unequal step heights in the pole voltage of the 5-level NPC VSI. These unequal steps in the pole voltage of the two VSI of open end drive restrict the use of existing technique for operating an OEIM drive from a single dc source. In this article, a carrier based PWM technique is proposed to restrict the circulating current caused by CMV even when the voltage steps are unequal.