Improved Scheme of Current Differential Protection for MMC-HVDC Connected to AC Power Grid

The scale of new energy accessing to the power grid is increasing day by day, it is necessary to widely use flexible DC transmission technology to improving the adaptability of the power grid. When AC fault occurs, the feature of short-circuit current in MMC side change significantly and fundamentally compared with traditional synchronous generators. Therefore, the characteristics of phase angle of fault current are different from AC side, at the same time, the sensitivity of differential protection (DP) decreases or even refuses to operate. In present study, the amplitude difference of negative sequence current (NSC) on both sides is used as an auxiliary criterion to solve the problem of reduced sensitivity of current DP under asymmetric fault, and the feasibility of the scheme is verified through simulation in PSCAD.