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A Flying Capacitor Hybrid Modular Multilevel Converter with Reduced Number of Submodules and Power Losses
Year: 2023 | Author: Jian Liu | Paper: T4.4
Fig.1. Topology of FC-HMMC.
The basic working principle of the FC-HMMC is that two HV insulated-gate bipolar transistors (IGBTs) work at the line frequency. Moreover, once the trapezoidal-shaped current of the HV switch is designed, soft-switching can be realized at any modulation index or power factor (PF). Considering line-frequency operation, the difficulty of series connection in a two-level converter is reduced significantly. This characteristic means a thyristor-based FC-HMMC is also feasible, which could further reduce the conduction losses and cost.
The basic CL waveforms with the designed trapezoidal current allocation at unity power factor are plotted in Fig. 2. It is notable that the third harmonic is injected into the AC voltage in order to realize the higher DC bus voltage utilization. As shown, the energy of the FC-HMMC can be balanced for the CL and the FC by using the proposed current allocation. Compared to the standard MMC, the magnitude of energy ripple in the FC-HMMC is reduced to 42%, implying a total capacitance reduction of 37%. Therefore, the FC-HMMC is quite promising for MV ac-dc conversion, such as renewable energy grid-connected interfaces and MV traction converters. But it should be noted that high-PF operation is more suitable for such topologies due to concerns over capacitor energy storage.
Fig. 2. Phase current reconstruction principle.
