Current Sharing Control of Non-isolated High Power Modular Bi-directional DC/DC Converter for HVDC-Flexible System in Offshore Wind Farm

The flexible HVDC power transmission system (HVDC-Flexible) will be a kind of important power transmission modes for offshore wind farms in the future. In this paper, a novel multi-terminal HVDC-Flexible power transmission system for offshore wind farms is introduced, and its DC grid side converter (GSC) is mainly studied. In order to achieve high power and high gain, GSC adopts the topology structure of non-isolated cascaded DC/DC converter modules in parallel. The causes of currents imbalance in the parallel topology structure are analyzed based on the mathematical modeling of GSC, and the current-sharing control strategy which is independent of the parameters of the devices in the modules is proposed. Finally, the simulation experiments are carried out. The simulation results show that the proposed strategy can realize the current-sharing control of the parallel system under the premise of realizing the voltage stability control of the low voltage side of GSC and bidirectional energy transmission, which verifies the effectiveness of the proposed currentsharing control strategy.