Topology and Control Strategy of Push-pull DC Autotransformer Suitable for Interconnecting LCC-HVDC and VSC-HVDC

By combining the topological deductive law of DC autotransformer and modular multi-level converter, this paper proposes a new type of DC transformer topology suitable for interconnecting line commutated converter based HVDC system (LCC-HVDC) and voltage source converter based HVDC system (VSC-HVDC), namely push-pull DC autotransformer (PPDAT). PPDAT establishes a certain degree of direct electrical coupling between LCC-HVDC and VSC-HVDC, which has the advantages of high transmission efficiency and high operational reliability. And PPDAT has the abilities to track LCC-HVDC voltage polarity changes and isolate DC faults due to the adopt of full bridge sub-module in the upper bridge arm. In this paper, the topological structure, operating principle, steady state control strategy and DC fault protection strategy of PPDAT are analyzed in detail. Finally, the correctness of theoretical analysis is verified by the simulations based on PSCAD/EMTDC.