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AC (alternating current) system backup protection setting calculation is an important basis for ensuring the safe operation of power grids. With the increasing integration of modular multilevel converter based high voltage direct current (MMC-HVDC) into power grids, it has been a big challenge for the AC system backup protection setting calculation, as the MMC-HVDC lacks the fault self-clearance capability under pole-to-pole faults. This paper focused on the pole-to-pole faults analysis for the AC system backup protection setting calculation. The principles of pole-to-pole faults analysis were discussed first according to the standard of the AC system protection setting calculation. Then, the influence of fault resistance on the fault process was investigated. A simplified analytic approach of pole-to-pole faults in MMC-HVDC for the AC system backup protection setting calculation was proposed. In the proposed approach, the derived expressions of fundamental frequency current are applicable under arbitrary fault resistance. The accuracy of the proposed approach was demonstrated by PSCAD/EMTDC (Power Systems Computer-Aided Design/Electromagnetic Transients including DC) simulations.

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Superconducting fault current limiter (SFCL) is a promising technology to suppress fault current in multi-terminal VSC-HVDC. However, the location and size of SFCLs determination have become a problem to be studied. This study proposes a location and size determination method of SFCLs in multi-terminal VSC-HVDC. In the proposed method, the current reduction ranking is developed to quantify the current limiting effect, and iterative process is utilised to deal with the non-linear characteristics of current reduction. With the iterative current reduction ranking, the location and size could be determined collaboratively under mathematical guidance, ensuring the optimality of current limiting effect. Fast calculation of current reduction is investigated to accelerate the determination process. Finally, the effectiveness and feasibility of the proposed method are validated in a four-terminal VSC-HVDC.

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