A Cascaded Converter Interfacing Long-Distance HVdc and Back-to-Back HVdc Systems

This paper proposes a cascaded converter dedicated to a long-distance high-voltage direct current (HVdc) infeed and asynchronous back-to-back interconnection of receiving grids. The cascaded converter consisted of modular multilevel converters (MMCs) in series and parallel connection, meeting the high dc voltage and power demand of the HVdc system. It realizes the hierarchical feeding and asynchronous interconnection of receiving grids, optimizing the multi-infeed short-circuit ratio (MISCR) and improving the flexibility of the receiving grids. The topology and operating characteristics of the cascaded converter are introduced in detail. The MISCR and the maximum power infeed of the cascaded converter-based HVdc systems are analyzed. Various feasible operating modes with online switching strategies of the cascaded converter are studied to improve the operational flexibility of the system. The simulation results verify the effectiveness of the control strategy of the HVdc system embedding the cascaded converter. The dc faults clearing strategy and operating modes switching strategies are also validated.