| High voltage (3.3–4.5 kV) insulated gate bipolar transistor (HVIGBT) modules have recently been developed for megawatt AC motor drive applications. But switching losses and safe operating area (SOA) limitations restrict the utilization of these devices to a fraction of their potential power rating, in part because their design has decreased on-state voltage at the expense of slower switching speed. This work investigates the suitability of soft switching technology for high power, relatively slow switching inverters at medium voltage levels, in particular for a 1.0 MVA induction motor drive voltage source inverter (VSI), as a means of improving HVIGBT utilization. The goal of applying lower switching loss and a reduced SOA requirement primarily toward increasing the output volt-amp capability of the VSI, rather than increasing the switching frequency, sets this study apart from previous work.; Limitations of HVIGBTs under hard switching are first identified. Inverter circuits that offer increased VSI output power are then discussed: the actively clamped quasi resonant DC link inverter (ACQRLI) of Salama and Tadros, the auxiliary resonant commutated pole inverter (ARCPI) of De Doncker and Lyons, and a new quasi three-level inverter (Q3I) hard switching method based on a three-level inverter topology. These circuits are analyzed in depth and evaluated for cost and integrability with a modular inverter. Design rules and trade-offs are developed, and fundamental limitations in converter operation are illuminated. In addition, fault modes are evaluated, and compatibility with typical high power modulation schemes, e.g., space vector pulse-width modulation (SVPWM) and direct self control (DSC), is discussed.; The soft switching circuits substantially reduce SOA requirements and switching loss compared to hard switching. The Q3I is proposed as a lower cost alternative to the soft switching circuits. The reduction of switching loss has not been precisely quantified, but an increase in output power of 30 percent with the ARCPI is estimated. The ARCPI is shown to provide the greatest potential for improving inverter power throughput without unduly restricting modulation capability. Experimental prototypes of the ARCPI and Q3I were built and tested. Laboratory results and inverter design considerations are discussed. |
