Generator loss of excitation fault refers to the sudden disappearance or partial disappearance of the excitation of the generator. The causes of loss of excitation include: rotor winding failure, exciter failure, false tripping of automatic deexcitation switch, damage to certain components or circuit failures in the semiconductor excitation system, and incorrect operation.

Due to asynchronous operation, the Rotor machine speed of the generator is greater than the synchronous speed. Due to the slip, the stator winding current increases, and the rotor winding generates induced current, causing additional heating of the stator and rotor windings. Analysis shows that the loss of excitation of a generator can cause varying degrees of harm to the power system and the generator itself, which can be summarized in the following aspects.

Hazards to the generator itself:

(1) After the generator loses excitation, the leakage magnetic field at the end of the stator increases, causing overheating of the end components and the end iron core.

(2) After asynchronous operation, the equivalent reactance of the generator decreases. Therefore, the reactive power absorbed from the system increases, causing the stator winding to overheat.

(3) The differential current in the generator rotor winding generates additional losses in the rotor winding, causing the rotor winding to heat up.

(4) For large direct cooling turbogenerators, the maximum average asynchronous torque is relatively small, the inertia constant is also relatively reduced, and the rotor is significantly asymmetric in both vertical and horizontal axes. Due to these reasons, the torque and active power of a generator with loss of excitation under heavy load will undergo severe oscillations. This impact is more severe on hydroelectric generators.

Hazards to the power system:

(1) After the generator loses excitation, due to the swing of active power and the decrease of system voltage, it may cause a loss of synchronization between adjacent normally operating generators and the system, causing system oscillation.

(2) The loss of excitation of the generator causes a large amount of reactive power shortage in the system. When the reactive power reserve in the system is insufficient, it will cause a voltage drop. In severe cases, it can cause voltage collapse and system collapse.

(3) The loss of excitation of a generator causes a voltage drop, and other generators in the system will increase their reactive power output under the action of the automatic excitation device. As a result, certain generators, transformers, and transmission lines may experience overcurrent, and backup protection may expand the fault range due to overcurrent actions