Which protection scheme trips a generator if there is a loss of excitation?

• A. Differential protection
• B. Overcurrent protection
• C. Loss-of-field protection
• D. Reverse power protection

Answer: (C)

Loss of excitation means the field winding loses its current, so the generator’s internal emf collapses and it can no longer regulate voltage or stay synchronized with the grid. When the excitation is lost, the machine may draw abnormal current or fall out of step with the system, risking damage to the rotor, windings, and connection to the bus. The protection that specifically monitors the field current and trips the machine when excitation is lost (or falls away rapidly) is designed for this situation. It acts quickly to prevent the unsafe condition from developing. Other protections serve different faults—differential protection checks for internal faults, overcurrent protection trips on excessive current, and reverse power protection trips if the generator starts to act as a motor.