Which protection is most directly associated with detecting and interrupting backward power flow from the grid into the generator, indicating islanding?

• A. Loss-of-field protection
• B. Differential protection
• C. Stator-to-ground protection
• D. Reverse power protection

Answer: (D)

Reverse power protection focuses on the direction of real power flowing through the machine. In normal operation, the generator delivers power to the grid, so the flow is out of the machine. If the grid goes down or a switch opens but the generator is still connected and may be energized by the grid or by local sources, power can start flowing back into the generator. Detecting this reverse flow is a direct indication of islanding, where part of the system remains energized while isolated from the main grid. The protection then trips to interrupt the connection and prevent backfeeding or damage.

The other protections don’t directly monitor the direction of power flow. Loss-of-field protection relates to losing excitation, differential protection looks for internal faults by comparing currents, and stator-to-ground protection detects ground faults in the stator winding. None of these specifically indicate islanding via reverse power flow.