Reasons for Diesel Power Generators Overexcitation
by Starlight Generator dieselgeneratortechThe control of generator
stator voltage is realized by adjusting the magnitude of rotor excitation
current. The excitation corresponding to the amount of electricity generated is
normal excitation. When the stator operating voltage is higher than the rated
voltage, it is called overexcitation. Conversely, the stator operating voltage
below the rated voltage is called underexcitation. In practice, there is a
definite positive and negative value above or below the rated voltage.
Different generators have different excitation control system and different
excitation values.
Generators and
transformers are composed of iron core windings. If the applied voltage of
windings is U, the turn number is W, the cross section of iron core is S, and
the magnetic density is B, then U = 4.44fWBS. Because W and S is definite
number, so it is written in B=KU/f, K=1./4.44WS, for each particular generator
or transformer, K is a definite number. We can know from B=KU/f: the increase
of voltage and the decrease of frequency can lead to the increase of magnetic
density B.
For diesel generator,
when overexcitation multiple number n=B/BN=(U/UN)/(f/fN)=U*/f* is less than 1,
it will cause damage by overexcitation. It is mainly manifested in the increase
of leakage magnetic field in the back of stator core of generator, the
induction of electric potential in the positioning bar of stator core, and the
closed loop formed by stator core, which flows through the current. It not only
causes serious overheating, but also may cause spark discharge at the contact
surface between positioning bar and stator core, which is very disadvantageous
to hydrogen-cooled generating sets.
When operating diesel power
generators, the reasons for overexcitation maybe as follows:
1.Before the generator
is parallel to the system, the excitation is caused by the incorrect increase
of the excitation current due to the wrong operation. For example, misjudgement
is caused by TV disconnection of generator.
2.During the start-up of
the generator, the generator accompanies the low-speed heater of the steam
turbine rotor. If the voltage is raised to the rated value by mistake, the
overexcitation will be caused by the low-frequency operation of the generator.
3.During shutdown diesel
generator set, main valve closed, outlet circuit breaker disconnected, and
demagnetization circuit breaker refused to move. At this time, the speed of
steam turbine decreases, the automatic excitation regulator strives to keep the
terminal voltage equal to the rated value, so that the generator is subjected
to overexcitation.
4.When the generator
outlet circuit breaker trips, if the automatic excitation regulator operates
manually failure or fails automatically, the voltage and frequency will
increase, but the generator overexcitation will be caused by the slow increase
of frequency.
5.If the generator does
not reduce excitation in time during load rejection, the overvoltage will
occur; even if the terminal voltage can maintain the previous value in the
generator-transformer mode, the overvoltage will occur because the transformer
is no-load.
If the allowable
overexcitation multiple number of diesel generator is lower than that of
transformer, it will be more vulnerable to the harm of overexcitation.
Therefore, large diesel generators need to be equipped with overexcitation
protection with perfect performance. For the generator set with circuit breaker
installed at the generator outlet, in order to protect the generator and
transformer under various operation modes, the overexcitation protection of the
generator and transformer should be set separately.
Overexcitation
protection should be adjusted according to the overexcitation curve provided by
generator manufacturer under full load condition, the setting principle is to
make the excitation curve of the adjusted excitation protection relay below the
generator overexcitation curve as far as possible. That is to say, when the
diesel generators operates under overexcitation conditions, the excitation
protection relay should be able to reliably jump off the circuit breakers on
each side of the main transformer in a shorter time than it can withstand. At
the same time, generators should have a certain level of excitation resistance
under ensuring normal operation, so start-up value of excitation protection
relay is generally set to slightly greater than the normal maximum operating
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Created on Sep 24th 2019 20:27. Viewed 359 times.