Distance protection computes apparent impedance from voltage and current at the line end and compares it with zones set as fractions of the line impedance. Zone 1 typically covers 80 to 85 percent of the line and trips with no intentional delay, zone 2 reaches past the far end with about 0.3 to 0.4 s delay, and zone 3 backs up adjacent lines. Because impedance is proportional to distance, the relay also estimates fault location for the line patrol.
Distance protection says zone 1, 23.4 km from Ridgeway, send the patrol to the river crossing first.
Also heard as
- impedance protection
- 21
Do not mix it up with
- Differential protection: Differential protection compares currents at the ends of a zone and gives no fault distance.
Related terms
Reach
Extent of line or network a distance relay zone is set to cover, expressed as an impedance.
Protection relay
Device that watches currents and voltages and orders a circuit breaker to trip when it detects a fault.
Auto-reclose
Automatic closing of a breaker shortly after a fault trip, restoring supply when the fault was only transient.