Effect of Saturated Current Transformers On Distance Relays

A distance relay detects a fault by measuring the impedance of a transmission line. Impedance is the ratio of voltage to current. The distance relay is set up so the cable voltage holds the relay in place and the current causes it to energize.

http://www.electrical4u.com/distance-relay-or-impedance-relay-working-principle-types/

The farther the fault the higher the voltage and lower the current. For example, a very near fault would result in high current and low voltage. The relay is designed to detect a fault out to a specific distance which would have a defined ratio of voltage and current. If the actual distance that the relay detects faults is less than the design distance, this is termed under reach. Likewise, if the actual distance is greater than the design distance, it is over reach.

https://www.quora.com/What-is-over-reach-and-under-reach-of-a-protecting-relay

Since the current to voltage ratio decreases with distance to the fault, if the current is reduced due to saturation of the CT, the farthest fault distance that can be detected is reduced from the design distance, the definition of under reach.