Causes and harms of generator loss of magnetism

First, the cause of generator loss of magnetism

During the normal operation of the generator, the excitation suddenly disappears completely or partially, which is called generator demagnetization. The reason why the generator loses excitation can be summarized as open circuit or short circuit of the excitation circuit, including the excitation of the exciter, excitation or excitation circuit, accidental excitation switch, improper switching excitation, loss of excitation power of the excitation system, rotor winding or excitation circuit. The open circuit or the rotor winding is severely short-circuited, the semiconductor excitation system is faulty, and the rotor slip ring is ignited or blown.

1. Excitation variable fault trip causes generator to lose magnetism

Due to the insulation manufacturing defects of the transformer, or the insulation defects gradually deteriorated during operation, a discharge phenomenon occurs, causing the excitation change protection action to trip, and the demagnetization protection action causes the unit to trip. Strictly implement procedures and standards, conduct regular tests, implementation, and troubleshooting. In accordance with relevant regulations and standards, the implementation of regular tests on insulation professionally will be carried out.

2, the degaussing switch trip causes the generator to lose magnetism

The reasons for the de-excitation switch trip include: ( 1) mis-excitation of the de-excitation switch trip command on the DCS; (2) the exit relay fault issuing the de-excitation switch trip command; (3) the control unit electrical stand-off degauss switch trip button contact pull-out Tripping command; (4) Excitation small room local control panel manually separate the de-excitation switch; (5) De-excitation switch control loop cable insulation drop; (6) Switch body mechanical tripping de-excitation switch; (7) DC system instantaneous grounding Causes the de-excitation switch to trip.

3. Excitation slip ring ignition causes generator loss of magnetism

The cause of the accident is that the pressure of the carbon brush compression spring is uneven, resulting in uneven distribution of some carbon brush currents, resulting in excessive current of individual carbon brushes, causing heat. In addition, the carbon brush is dirty, polluting the contact surface of the carbon brush and the slip ring, causing the contact resistance of some carbon brushes and slip rings to increase and then sparking. In addition, the wear of the carbon brushes of the positive and negative electrodes is not balanced, and the wear of the negative electrode is always higher than that of the positive electrode. Severe, due to the serious wear and tear, the surface roughness of the slip ring is increased, and the slip ring is fired because it is not controlled in time.

4, DC system grounding causes the generator to lose magnetism

After the positive current is grounded in the DC system, due to the distributed capacitance of the long cable, the voltage across the capacitor cannot be abruptly changed, causing the external circuit of the generator to extinguish the magnetic circuit. The long cable capacitance current flows through the external tripping outlet intermediate relay, and the relay action jumps to develop the motor. The magnetic switch causes the generator to lose its magnetic protection action.

5, the excitation adjustment system failure causes the generator to lose magnetism

The generator excitation system regulator EGC board fails, causing the generator excitation regulator rotor overvoltage protection action, causing the loss of magnetic protection action to trip.

6. The rectifier cabinet is completely stopped, causing the generator to lose magnetism.

During the start of the electric pump, the system voltage is reduced, and the excitation system issues an auxiliary power failure alarm. The auxiliary circuit of the switching circuit relay is too large, causing the power supply to fail to switch, and the rectifier cabinet fan cannot operate normally, causing the rectifier cabinet to over-temperature trip. Magnetic protection action, the unit is out of service. The silver plating layer of the AC side power switch contact of the rectifier cabinet is thin or of poor quality. During operation, copper and air contact the oxide layer, which causes the contact resistance of the contact to increase. As the current increases, the temperature rises and the contact is overheated. During the process, the demagnetization protection action is caused and the unit trips.

Second, the damage of generator loss of magnetism

1. The harm of generator loss of magnetism to the power system

( 1) When the generator is demagnetized, the low-excitation or de-energized generator will absorb the reactive power from the system, which will cause the voltage of the power system to drop. If the power system capacity is small or the reactive power reserve is insufficient, That would cause the generator terminal voltage, the bus voltage on the high voltage side of the step-up transformer, or other nearby voltages to be lower than the allowable value, which will destroy the stable operation between the load and the power supply, and even cause a voltage collapse in the power system. phenomenon.

( 2) When the generator is low-excited or demagnetized, the voltage drops. Other generators in the system will increase their reactive power output under the action of automatically adjusting the excitation device, which will cause some electrical components in the system. If the transformer or the transmission line generates an overcurrent, the backup protection action removes the overload component and expands the fault range.

( 3) When the generator is low-excited or de-magnetized, due to the swing of active power and the drop of the system voltage, it may cause the system to oscillate between the adjacent normal running generator and the system or between the parts of the power system. And a lot of load shedding.

2. The damage of the generator to the generator itself

After the generator is demagnetized, it will not only cause great harm to the power system, but also cause certain harm to the generator itself:

( 1) A frequency difference current occurs in the generator rotor due to a slip occurring when a demagnetization occurs. The loss generated by the difference frequency current in the rotor return will overheat the rotor if it exceeds the allowable value. The differential current flowing through the surface of the rotor also causes severe local overheating or even burns on the contact surface of the rotor body with the wedge and the guard ring.

( 2) After the generator with low excitation or loss of magnetism enters the asynchronous operation state, the equivalent reactance of the generator decreases, and the reactive power absorbed from the system increases continuously. After the heavy load is demagnetized, the generator stator will overheat due to the overcurrent generated.

( 3) For large steam turbine generators that directly cool and utilize high power, the torque and active power of such generators will undergo severe periodic oscillation after heavy load loss. At this time, electromagnetic torque, which is large or even exceeds the rated value, is periodically applied to the generator shaft and transmitted to the frame through the stator. At this time, the slip will also make periodic changes to make the periodicity of the generator severely overspeed.

( 4) When the generator is under excitation or demagnetization, the leakage of the stator end is enhanced, which will cause overheating of the end parts and the side core.