Design Considerations at the High Frequency Link of an MMC-driven Power Electronic Transformer for Wind-energy Systems

Back-to-Back MMC based Power Electronic Transformer has recently been proposed for the integration of wind energy systems, renewables and storage to medium voltage grid with advantages of decreased transformer size, increased efficiency, potential for virtual inertia to the grid and others. The unique arrangement of the High-frequency link MMC(HF-MMC) interfaced to the two-level voltage source converter(2L-VSC) at the HF-link presents the need to synthesize design equations targeting the required performance during operation. Design of the HF link parameters are discussed to simultaneously satisfy maximum power transfer, soft-switching at 2L-VSC, HF-link current THD, unity displacement power factor(DPF) operation at the HF-link and DC bus utilization. Expressions for the soft-switching current for the 2L-VSC, minimum and maximum transformer turns ratio, power transfer at unity DPF operation have been derived to aid in the design process and provide insights into the operation of the topology. Extensive simulation results in MATLAB/SIMULINK and real-time simulations in OPAL-RT HIL have been presented to validate the design and analysis.