A novel modular multilevel step-up DC/DC converter for offshore systems

This paper presents a novel modular multilevel DC/DC converter suitable for integrating the oif-shore wind farm with the HVDC transmission system. The proposed converter consists of a wind farm-side three-phase MMC inverter and a series-connected rectifier linked by a special decoupled medium frequency transformer. The transformer decouples the three-phase input voltage into three isolated single-phase voltages with the 120-degree displacement. As the system requires only a unidirectional power flow, this enables the reduction of switching losses by using simple diode bridge rectifiers modules. The modular structure of the proposed system reduces also the voltage and current stresses of the power devices. A simple control strategy based on the stationary reference frame without involving the complex Park transformation is proposed in this paper. The system is managed through controlling the primary side converter only without any feedback from the output side. A generalized control strategy to extend the system into larger scale is also presented. Simulation results using MATLAB/Simulink are reported to demonstrate the feasibility and the effectiveness of the proposed converter.