Accurate estimation on AC short-circuit current for grid-connected MMC-HVDC system considering FRT and various control modes

The modular multilevel converter based high voltage direct current (MMC-HVDC) system aggravates the shortcircuit current of the AC system. Accurate fault current estimation for the grid-connected MMC-HVDC system under various control modes is essential for equipment selection and protection systems design. Therefore, this paper proposes a universal and accurate algorithm to estimate the AC short-circuit current in MMC-based hybrid AC/DC grids. First, the MMC AC-side fault current response characteristics in different control modes of the MMC are analyzed. Subsequently, the transient and steady-state fault current expressions considering various fault ride-through (FRT) strategies are presented. Further, based on the hybrid parameter matrix, the equivalent multiport network for the MMC-based AC/DC transmission system is established. Then, the fault current of the MMC is iteratively calculated. Finally, the effectiveness and adaptability of the proposed method is validated, and the influences of the AC system parameters for the fault current are studied.