Reach is the impedance setting of a distance protection zone, and by extension the length of line it covers. Underreach means the relay sees a fault as further away than it really is, often because of infeed from another source or arc resistance, and so fails to trip in the expected zone. Overreach is the reverse and risks tripping for faults on the next line, which is why zone 1 is set deliberately short of the remote busbar.
With the wind farm infeeding at the tee, zone 2 underreaches badly, we need a permissive scheme or more reach.
Also heard as
- zone reach
Related terms
Distance protection
Line protection that measures impedance to a fault and trips fastest for faults close to the relay.
Protection relay
Device that watches currents and voltages and orders a circuit breaker to trip when it detects a fault.
Selectivity
Arranging protection so only the device nearest a fault operates and the rest of the system stays in service.