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Excitation Principle and Function of Synchronous Generator in Genset

Oct. 23, 2018

The synchronous generator in diesel genset is an electromagnetic induction principle that uses a wire to cut a magnetic line to induce an electric potential, and changes the mechanical energy of the prime mover into an electric energy output. The synchronous generator consists of two parts, a stator and a rotor. The stator is an armature that emits electric power, and the rotor is a magnetic pole. When the rotor field winding is DC-connected, a constant magnetic field is established. The stator is composed of an armature core, a uniformly discharged winding, a base and an end cover. The rotor is usually a hidden pole type consisting of a field winding, a core and a shaft, a guard ring, a center ring, and the like.

Excitation Principle and Function of Synchronous Generator in Genset.jpg


The field winding of the rotor is energized with a direct current, producing a magnetic field close to a sinusoidal distribution (referred to as the rotor field) whose effective field flux intersects the stationary armature winding. When the rotor rotates, the magnetic field of the rotor rotates with the same rotation every revolution. The magnetic lines sequentially cut the windings of each phase of the stator, and induce an alternating current potential in the stator windings. The stator magnetic field interacts with the rotor magnetic field to generate braking torque. The generator can send active and reactive power. The strength of the rotor magnetic field directly affects the voltage of the stator winding. Therefore, the rotor current should be adjusted by adjusting the generator terminal voltage or regulating the reactive power of the generator.


The excitation system supplies the DC power supply of the excitation winding of the synchronous generator. The effect of the excitation system on the synchronous generator can be reflected from the following aspects.



1. Adjust the excitation can maintain voltage stability.

2. It can make each unit between the reactive power reasonable distribution.

3. By adopting perfect excitation system and its automatic adjusting device, the transmission power limit can be raised and the range of static stable operation can be enlarged.

4. In the event of short circuit, forced excitation is helpful to improve dynamic stability.


What is the structure and principle of constant voltage excitation system for synchronous generator.


The excitation system of synchronous generator with AC exciter and rectified by rotary rectifier is called brushless synchronous generator excitation system in modern times. Below will introduce the structure and principle of the constant voltage excitation system of the next synchronous generator.


The motor adopts the capacitor phase shift phase compound excitation device.


A secondary coil is installed in the stator of the generator for special excitation. The generator stator sub-coil is used as the excitation current after rectifying, and AC induced by the exciter is rectified to DC through the rotary collator and fed into the generator as the excitation current.


The AC side current component of the rectifier is provided by the current transformer CT in the neutral cabinet, while the voltage component is provided by the secondary coil through the capacitor in the excitation block, and then synthesized by rectifier, exciter, rectifier and generator to form the excitation current.


When the load current or power factor COS is changed, the magnitude of the excitation current s changed by changing the current component or the voltage component to achieve the purpose of voltage regulation. When starting, DC is directly supplied by BWA (B\ C\ D). After rectifier, DC is like AC excitation, and DC passing through rectifier will not be affected.


Automatic voltage regulator


The automatic voltage regulator, matches the brushless three phase AC synchronous generator, through which the excitation current of the exciter is controlled, and then the excitation current of the generator is controlled, so that the output voltage of the generator is constant.


Regulation principle of automatic voltage regulator

When the main generator voltage decreases, the voltage output of the voltage measurement circuit through the voltage transformer PT decreases and the pulse width of the pulse width modulation voltage output voltage increases. The excitation current of the power amplifier tube with long turn-on time increases and the magnetic field of the main generator increases, which makes the output voltage of the generator rise again and stabilize at the original setting valve. Vice versa, so that the generator output voltage back down, stable in the original set value.


The function characteristics of forced excitation of diesel generator set and points for attention.


In the process of application of diesel generator set, some users will force excitation to diesel generator set. So what is forced excitation? What factors limit the forced excitation time?


Why should the generator set be excited?


In order to improve the stability of diesel generator set operation system, the voltage can quickly return fault is removed, and when the voltage drops to a certain value. The excitation of the generator can be increased immediately, so the generator should be forced to excite.


What factors limit the forced excitation time?


The action of line excitation is to connect the magnetic field adjustment resistance of the exciter loop by the relay automatically, or the excitation device of the generator set to adjust the exciter automatically and quickly so that the exciter works under the maximum voltage. Supply generator sets with sufficient excitation current.


The forced excitation of a generator can play its due role when the forced excitation multiple is higher, the excitation voltage rises faster, and the forced excitation time is sufficient. Therefore, when the excitation of the generator is increased to the maximum due to the forced excitation. No interference with the forced excitation shall be made within one minute, and immediately after one minute measures shall be taken to reduce the stator and rotor currents of the generator to the normal allowable values.


What should we pay attention to after the forced excitation action?


After forced excitation, if not returned, the magnetic field resistance will be connected for a long time, and the rotor will suffer high voltage and be damaged when the generator is in normal operation.


It can be seen that if you do not return, you will do more harm to the diesel generator set. Therefore, it is suggested that according to the operation condition of diesel generator set, under the premise of ensuring normal operation, the forced excitation device that does not return can be excised, the reason can be found out, and then put into operation after troubleshooting.



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