I hope with it can bring up some guide line and basic knowledge about excitation system for generator.I suggest for more detail information and understanding,please contact your local generator supplier.Below are listed types of excitation system for generator :-
What is the types of excitation system?
1) A.C. excitation.
This type of the exciter operated with a.c. supply and rated frequency around 50 until 250 Hz.The output provide with slip ring to be rectified for internal mounted diode cubicles.This types can replaced for commutator on the d.c with solid state rectifier.
The output from rectifier supply to the generator rotor with slip ring for the d.c excitation system. Usually a pilot exciter made from a permanent magnet for supply to main exciter.
For the main exciter,it commonly made in three phase used the combination of diode in a bridge formation.The bridge arm have a several piece of diodes in parallel condition.
The method for short circuit protection is each diode individually fused to remove it from the electrical circuit in when it happen.
2) Brushless excitation.
This excitation system have a similar function between A.C excitation with advance application.The rectifier diode installed on the exciter shaft.The main exciter has its output armature winding on the rotor with its field on the stator.
Normally for rotor output frequency rated around 100 and 250 Hz and it is connected to the shaft mounted diodes.The rotor of main generator can supply the output with directly application.
The pilot exciter can be a permanent magnet generator with frequency operation until 400 Hz.For the small generator,supply can provide from generator terminal to the exciter field.
3) Thyristor excitation.
For this type of excitation system it used thyristor rectifier to directly control the generator filed current and it can response with faster system by controlling the exciter field current.Thyristor excitation fed system is necessary to provide slip rings and brushes to connect to the machine rotor.
This function is important to alert and response to the error or faulty for system stability due to critical application.But it have disadvantage because this system not been developed commercially and not common system for generator at around the world.It also difficult to providing the reliably control signals to them from stationary equipment.
Excitation power may be provided with direct application to the coupled main and pilot exciters or it can fed from main generator terminals using a step down transformer.
Normally the transformer ratio is design to provide full output during the occurrence of a system fault and it reduces the machine terminal voltage for protection system.