A Novel Floating Capacitor Voltage Control Technique for 9-Level T-Type Inverter with Reduced Device Count

Abstract

A three-phase 9-level multilevel inverter (MLI) using single dc source and reduced device count is discussed in this paper. The hybrid MLI has a 3-level T-type inverter cascaded by a floating capacitor (FC) fed H-bridge (HB) unit in each of the phase. The number of pole voltage levels are dependent on the ratio of dc link capacitor voltage with respect to the FC voltage. The ratio being 3:1, the inverter can generate nine output levels but with lack of pole voltage redundancies. Thus the commonly adopted redundant state switching method cannot be used for FC voltage balancing. A novel carrier based modulation technique is proposed here to address this issue. This technique balances the FC voltage by shifting the phase reference to a nearest carrier region whose voltage levels aids in achieving the desired FC voltage. The voltage balancing capability of the proposed technique is tested under different operating conditions and is illustrated using simulation results. It is found that the FC voltages remain stable during transient and steady-state conditions of the inverter and the inverter is able to provide a dc bus utilization of 0.607 times the dc link voltage at unity power factor.