Differential protection sums the currents at every boundary of a protected item, such as a transformer, generator, busbar or cable, and operates when the difference exceeds a biased threshold. Faults outside the zone produce almost no difference, so the scheme is fast and inherently selective. A bias characteristic tolerates CT mismatch, tap position and saturation, and transformer schemes add second-harmonic blocking so inrush does not cause a trip.
Transformer went out on differential protection with no Buchholz alarm, check the CT wiring before anyone calls it a winding fault.
When an 87 operates on a newly commissioned site, the first suspect is CT polarity or ratio, not the plant.
Also heard as
- 87
- diff
Related terms
Protection relay
Device that watches currents and voltages and orders a circuit breaker to trip when it detects a fault.
Inrush current
Large transient current drawn when a transformer, motor or capacitive load is first switched on.
CT
Current transformer: an instrument transformer that scales large primary currents down to a standard secondary.
Bus
Rigid conductor or common node in a substation or switchboard where several circuits connect and share power.