A Generalized Modulation Strategy for a Cascaded H-Bridge Multilevel Inverter under Unequal DC Sources

This paper proposes a generalized pulse-width modulation strategy for a cascaded H-bridge multilevel inverter (CHBMI) that can produce balanced line output voltages and currents even if its phase output voltages remain unbalanced because of unequal DC sources. The three-phase balanced line voltages were obtained by properly readjusting the pulse-width modulation reference phase angles during the compensation mechanism. The findings of the proposed strategy were derived from the 3D space vector analysis of its reference voltage vector projected in alpha beta 0, alpha 0, beta 0, and alpha beta planes under unequal DC sources. Such an analysis is beneficial not only to examine the maximum achievable balanced line voltages by inverter, but also to determine the achievable limits of homopolar component to be injected to maintain balanced output currents under unbalanced DC sources. Suitability and effectiveness of the proposed strategy were verified through simulation and experimental results. The proposed solution has wide applicability in battery-powered and solar cell-powered CHBMI systems.