Research on the Short-Circuit Current of MMC-HVDC under DC Pole-to-pole Fault Considering the Impacts of DFIG

Integrating a doubly-fed wind farm into the grid through high-voltage direct current (HVDC) transmission using a modular multilevel converter (MMC) is a common approach for wind power integration. During DC pole-to-pole faults in MMC-HVDC, the terminal voltage of the doubly fed induction generator (DFIG) symmetrically drops and triggers low voltage ride-through (LVRT) control to provide reactive power. However, due to different control methods between DFIG converter and MMC, there exists significant interaction between their transient processes. While previous research has focused on fault characteristics of MMC under pole-to-pole faults, the influences of DFIG have been excluded. This paper analyzes the timing characteristics of both MMC blocking and DFIG LVRT control, establishes fault equivalent circuits for DFIG and MMC-HVDC before and after MMC blocking, deduces short-circuit current expressions for MMC considering the impacts of DFIG under pole-to-pole faults, and analyzes both the influences of DFIG as well as characteristics of short-circuit currents in MMC. The proposed theoretical analysis was verified by PSCAD/EMTDC.