The impact of induction generators on operation characteristics of zone 3 distance protection is studied. The theoretical impedance of a induction generator is quantified with fixed stator voltage. The actual impedance is derived with a newly proposed load flow model which differentiates the stator and the rotor circuits. From the sensitivity model of bus voltage to mechanical input power， and that of operation margin of zone 3 relay to bus voltage， the linear relationship of operation margin to input wind power is derived. Numerical results show that: the induction generator has a characteristic of negative resistance and positive reactance， yielding the apparent impedance of the far-end wind farm bus similar to the backward fault; the increment of mechanical input power corresponds to lower stator voltage， lower machine impedance magnitude， and lower operation margin; the load increment near wind farms may notably decrease the operation margin of zone 3 impedance relay of the far-end wind farm bus.