A Transformerless Bipolar Multistring DC-DC Converter Based on Series-Connected Modules

This paper proposes a transformerless bipolar multistring dc-dc converter based on series-connected submodules. It is intended for converting dc voltage and managing power flow for medium-voltage high-power dc networks. This new converter has a flexible structure of multistrings and each string consists of four valves with their middle points crossly connected through two cost-effect dc capacitors to form power balancing channels between valves. Its transformerless solution and single-stage configuration greatly reduce the converter volume, weight, cost, and power loss. In this paper, voltage ratio range and its influence on submodule design are analyzed. Moreover, a control strategy with focus on current sharing control and submodule capacitor voltage control is proposed. A simulation model rated at +/- 10 kV 2 MW verifies the feasibility of the proposed dc-dc converter. Experimental results obtained from a 100-V 1.5-kW laboratory setup also confirm the validation of the proposal.