A large modern wind farm should keep connected to the grid in the case of a grid fault whilst it should be isolated for an intertie fault. To achieve this function, this paper proposes a distance relaying algorithm suitable for intertie protection of a wind farm in the time domain. The proposed algorithm estimates impedance based on a differential equation method because the frequencies of the voltage and the current deviate from the nominal frequency. To discriminate an intertie fault from a grid fault, the algorithm uses a voltage blocking scheme because the magnitude of the voltage at the point of common coupling for an intertie fault becomes less than that for a grid fault. The performance of the algorithm is verified using the PSCAD/EMTDC simulator under various fault conditions. The results indicate that the algorithm can extend the reach of Zone 1 up to 100% of the length of an intertie and thus discriminate successfully the intertie fault from the grid fault.