[Summary] Solutions to the demagnetization, out-of-step, and oscillation of the generator set

excitationGenerator setThe three most common problems are: loss of magnetism, loss of synchronization, and oscillation. Although Jiangsu Haixing Power has explained these problems before, many customers still have questions. Therefore, this article summarizes the solutions.

Loss of magnetism
PhenomenonGenerator setDuring operation, after the generator loses excitation due to open circuit, short circuit of the excitation circuit, small excitation current or rotor circuit failure, the relevant indications of the generator and excitation system will react as follows:
1. The rotor ammeter and voltmeter indicate zero or close to zero;
2. The stator voltmeter indication is significantly reduced;
3. The electronic ammeter indication rises and shakes;
4. The indication of the generator’s active power meter decreases and swings;
5. The generator active power meter indicates a negative value.
The generator loses excitation current during operation, causing the rotor's magnetic field to disappear. This may be caused by the excitation switch accidentally tripping, the exciter or semiconductor excitation system malfunctioning, the rotor circuit disconnection, etc. When demagnetization occurs, the rotor magnetic field disappears, the electromagnetic torque decreases, excess torque appears, and synchronization is out of synchronization. The rotor and stator have relative speeds, and the stator magnetic field cuts the rotor surface at slip speed, causing current to be induced on the rotor surface. The interaction between this current and the nail's rotating magnetic field produces a torque, often called an asynchronous torque. This asynchronous torque is also a resistance torque here. It acts as a brake. The generator rotor does work in the process of overcoming this torque. The mechanical energy is turned into electrical energy, which can continue to send reactive power to the system. The rotation speed of the generator will not increase, because the higher the rotation speed, the greater the asynchronous torque. In this way, the synchronous generator is equivalent to becoming an asynchronous generator. In the asynchronous state, the motor absorbs reactive power from the system, supplies the stator and then the rotor to generate a magnetic field, and sends reactive power to the system. If this motor can generate a large asynchronous torque with a small slip, then in the demagnetization state It can also carry larger loads, even with the same load. Two points should be noted in this state: first, the stator current cannot exceed the rated value; second, the temperature of the rotor part cannot exceed the allowable value.

'Impact' What are the adverse effects after the generator set loses magnetism? This issue should be explained in two aspects: one is the impact on the generator itself, and the other is the harm to the system.
First, the harm to generators is mainly manifested in the following aspects:
1. Due to the occurrence of slip, a differential frequency current will be induced on the rotor surface. The differential frequency current produces additional losses in the rotor circuit, which increases the heating of the rotor, and in severe cases can cause the rotor to burn out. Especially for large units with direct cooling and high utilization, the heat capacity margin is relatively reduced, and the rotor is prone to overheating;
2. After the demagnetized generator switches to asynchronous operation, the equivalent reactance of the generator decreases, and the reactive power sent from the system to the generator increases. The active power of the belt before loss of magnetism is greater. The greater the slip, the smaller the equivalent reactance, and the greater the reactive power sent by the system. Therefore, loss of magnetism under heavy load will cause the generator stator to overheat due to overcurrent in the stator winding;
3. During asynchronous operation, the torque of the generator changes, so the active power undergoes serious periodic changes, causing the generator, rotor and base to be impacted by abnormal mechanical forces, threatening the safety of the unit;

Second, after the generator loses excitation, the impact on the system is as follows:
1. The generator after demagnetization will absorb reactive power equivalent to the rated capacity from the power system, causing the voltage of the power system to drop. If the reactive power reserve capacity of the power system is insufficient, the voltage of part of the system adjacent to the demagnetized generator will be lower than the allowable value, threatening the stable operation of the load and various power sources, and even causing the voltage of the system to collapse and collapse. This is a sign of generator failure. Serious consequences caused by magnetism;
2. The voltage drop of the system caused by the loss of excitation of a generator will cause the excitation regulator of the adjacent generator to operate and increase its reactive power output. Therefore, these generators, transformers and lines will cause overcurrent, leading to large-scale power outages and expanding the The scope of the failure.

[Solution] After the generator set loses excitation (no excitation), what should the operator do?
The generator set will run asynchronously without excitation. Judging from the test results, the unit can operate with load, but only 50%-60% of the rated capacity. The running time of small and medium-sized units does not exceed 30 minutes. Large units (above 200MW) ) can only run for 15 minutes. Large-scale units are equipped with loss-of-excitation protection devices. The loss of excitation protection device is equipped with a voltage disconnection locking device and a low-voltage relay. When the low-voltage relay does not operate (the bus voltage is not lower than the allowable value), the loss of excitation protection will not operate.
1. When the generator set loses excitation, the loss of excitation protection operates, the 'generator loss protection trip' signal is sent, and the main switch of the generator trips, indicating that the protection has been activated to demagnetize and handle it as a generator accident trip (timely Check the power switching situation in the factory);
2. If the loss of excitation protection refuses to operate, immediately manually deload the generator;
3. During the demagnetization process of the generator set, attention should be paid to adjusting the stator current and reactive power of other normally operating generators.
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Oscillation, loss of step
The main causes of 'phenomenon' causing generator oscillation include: sudden change in load; removal of the output line and transformer between the two power supplies; sudden tripping of the generator, especially large-capacity units; sudden change in input torque of the prime mover; sudden short-circuit failure in the system, etc. Short circuit fault is usually the main cause of generator oscillation. When the synchronous generator is operating normally, the connection between the stator magnetic poles and the rotor magnetic poles can be seen as elastic magnetic lines of force. When the load increases, the power angle will increase, which is equivalent to lengthening the magnetic field lines; when the load decreases, the power angle will decrease, which is equivalent to shortening the magnetic field lines. When the load suddenly changes, due to the inertia of the rotor, the rotor power angle cannot immediately stabilize at the new value, but has to swing several times around the new stable value. This phenomenon is called oscillation of the synchronous generator. There are two types of oscillation: one is that the amplitude of the oscillation becomes smaller and smaller, the swing of the power angle gradually attenuates, stabilizes at a new power angle, and still runs stably at synchronous speed, which is called synchronous oscillation; the other is The amplitude of the oscillation becomes larger and larger, and the power angle continues to increase until it goes out of the stable range, causing the generator to lose synchronization and the generator enters asynchronous operation, which is called asynchronous oscillation.
1. The indication on the stator ammeter exceeds the normal value, and it moves violently back and forth. This is because the angle between the parallel potentials changes, and an electromotive force difference occurs, causing a circulating current to flow between the generators. Due to the swing of the rotor speed, the angle between the electromotive force is sometimes large and sometimes small, and the torque and power are also sometimes large and sometimes small, resulting in a circulating current that is sometimes large and sometimes small, so the stator current pointer swings back and forth. This circulating current plus the original load current may exceed the normal value.
2. The pointer indications of the stator voltmeter and other bus voltmeters are lower than the normal value and swing back and forth. This is because the angle between the potential of the out-of-step generator and other generators is changing, causing voltage swings. Because the current is larger than normal, the voltage drop is also large, causing the voltage to be low.
3. The active load and reactive load swing greatly and violently. This is because the power sent out by the generator during the oscillation process when it is not out of step is sometimes large and sometimes small, and when it is out of step, it sometimes sends out active power and sometimes absorbs active power.
4. The pointers of the rotor voltage and ammeter swing around the normal values. When the generator oscillates or loses step, an alternating current will be induced in the rotor winding and fluctuate with the fluctuation of the stator current. This current is superimposed on the original excitation current, causing the rotor ammeter pointer to swing near the normal value.
5. The frequency meter swings high and low. When oscillating or out of step, the output power of the generator changes continuously, and the torque acting on the rotor also changes accordingly, so the rotation speed also changes accordingly. .
6. The generator emits a rhythmic sound, which is in time with the swing rhythm of the meter pointer.
7. The low-voltage relay overload protection may trigger an alarm.
8. In the control room, the rhythmic movement and release sounds of the relevant relays can be heard, and their rhythm is in sync with the swing rhythm of the meter.
9. The pointer of the balance meter of the turbine generator speed regulator swings; there may be a signal that the shear pin is sheared; the oil pump motor of the oil pressure tank starts frequently.

'Cause' According to operating experience, the causes of generator oscillation and loss of synchronization are:
1. Static stability is destroyed. This often occurs when the operating mode is changed so that the delivered power exceeds the allowable power at that time.
2. The impedance between the generator and the power grid suddenly increases. This situation often occurs when a short circuit occurs somewhere in the power grid that is connected to the generator, and some parallel components are cut off, such as one circuit in a double-circuit line is disconnected, one of the parallel transformers is cut off, etc.
3. The power of the power system suddenly becomes unbalanced. For example, a large-capacity unit suddenly sheds load and a tie line trips, causing a serious power imbalance in the system.
4. The large unit loses magnetism. The large unit loses magnetism and absorbs a large amount of reactive power from the system, causing insufficient reactive power in the system and a significant drop in system voltage, causing the system to lose stability.
5. The prime mover speed control system fails. The prime mover speed control system fails, resulting in a sudden change in the input torque of the prime mover, a sudden increase or decrease in power, causing the generator torque to lose balance and causing oscillation.
6. The potential is too low or the power factor is too high when the generator is running. (
7. The asynchronous parallelization between power supplies failed to bring in synchronization.

The difference between oscillation caused by single machine out-of-synchronization and systematic oscillation
1. The meter swing range of the out-of-step unit is larger than that of other units;
2. The active power meter pointer of the out-of-step unit swings in the opposite direction to that of other units. The active power meter of the out-of-step unit may swing at full scale, while other units swing around the normal value.
3. When the system oscillates, the swings of all generator meters are synchronous.

'Solution' When oscillation or out-of-step occurs, you should quickly determine whether it is caused by misoperation of the factory, and observe whether a certain generator has lost excitation. If the factory is in normal condition, you should know whether the system has malfunctioned to determine the cause of oscillation or out-of-synchronization. The treatment of generator oscillation or demagnetization is as follows:
1. If it is not caused by the loss of excitation of a certain generator, the excitation current of the generator should be increased immediately to increase the electromotive force of the generator, increase the power limit, and improve the stability of the generator. This is because the increase in excitation current increases the pulling force between the stator and rotor magnetic poles, weakens the inertia of the rotor, and pulls the generator into synchronization when it reaches the equilibrium point. At this time, if the generator excitation system is in a strong excitation state, there should be no intervention within 1 minute.
2. If it is caused by the high power factor of a single machine, the active power should be reduced and the excitation current should be increased. This can not only reduce the rotor inertia, but also increase the unit's stable operation capability by increasing the power limit.
3. When the oscillation is caused by a system fault, the excitation current of each generator should be increased immediately and handled according to the factory's position in the system. If the plant is at the sending end and it is a high-frequency system, the active power of the unit should be reduced; conversely, if the plant is at the receiving end and it is a low-frequency system, the active power should be increased and emergency pulling measures should be taken to increase the frequency.
4. If the oscillation is caused by the loss of synchronization of a single machine, if the above measures are taken and the synchronization state is not achieved in time, the unit and the system can be decoupled according to the on-site regulations, or the two parts of the system in the same period can be decoupled according to the dispatching requirements.
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