Multilevel converter topology using two types of current-source inverters

A multilevel power-converter topology based on two types of current-source inverters (CSIs) has been proposed for large induction motor drives. The topology utilizes the combination of the load-commutated inverter (LCI) using thyristors and the CSI using gate turn-off thyristors (GTOs). As a result, the multilevel-inverter operation takes advantage of a soft switching of the LCI and hard switching of the CSI. The output capacitor is required to generate the leading power factor for load commutation of the LCI. It is shown that the proposed multilevel operation contributes to the leading-power-factor generation by providing the effective phase shift. Thus, the leading power factor required for the load commutation of the LCI is obtained with significantly smaller capacitors at the inverter's output terminals. As a result, the proposed approach can employ the LCI and utilize its soft-switching operation, yielding a cost-effective solution compared with the conventional multilevel CSI using two GTO CSIs. The switching losses are curtailed due to the natural commutation of the LCI and the six-step operation of the GTO CSI. Therefore, the proposed multilevel inverter can both increase the output power level and decrease the, output capacitor size for LCI. The simulation and experimental results have been shown to verify the feasibility of the proposed multilevel topology and its operation.