Hybrid multi-terminal integration of parallel FB-MMC tapping station into LCC-HVDC transmission link

Tapping bulk HVDC links to serve intermediate loads or to pick up renewable energy resources always presents formidable technical and economic challenges to system designers. It is seen as quite difficult, and so is DC current coordination between multi-terminals. In order to solve this problem, a novel hybrid HVDC system with full bridge modular multilevel converter (FB-MMC) based tapping station is proposed. Control and coordination strategies for tapping station are investigated emphatically. Firstly, coordination for 3-terminal hybrid system is put forward which is more concise compared to line commutated converter based 3-terminal system. Secondly, on premise of being capable of operating under lower DC voltage with over modulation and free from effect of AC voltage drop, DC fault ride-through strategy and control method of FB-MMC station are improved. Finally, using PSCAD/EMTDC models, three cases, i.e. DC fault, power reversal and AC system fault at tapping station, are studied. Compared with line commutated converter based 3-terminal system, simulation results indicate that the proposed scheme can simplify coordination between terminals, optimize overall system performance and is more favorable for resisting AC voltage drop.