Adaptive distance protection for grounded fault of lines connected with doubly-fed induction generators

Crowbar circuit is a preferred solution to maintaining wind generators connected to the grid during a disturbance. However, the activation of crowbar protection will lead to an obvious difference between the positive sequence impedance of wind generators and the negative sequence one. This study analyses the influence of this unique fault characteristic of doublyfed induction generators (DFIGs) on the three zones of distance relays. When the transmission line emanates from a DFIG-based wind farm, there exists a high probability that the conventional protective zones of distance relays fail to operate. Hence, an adaptive branch coefficient is proposed to calculate the zone 2 setting according to the fault type and equivalent sequence impedances of wind generators. The proposed adaptive setting principle realises a better coverage of zone 2 and enhances the reliability and sensitivity of the relays. Simulation results verify the improvement of the protective zone of distance relays.