Phase Differential (87 T ) Protection
Current magnitude balancing within a differential relay is accomplished through the selection of the appropriate transformer tap settings within the relay.
Older electromechanical (EM) relays used five or six discrete tap settings to balance current magnitudes on the primary
(H winding) and secondary (X winding) of the transformer.
They could balance a current mismatch of approximately 3 : 1. Digital relays have continuous settable tap settings and can balance a 10 : 1 current mismatch, making them more accurate and providing the flexibility to handle larger mismatches.
Typically, the tap settings on the primary and secondary are selected by determining the full-load current at the self-cooled rating of the transformer and then checking to make sure that the relay current coil ampere rating is not exceeded under
emergency loading conditions.