Generator Loss of Excitation

by Starlight Generator dieselgeneratortech

The effect of generator loss of excitation on the system:

1. Low-excitation and magnetic-loss generators absorb reactive power from the system, causing the voltage of the power system to decrease. If the reactive power reserve in the power system is insufficient, the voltage in some adjacent points in the power system will be lower than the allowable value, which destroys the stable operation between the load and each power supply, and even collapses the power system voltage.

2. When a generator is demagnetized, due to the voltage drop, other generators in the power system will increase their reactive power output under the action of the automatic adjusting excitation device, thereby causing some generators, transformers or circuits overcurrent, its backup protection may be mis-operated due to overcurrent, which will widen the scope of the accident.

3. After a generator loses its magnetism, due to the swing of the generator's active power and the decrease of the system voltage, it may cause the step-out between the adjacent normal operation generator and the system, or between the parts of the power system, causing the system to oscillate.

4. The larger the rated capacity of the generator, the larger the reactive power shortage caused by low excitation and demagnetization, and the smaller the capacity of the power system, the smaller the ability to compensate for this reactive power shortage. Therefore, the greater the ratio of the unit capacity of the generator to the total capacity of the power system, the more serious the adverse effect on the power system.

The main effects of generator loss of magnetism on the generator itself are as follows:

1. Due to the slip after the generator loses magnetism, the differential frequency current occurs in the rotor circuit of the generator, and the differential frequency current causes loss in the rotor circuit. If the allowable value is exceeded, the rotor will be overheated. Especially for high-power large gensets that are directly cooled, the thermal capacity margin is relatively low and the rotor is more prone to overheating. The differential frequency current of the rotor surface layer can also cause severe local overheating or even burns on the contact surface of the rotor body wedge and the guard ring.

2. After the loss-generating generator enters the asynchronous operation, the equivalent reactance of the generator is reduced, and the reactive power is absorbed from the power system. The greater the active power before the demagnetization, the larger the slip, the smaller the equivalent reactance, and the greater the absorbed reactive power. After losing magnetism under heavy load, the generator stator will overheat due to overcurrent.

3. For large turbo-generators with direct cooling and high power, the maximum value of the average asynchronous torque is small, the inertia constant is relatively low, and the rotor is also obviously asymmetrical in terms of the vertical axis and the horizontal axis. For these reasons, after the loss of excitation under heavy load, the generator torque and active power are subject to severe periodic oscillation.

In this case, the motor torque, which has a large or even exceeding the rated value, is periodically applied to the shafting of the generator and transmitted to the base through the stator. At this time, the slip is also periodically changed, and its maximum value may reach 4% to 5%, and the generator periodically overspeeds. These situations directly threaten the safety of the generating set.

4. During the demagnetization operation, the leakage of the stator end is enhanced, which will cause the end parts and slide section core to overheat.