A multilevel power conversion scheme for nonisolated DC sources

This paper is concerned with the design of a 10-15 MW power conversion scheme required to interface rechargeable, nonisolated DC Sources to the utility grid at medium-voltage. In order to meet application requirements, a 7-level H-bridge inverter topology using series-cascaded single-phase power transformers is proposed. Equalised-power selective harmonic elimination PWM is applied to the inverter, thereby maintaining a low switching-frequency whilst meeting utility grid harmonic requirements. Transformer-cascading allows three-phase voltage generation using H-bridges connected to a single DC-link, thereby cancelling the twice-fundamental current pulse associated with such an inverter, and hence reducing the current ripple on the DC-link. Bidirectional multiphase DC-DC boost converters are utilized to maintain a stabilized voltage on the DC-links and control the power flow from and to the DC sources. A low switching-frequency is also employed on the DC-DC converters and current interleaving reduces the DC source current ripple. A 5 kVA laboratory prototype based on the high power design has been built and is currently undergoing testing. Experimental results are presented in this paper.