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'Summary' Solutions for demagnetization, loss of step and oscillation of generator sets

Views: 0     Author: Site Editor     Publish Time: 2011-05-06      Origin: Site

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The three most common problems in excitation generator sets : demagnetization, loss of step, and oscillation. Although Jiangsu Haixing Dynamics explained these issues separately before, many customers still have questions, so we will summarize this solution.

'Demagnetization'
'Phenomenon' During the operation of the generator set , after the generator is demagnetized due to the open circuit, short circuit, small excitation current or rotor circuit failure, the relevant meter of the generator and the excitation system are as follows:
1. The rotor ammeter and voltmeter indicate zero or close to zero;
2. The stator voltmeter indicates significantly lower;
3. The electronic ammeter indicates higher and shake;
4. The generator's active power meter indicates lower and swing;
5. The generator's active power meter indicates negative value.
The generator loses the excitation current during operation, causing the magnetic field of the rotor to disappear. This may be caused by accidental tripping of the excitation switch, failure of the excitation machine or semiconductor excitation system, and disconnection of the rotor circuit. When the demagnetization occurs, the rotor magnetic field disappears, the electromagnetic torque decreases, and excess torque occurs, which is out of synchronization. The rotor and the stator have a relative speed. The stator magnetic field cuts the rotor surface at a slip speed, so that the rotor surface induced current. This current and the rotational magnetic field of the nail produce a torque, which is often called asynchronous torque. This asynchronous torque is also a resistance torque here. It acts as a braking function. The generator rotor does work in the process of overcoming this torque, turning mechanical energy into electrical energy, and can continue to send reactive power to the system. The speed of the generator will not increase, because the higher the 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 the motor can generate a large asynchronous torque under a very small slip, then it can still carry a large load in the demagnetized state, and even the load it carries remains unchanged. Two points should be paid attention to in this state: one is that the stator current cannot exceed the rated value; the other is that the temperature of the rotor part cannot exceed the allowable value.

'Impact' What adverse effects will the generator set have after the magnetization is lost? 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 the generator is mainly reflected in the following aspects:
1. Due to the occurrence of slip, the frequency current of the frequency will be induced on the rotor surface. The differential frequency current generates additional losses in the rotor circuit, which increases the heat generation of the rotor, and in severe cases, the rotor may burn. Especially for large units with high utilization, the heat capacity margin of the unit is relatively reduced and the rotor is prone to overheating;
2. After the demagnetization generator is turned into asynchronous operation, the equivalent reactance of the generator is reduced, and the reactive power sent from the system to the generator increases. The greater the active power of the demagnetized front belt. The greater the slip, the smaller the equivalent reactance, and the greater the reactive power sent by the system. Therefore, demagnetization under heavy loads will cause the generator stator to overheat due to overcurrent of the stator winding;
3. During asynchronous operation, the torque of the generator changes, so the active power will undergo serious periodic changes, causing the generator, rotor and base to be impacted by abnormal mechanical forces, which will threaten the safety of the unit;

Second, after the generator is demagnetized, 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 some systems adjacent to the demagnetized generator will be lower than the allowable value, threatening the stable operation of the load and the power supply, and even causing the voltage of the system to collapse and collapse. This is a serious consequence of the generator demagnetization;
2. The demagnetization of a generator causes the voltage of the system to drop, which will cause the adjacent generator excitation regulator to operate and increase its reactive output. Therefore, these generators, transformers and lines cause overcurrent, resulting in large-scale power outages, and expand the range of faults.

'Solution' What should the operator handle the generator set after it loses magnetism (no excitation situation)?
The generator set will operate asynchronously without excitation. Judging from the test situation, the unit can operate with load, but only 50%-60% of the rated capacity. The operating time of small and medium-sized units does not exceed 30 minutes, and large units (more than 200MW) can only operate for 15 minutes. Large units are equipped with demagnetization protection devices. The demagnetization 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 demagnetization protection will not operate.
1. When the generator set loses magnetism, the demagnetization protection acts, and the 'generator demagnetization protection trips' signal is issued, and the generator main switch trips to indicate that the protection has been demagnetized and demagnetized. The trip is handled according to the generator accident (timely check the factory power switching situation);
2. If the demagnetization protection refuses to move, the generator will be demagnetized immediately;
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.
Related articles: Symptoms of demagnetization of excitation generator sets Jiangsu Haixing Power Rejuvenation

'Obliography, Loss of Steps
' The main reasons for generator oscillation are: sudden load change; removal of output lines and transformers between the two power supplies; sudden tripping of generators, especially large-capacity units; sudden change in the input torque of prime mover; sudden short circuit failure in the system, etc. Short circuit failures are usually the main cause of generator oscillation. When the synchronous generator is operating normally, the stator pole and the rotor pole can be regarded as an elastic magnetic line connection. When the load increases, the work angle will increase, which is equivalent to stretching the magnetic force line; when the load decreases, the work angle will decrease, which is equivalent to shortening the magnetic force line. When the load suddenly changes, due to the inertia of the rotor, the rotor work angle cannot be stabilized immediately at the new value, but it has to swing several times around the new stable value. This phenomenon is called the oscillation of the synchronous generator. There are two types of oscillation: one is that the amplitude of the oscillation becomes smaller and smaller, the oscillation of the work angle gradually decays, and it is stable at a certain new work angle, and it is still running at a synchronous speed, which is called synchronous oscillation; the other is that the amplitude of the oscillation becomes larger and larger, and the work angle continues to increase until it breaks out of the stable range, causing the generator to lose its steps, and the generator enters asynchronous operation, which is called asynchronous oscillation.
1. The stator ammeter indicates that it exceeds the normal value and is moving vigorously back and forth. This is because the angle between the parallel potentials changes, and an electromotive force difference occurs, causing the generators to flow through the circulation. 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 sometimes large and sometimes small, which causes the circulation to be sometimes large and sometimes small, so the pointer of the stator current swings back and forth. This circulation plus the original load current may exceed the normal value.
2. The stator voltmeter and other busbar voltmeter pointers indicate lower than normal value and swings back and forth. This is because the angle between the out-of-step generator and other generators changes, causing voltage swing. Because the current is larger than normal and the voltage drop is also larger, the voltage is lower.
3. The active load and reactive load swing violently. Because the power sent by the generator during the oscillation without losing steps is sometimes large or small, and sometimes it is delivered when losing steps is sometimes active, and sometimes it is absorbed
. 4. The rotor voltage and the pointer of the ammeter swing near the normal value. When the generator oscillates or loses steps, alternating current will be induced in the rotor winding and fluctuates 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 at high and low. When oscillating or losing steps, the output power of the generator continues to change, and the torque acting on the rotor also changes accordingly, so the speed also changes accordingly. .
6. The generator makes a rhythmic sound and beats in conjunction with the meter pointer swinging rhythm.
7. The low-voltage relay overload protection may be alarmed.
8. In the control room, you can hear the rhythmic movements and releases of the relay, and the rhythm is in line with the meter swing rhythm.
9. The balance meter pointer of the water turbine generator speed regulator swings; there may be a signal of cutting the pin; the oil pump motor of the oil pressure tank starts frequently.

'Cause' According to operating experience, the reasons for generator oscillation and loss of steps are
1. Static and stable damage. This often occurs when the operation mode changes, causing the delivery power to exceed 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 in the power grid where the generator is connected, and some parallel components are cut off, such as one back of the double-return circuit is disconnected, and one of the parallel transformers is cut off.
3. The power of the power system suddenly becomes unbalanced. If a large-capacity unit suddenly drops the load and a certain contact line trips, causing serious imbalance in the system power.
4. The large unit loses magnetism. The large unit loses magnetism, absorbs a large amount of reactive power from the system, causing the system to be insufficient reactive power, and the system voltage drops significantly, resulting in the system losing stability
. 5. The prime mover speed regulation system fails. The prime mover speed regulation system fails, causing the prime mover input torque to suddenly change, the power rises or falls suddenly, causing the generator torque to lose balance and cause oscillation
6. The potential of the generator is too low or the power factor is too high when the generator is running. (
7. The synchronous synchronization between power supplies cannot be pulled in.

The difference between oscillation caused by a single-machine failure and systematic oscillation
1. The meter swing amplitude of the meter of the stepless unit is larger than that of other units;
2. The swing direction of the active power meter of the stepless unit is exactly the opposite of that of other units. The swinging of the active power meter of the stepless unit may be full of scale, and other units swing near the normal value.
3. During systematic oscillation, the swing of all generator meters is synchronized.

'Solution' When oscillation or loss of steps occurs, you should quickly determine whether it was caused by the factory's misoperation and observe whether a generator has lost magnetism. If the factory is in normal circumstances, you should know whether the system has failed to determine the cause of oscillation or loss of steps. The oscillation or demagnetization of the generator is as follows:
1. If it is not caused by a certain generator, the excitation current of the generator should be increased immediately to increase the generator electromotive force, increase the power limit, and improve the generator stability. This is due to the increase of excitation current, the tension between the fixed and rotor poles increases, weakening the inertia of the rotor, and pulling into synchronization when the generator reaches the equilibrium point. At this time, if the generator excitation system is in a strong excitation state, no intervention should be made within 1 min.
2. If it is caused by a single machine's high power factor, the active power should be reduced and the excitation current should be increased. This not only reduces the rotor inertia, but also increases the unit's stable operation ability due to the increase of the power limit.
3. When the oscillation is caused by a system failure, the excitation current of each generator should be increased immediately and processed according to the factory's position in the system. If our factory is at the delivery end and is a high-frequency system, the active power of the unit should be reduced; on the contrary, if our factory is at the receiving end and is a low-frequency system, the active power should be increased and emergency road pulling measures should be taken to increase the frequency.
4. If the oscillation caused by a single-machine failure and the above measures are taken in time and still not entered the synchronization state, the unit can be de-arranged with the system according to the on-site regulations, or the two parts of the system at the same time can be de-arranged according to the scheduling requirements.
Related articles: After-sales review: Solve the problems of oscillation and loss of steps in synchronous generator sets


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