What Causes Diesel Generator Overexcitation?
The 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 value.