A Three-Port PSFB/DAB-MMC PET With Inertia Enhancement Under LVDC Disturbance

DC distribution system has been popular in research and industry applications in recent years. The topology of the three-port modular multilevel converter (MMC) was proposed to interconnect the medium voltage DC (MVDC) distribution system, the medium voltage AC (MVAC) distribution system and the low voltage DC (LVDC). Compared to classical two-port MMC, a three-port MMC substitutes the tranditional submodules with bidirectional isolated converters. There are two kinds of existing three-port MMC topologies. One topology uses dual active bridge (DAB) converters to transfer power from the medium voltage side to the LVDC system, which is called DAB-MMC. The other topology utilizes bidirectional phase shift full bridge (PSFB) converters as submodules, which is called isolated MMC (I-MMC). Compared to the DAB-MMC, the I-MMC reduces the cost of the submodule capacitors and the power switches. But the LVDC inertia of the I-MMC is low due to the common submodule capacitors, especially during disturbances. In this article, a hybrid three-port MMC topology with inertia enhancement is proposed. With the combination of the PSFB and DAB submodules, the proposed MMC increases the equivalent inertia of the LVDC system using a novel virtual control method. The simulation results in MATLAB/Simulink and real-time RT-LAB prove the feasibility and the accuracy of the proposed inertia enhancing method.