In differential relay protection schemes, two or more currents are compared to ensure rapid detection of fault current within a certain "zone." The basic concept is that under normal conditions, the "sum of currents in" must equal "sum of currents out,"allowing for some error margin tolerance (restraint). When current is feeding a fault in between these current sensors, much more current enters the zone than current leaving (if any at all). For example, an unfortunate cat creates a fault current path from A-phase to the case or ground of a transformer.
This type of protective circuit can issue a trip or "open" signal to switches that quickly isolate and extinguish the fault that could reach thousands of amperes, limiting damage to equipment, and possibly saving human life, though it only takes a fraction of a second many cases for lethal consequence: only safety work processes and procedures can mitigate such danger.
In such differential circuits, there should be a single ground for each zone located nearest the respective relay. This must be electrically verified through a single-ground test during commissioning.
During a fault condition, multiple grounds allow a different ground potential rise for each current transformer. The result is a significant current flow through the current sensing (CT) circuit that is not representative of the primary current.
This ground loop typically creates a potential across the operating coil or input of the differential relay causing the relay to pick up as though a fault exists in the relay's protective zone. Tripping a differential relay due to a fault external to the zone of protection is one of the most common types of nuisance trips.
These nuisance trips may not only shut down the load, but may require a maintenance crew to spend days in testing to determine that no real problem exists in the differential zone. Further, the actual problem may go uncovered until the system is re-energized into the original fault.
Further reading:
http://www.powellind.com/main/Uploadpdf/ptb%2087%20current%20transformer%20grounding.pdf"
Nelson Abreu &
guest writer Jeff Velena, Southern CA
