| Summary: | This paper proposes a DC fault current calculation method to reveal the fault current evolution mechanism of the modular multilevel converter (MMC)-based HVDC grid. The new method includes the fast switching of the current-limiting devices and the transient characteristics of the metal oxide arresters (MOA), which are not reported previously. Specifically, begin from a single-end MMC, two peak-valley points of inflection in the DC fault current curve are pointed out when switching the current-limiting devices into the fault branch with a follow-up action of MOA when the branch current reaches zero. The two points are selected as important indicators in the design of the coordinated protection scheme and switching logics. Then, the impact of the total DC reactance distribution in the DC-side reactor and the fault current-limiter (FCL) together with the initial current values on the two points of inflection are discussed. The applicability of the above approaches on HVDC grid is studied and an overall fault current calculation procedure with six steps including the tripping of the DC circuit breakers (DCCB) are proposed for HVDC gird applications. The proposed calculation method is validated by Electromagnetic Transient (EMT) simulation for pole-to-pole fault, different fault locations, and DC reactance values.
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