Analysis of a transformer-less, multi-level DC-DC converter for HVDC operation

HVDC systems require DC step up and DC step down units. The traditional approach is the application of twelve-pulse thyristor bridges with transformers. The developments of fast switching IGBT devices permit the development of transformer-less, multi-level converters. A multi-level circuit was suggested by Limpaecher (Ref. I). This paper presents a detailed simulation of the proposed circuit together with the analysis of its performance. The converter consists of a set of capacitors, air care inductors and solid state switches arranged in a ladder network. In the step-up mode, the closing of solid state switches resonantly charges the capacitors in parallel through an air-cared inductor. Then solid slate switches connect the capacitors in series and discharge them through an air-core inductor to the load. In the step-down mode the capacitors are charged in series and discharged in parallel. The circuit has three modes of operation in each cycle: charge, inversion, and discharge. The circuit operation is analyzed in each mode using SPICE simulations. The selection of the components is discussed and output voltage regulation is analyzed The results show that the proposed circuit promises significant reduction of losses, because of the zero current switching. The investment cost is reduced because of the elimination of transformers.