What does differential protection monitor in a generator?

• A. The difference between currents entering and leaving the generator or transformer; detects internal faults.
• B. Voltage fluctuations across the stator windings.
• C. The speed of the generator rotor.
• D. Ambient temperature around the equipment.

Answer: (A)

Differential protection watches for current imbalance across the protected zone. In a generator, that means comparing the current entering the machine with the current leaving it (or, equivalently, the currents at the two sides of the generator/transformer boundary). If an internal fault occurs, fault current is generated inside the machine and does not fully pass out of the protected area, causing a noticeable difference between the two currents. The differential relay uses current transformers on both sides and trips when the difference exceeds a set threshold, fast enough to isolate the fault and limit damage.

The other options aren’t about current balance. Voltage fluctuations across stator windings relate to voltage protection, not the current difference used here. Rotor speed is a dynamic parameter used by speed or frequency protections. Ambient temperature is for thermal protection.