Research on Multiphase Topology of F2F DC Transformer and the Control Method

With the development of HVDC system and medium voltage DC distribution network, DC transformer is needed for voltage level conversion and power transmission. In this paper, a multi-phase face-to-face (F2F) DC transformer topology based on modular multilevel converter (MMC) is proposed to connect two DC systems with different voltage levels and stabilize the lower bus voltage. When the two systems have fixed voltage levels, the topology can increase the transmission power and reduce AC transformer insulation requirements. In this paper, the topology of the proposed DC transformer is introduced, and the control and modulation strategy of the DC transformer are put forward. It can realize large voltage ratio of the system with small ratio AC transformer, and the voltage level conversion of the DC transformer does not depend on the AC transformer. At last, a simulation example is given in MATLAB/Simulink to verify the feasibility of the proposed topology.