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The Effect of Generator Loss of Excitation

Sep. 24, 2019

Generator loss of excitation fault means that the excitation current provided by the excitation system suddenly disappears completely or partially. After the synchronous generator is out of magnetic, it will be transferred to the asynchronous running state, and the reactive power will be absorbed from the original reactive power conversion.

The causes of generator loss of magnetism fault are: generator rotor winding fault, excitation system fault, automatic demagnetization switch trip and circuit fault.

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.


generator

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.

Why is it necessary to switch the auxiliary power with instantaneous stop when the generator loses its magnetism?

After the generator is demagnetized, the system will not operate normally, and the frequency and voltage will be affected. If the parallel method is adopted to switch the auxiliary power, it will affect the operation of the non-fault equipment and the system, and it may also cause non-synchronization and expand the abnormal operation range of the system. Therefore, the instantaneous stop method is used to switch the auxiliary power.

What is the effect of high or low terminal voltage on the generator itself?

The effect of high voltage on the motor:

a. It is possible to raise the temperature of the rotor winding beyond the allowable value;

b. The stator core temperature rises;

c. Local high temperature may occur in the stator structural components;

d. A threat to the insulation of the stator winding.

The effect of low voltage on the motor:

a. Reduce the stability of the operation, one is the stability of the parallel operation, and the other is the stability of the generator voltage regulation.

b. The stator winding temperature may rise.

What is the effect of high or low frequency on the generator itself?

The effect of high frequency on the generator:

The highest frequency is not more than 52.5Hz, that is, 5% of the rated value is exceeded, the frequency is increased, mainly due to the limitation of the mechanical strength of the rotating machine, the frequency is high, the rotational speed of the motor is high, and the centrifugal force on the rotor is increased, which is easy to make some parts of the rotor damage.


The effect of low frequency on the generator:

a. The low frequency causes the rotor speed to decrease, so that the air volume that is blown into the fan at both ends is lowered, the cooling condition of the generator is deteriorated, and the temperature of each part is increased.

b. The low frequency causes the temperature of the rotor coil to increase, otherwise the output power will be reduced.

c. The low frequency may also cause the turbine to break the blade.

d. When the frequency is reduced, in order to keep the terminal voltage unchanged, it is necessary to increase the magnetic flux, which makes it easy to saturate the stator core, the magnetic flux escapes, causing local high temperature of some structural components of the frame, and even spark.

e. When the frequency is low, the rotation speed of the auxiliary power motor is reduced, resulting in output decrease, and also has a bad influence on the safety of the user's electricity, product quality, efficiency, and the like.

f. The frequency is low and the voltage is low. This is because the size of the induction potential and the speed of the fuel pipe, while the low speed of the generator also reduces the output of the coaxial exciter, affecting the reactive power output.

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