The Enhanced DC Fault Current Calculation Method of MMC-HVDC Grid With FCLs

This paper introduces an enhanced dc fault current calculation method for modular multilevel converter-based high-voltage dc (HVdc) grid. The new method considers the fast switching of the fault current limiters (FCLs) and the nonlinear characteristics of the metal-oxide arresters (MOAs). The two peak-valley points of inflection in the dc fault current curves resulting from the insertion of the inductors and the energy dissipation in the MOAs are discussed. 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 reactor and the FCL reactor, and the initial inductor current values on these two points are analyzed. Finally, the complete fault current calculation method including the action of dc circuit breakers is proposed. The method is validated by electromagnetic transient simulations and has shown good applicability on both the overhead line-based and cable-based HVdc grids.