4. How do percentage differential relays increase the sensitivity of differential protection?
Percentage differential relays use operate and restraint coil which, during an external fault, high magnitude current flows through the restraint windings and only the differential current flows through the operating coil. The higher the current on a through fault, the more differential current required to operate the relay.
On an internal fault, the high magnitude differential current overcomes the restraint, and the relay operates
they can have increased sensitivity without affecting security.