Cascaded H-bridge multilevel inverters with discrete variation of DC sources

Abstract

ABSTRACTThis paper presents a new voltage regulation structure for multilevel inverters using discrete variation of DC sources. In the proposed method, discrete variation is achieved based on multi-tap transformers for applications where the DC sources are isolated and supplied by three-phase transformers, e.g., medium voltage (MV) motor drive applications. Moreover, by utilising the new solution, the number of required DC regulators is reduced significantly, for applications where the optimum modulation index with variable DC sources is used which suffer from large number of required converters. In addition, this paper proposes a new algorithm for optimising the switching angles, turn ratios of the multi-tap transformers and DC sources to maintain the voltage distortions at the standards levels for a wide range of output voltage regulation. To verify the feasibility of the new solution, a laboratory prototype is implemented based on a 2-cell cascaded H-bridge (CHB) multilevel inverter with unequal DC sources (with 9-level operation) while the proposed structure can be applied for other CHB multilevel inverters and switching strategies.