How is reverse power protection typically applied in a generator system?

• A. It trips if power flows backward from the grid into the generator, indicating islanding or decoupled operation.
• B. It trips when the voltage exceeds a preset limit.
• C. It monitors the rate of change of frequency.
• D. It measures phase-angle slip between the generator and the grid.

Answer: (A)

The main idea here is protecting the generator from being driven by the grid. Reverse power protection watches which way real power is flowing at the machines’ terminals. If power would flow backward into the generator—meaning the grid would supply the generator or the unit becomes decoupled and islands—the relay detects negative real power and trips after a short delay. This prevents the generator from acting like a motor or feeding energy into an open or de-energized system, which could be dangerous or damaging. Under normal parallel operation the generator exports power to the grid, so the real power flow is in the forward direction and there’s no trip. The other protections relate to voltage magnitude, how fast the frequency is changing, or the phase relationship with the grid, which protect different conditions and don’t address the direction of power flow.