1.School of Electric Power, South China University of Technology, Guangzhou 510640, China;2.College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China;3.Electric Power Research Institute of China Southern Power Grid Company Limited, Guangzhou 510663, China

For the scenario of large-scale renewable energy delivery through long-distance overhead lines, a flexible DC transmission system with energy storage system is proposed, which achieves functions such as suppressing power fluctuation of renewable energy, blocking DC fault, and actively supporting the power grid. The system is a hybrid DC transmission system with two terminals. The sending-end converter station adopts the MMC with energy storage system with the capability of blocking DC fault. The receiving-end converter station is an MMC with half-bridge submodules. For blocking the DC fault current of the MMC at the receiving end, a diode valve is installed on the grounding electrode line. In this paper, the operation principle of the flexible DC transmission system with energy storage system is analyzed. The development mechanism of bipolar short-circuit fault current of the MMC with energy storage system with the capability of blocking DC fault is studied. The corresponding control strategies for the active power support and the DC fault ride-through are provided. Finally, a corresponding simulation model is built, which verifies the feasibility of the proposed flexible DC transmission system with energy storage system and the effectiveness of the proposed control strategies. The simulation results show that, by using the proposed topology and control strategies, the flexible DC transmission system with energy storage system can achieve stable delivery of large-scale fluctuating electric energy, while providing effective active power support and realizing autonomous clearing of DC short-circuit fault during the power transmission process.