What is the function of a differential relay in a transformer bank?

• A. Trips when external faults occur on feeders.
• B. Monitors oil temperature for overheating protection.
• C. Detects internal faults by comparing currents on the transformer primary and secondary sides.
• D. Measures impedance on transmission lines.

Answer: (C)

Differential protection for a transformer bank detects internal faults by comparing the currents on the transformer’s primary and secondary sides. Current transformers on both sides feed a relay, and under normal operation the current going into the transformer equals the current coming out (adjusted for the turns ratio), so the differential signal is essentially zero. If an internal fault occurs—such as a turn-to-turn short, a winding-to-ground fault, or a winding-to-winding fault—the balance is disrupted and a tributary current pattern appears between the two sides. The differential relay senses this imbalance almost immediately and trips to isolate the transformer before damage spreads. This is distinct from monitoring oil temperature or measuring line impedance, and from external feeder faults, which are handled by other protections.