Frequency inconsistency in DFIG-based wind farm during outgoing transmission line faults and its effect on longitudinal differential protection

Based on the simplified circuit of induction machine, frequency response of the short-circuit current contributed by doubly fed induction generators (DFIG) is studied. According to the equivalent network of wind farm, the main frequency components of current and voltage on outgoing transmission line during faults are analyzed mathematically and verified by simulations, which show that the current frequency is inconsistent at the terminals of the outgoing transmission line. Following the frequency inconsistency feature, the adaptability of longitudinal differential protection is analyzed, which indicates the potential threaten to the reliability of longitudinal differential protection in large wind farms. And simulation results demonstrated that inconsistent frequency characteristics of wind turbines will deteriorate the sensitivity and reliability of longitudinal differential protection. Finally, several qualitative suggestions on improving the reliability of relay protection in wind farms are given.