Fault current calculation method for flexible HVDC grid considering the DC unified power flow controller

This paper proposes a generic short-circuit fault current calculation method for Flexible HVDC grids. The calculation procedure begins from modelling the fault circuit of the MMC converter station as an RLC circuit, and then constructs the fault network matrix of flexible DC grid during the fault occupation. Meanwhile, considering the dynamic characteristic of DC Unified Power Flow Controller (DC-UPFC), analytical formulas for fault current under different current limiting stages of DC-UPFC is obtained. The proposed approaches are validated by comparing with the electromagnetic transient (EMT) simulation results on PSCAD/EMTDC. Cases study showed that the calculation method can be easily used to evaluate the severity of a dc fault under different current limiting stages, which lays the foundation for subsequent research on the impact of DC-UPFC fault current limiting parameters and the coordination of DC circuit breaker (DCCB) operation timing.