A Capacitor Voltage Balancing Scheme for Single-Source Fed Switch Optimized Three-Phase Nine-Level Inverter

This article proposes a novel floating capacitor (FC) and dc-link capacitor voltage balancing scheme for a single-source three-phase nine-level inverter with reduced part count. Although nine levels are generated by keeping the dc-link capacitor voltage to FC voltage ratio as 3:1, it comes with complete absence of any useful pole voltage redundancies. Therefore, the conventionally adopted redundant-state-switching-based pulsewidth modulation technique cannot be applied for FC voltage balancing. Hence, a novel carrier-based modulation scheme is proposed here, which addresses the issue of FC voltage balancing by safely clamping the modulating reference to pole voltage levels that aid in FC voltage balancing. The dc-link capacitor voltages are also balanced using a unique way suitable for this inverter having nonredundant characteristics. Simulation and experimental results verify the capability of the proposed balancing scheme for the optimized nine-level inverter under different modulation and load conditions. It is found that a maximum fundamental phase peak voltage of 0.607 times dc-link voltage (at 0.91 modulation index) can be achieved at unity power factor and, thus, utilizing the dc bus of the inverter effectively.