Effects of Generator Demagnetization

The influence of generator loss of excitation on the system mainly includes:

1. Generators with low excitation and loss of excitation absorb reactive power from the system, causing the voltage of the power system to drop. If the reactive power reserve in the power system is insufficient, the voltage at some adjacent points in the power system will be lower than the allowable value, destroy the stable operation between the load and each power source, and even cause the power system voltage to collapse and collapse.

2. When a generator loses excitation, due to the voltage drop, other generators in the power system will increase their reactive power output under the action of the automatic adjustment excitation device, so that some generators, transformers, or lines overcurrent, its backup protection may malfunction due to overcurrent, which will expand the scope of the accident.

3. After a generator is demagnetized, due to the swing of the active power of the generator and the drop of the system voltage, it may cause out-of-synchronization between adjacent normal operating generators and the system, or between various parts of the power system. make the system oscillate.

4. The greater the rated capacity of the generator, the greater the reactive power deficit caused by low excitation and loss of excitation, and the smaller the capacity of the power system, the smaller the ability to compensate for this reactive power deficit. 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 impact on the power system will be.

Effects of Generator Demagnetization插图

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

1. Due to the slip after the engine loses magnetism, a differential frequency current appears in the generator rotor circuit, and the differential frequency current produces a loss in the rotor circuit. If it exceeds the allowable value, the rotor will overheat. Especially for large direct cooling high-power units, the thermal capacity margin is relatively reduced, and the rotor is more likely to overheat. The differential frequency current on the surface of the rotor may also cause serious local overheating or even burns on the contact surface of the rotor body slot wedge and the retaining ring.

2. After the demagnetization generator enters asynchronous operation, the equivalent reactance of the generator decreases and absorbs reactive power from the power system. The greater the active power before the demagnetization, the greater the slip, and the smaller the equivalent reactance, the greater the reactive power absorbed. After demagnetization under heavy load, the generator stator will overheat due to overcurrent.

3. For the directly cooled large steam turbine generator with a high power rate, the maximum value of the average asynchronous torque is small, the inertia constant is also relatively reduced, and the rotor is also obviously asymmetrical on the vertical and horizontal axes. For these reasons, after the demagnetization under heavy load, the torque and active power of this generator will fluctuate violently periodically. For the hydraulic generator, due to the small maximum value of the average asynchronous torque and the asymmetry of the rotor on the vertical axis and the horizontal axis, a similar situation will appear when it is running under heavy load with demagnetization. In this case, there will be a large motor torque that even exceeds the rated value to periodically act on the shafting of the generator and transmit it to the machine base through the stator. At this time, the slip also changes periodically, and its maximum value may reach 4%~5%, and the generator seriously overspeeds periodically. These situations directly threaten the safety of the crew.

4. During demagnetization operation, the magnetic flux leakage at the end of the stator increases, which will overheat the components at the end and the iron core of the side section.

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