Hybrid Multilevel Converter: Capacitor Voltage Balancing Limits and its Extension

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This paper presents theoretical analysis of a network fault tolerant hybrid cascaded multilevel converter when operated as one unit using multilevel pulse width modulation. The analysis establishes the modulation index range where the voltage balance of the H-bridge floating capacitors of the hybrid converter is attainable independent of load power factors. To realize the established modulation range, a new modulation strategy is proposed that exploits third harmonic subtraction modification of the reference voltage in order to extend the regions around zero voltage crossing where the cells capacitor voltage balancing can be achieved, with a minimum number of cells; independent of load power factor. The significance of the proposed modulation strategy is that it permits increased utilization of dc link voltage independent of operating condition over an extended modulation linear range. Therefore this hybrid converter is applicable to real and reactive power applications, with a higher power density than existing multilevel converters. The validity of the theoretical analysis and proposed extended modulation index linear range are confirmed using simulations and experimentations. The presented analysis and proposed modulation linear range extension can be extended to a hybrid converter with a large number of series H-bridge cells.