If a generator lags or leads in rotation, what protective action occurs?

If a generator lags or leads in rotation, what protective action occurs?

• A. Protective equipment just outside the generating station automatically opens
• B. The main breaker trips
• C. The turbine stops
• D. An alarm sounds

Answer: (A)

When a generator falls out of step with the grid—either ahead or behind in rotation—the system detects a loss of synchronism or excessive circulating power. Protective relays at the interconnection point are set to respond to this condition by quickly opening the line switch just outside the generating station. Opening that external switch electrically isolates the generator from the rest of the network, preventing damaging torque interactions, instability, or faults from spreading. This automatic line-opening action is the protective measure designed specifically for out-of-sync conditions, which is why it’s the best answer. The main breaker trip, stopping the turbine, or sounding an alarm can occur in other situations, but they are not the automatic protective action intended for a lagging or leading generator.

When a generator falls out of step with the grid—either ahead or behind in rotation—the system detects a loss of synchronism or excessive circulating power. Protective relays at the interconnection point are set to respond to this condition by quickly opening the line switch just outside the generating station. Opening that external switch electrically isolates the generator from the rest of the network, preventing damaging torque interactions, instability, or faults from spreading. This automatic line-opening action is the protective measure designed specifically for out-of-sync conditions, which is why it’s the best answer. The main breaker trip, stopping the turbine, or sounding an alarm can occur in other situations, but they are not the automatic protective action intended for a lagging or leading generator.