A distance relay is said to be over reach when the impedance presented to the relay less than the apparent a impedance to the fault.
Over reach will happen when two transmission lines are connected in parallel, and when one of them is disconnected and earthed from the two ends as shown in figure (28).
Mutual coupling between line causes the relay for the line in service to over reach. To over come this problem. Setting impedance of phase-Earth relay must be reduced to nearly (0.65% to 80%) from its original setting.
Ground Faults and Compensation Factor
In phase to ground faults, the setting is increased by a factor of
k0 * Z1 to be Z1(1+ k0)
k0 = (Z0- Z1 )/3Z1
Where:
k0 = Compensation Factor For Ground Faults
Z0 = Zero Sequence Impedance
Z1 = Positive Sequence Impedance
Z1 = Positive Sequence Impedance