Modular Multilevel DC-DC Transformer and its Modulation Strategy

A modular multilevel power electronics transformer for DC power grid is proposed in this paper. A novel 2-port submodule is designed and adopted in the DC-DC PET (power electronics transformer). The submodule has ten switches and a capacitor with three working states as an energy buffer. Submodules in the DC-DC PET are connected in input-series¬output-series (ISOS) structure. The capacitor is connected to the primary side and absorbs energy from primary side in State 1. When it is connected to the secondary side, the energy stored in it will be transferred to the secondary side in State 2. When it is isolated from both sides, there will be no energy exchange in State 3. If the modulation frequency is high enough, the capacitor voltage will maintain a constant value approximately. By rotating the submodule working states, the energy can be transmitted from the primary side to the secondary side. Therefore, the different voltage ratio can be achieved by arranging proper submodule numbers in different states simultaneously. The simulation are carried out in Matlab/Simulink and the proposed PET works as expected. The bidirectional power flow, voltage ratio adaption and primary-secondary isolation are achieved without any AC transformer.