A Nonisolated Bidirectional Soft-Switching Power-Unit-Based DC–DC Converter With Unipolar and Bipolar Structure for DC Networks Interconnection

In this paper, a novel nonisolated bidirectional soft-switching dc-dc converter based on series power units (PU-BDC) with unipolar and bipolar structure for dc networks interconnection is proposed. All semiconductor devices in the PU-BDC operate under soft-switching conditions in both directions with low voltage stress. By operating in the discontinuous conduction mode, all switches and diodes are turned on naturally under zero current. By introducing an auxiliary zero-current-transition commutation cell in each power unit, zero current turn-off of all switches and diodes is achieved to reduce the turn-off loss. Besides, the PU-BDC has a transformerless structure with small volume and low cost, which makes it more suitable for small-scale or medium-scale dc networks and distributed generation systems. In addition, two types of bipolar PU-BDC are proposed to extend its application to bipolar dc networks interconnection. The control system was also optimized for grid-connection applications by providing an output voltage control strategy, a voltage gain control strategy, and a power control strategy to maintain the voltage level or keep a constant voltage gain of the networks and performing precise power control. Finally, simulations of a unipolar PU-BDC and a bipolar PU-BDC with three power units were implemented, and a 1.6-kW experimental prototype of the unipolar PU-BDC with three power units was developed to verify the effectiveness of the converter.