Elements of excitation system
The basic blocks that are involved in the excitation system are shown in Figure.
1. Exciter:
Exciter provides the current required for the field winding of a synchronous generator to produce the rated terminal voltage at the generator terminals. Exciter can be of three types DC exciter, AC exciter and Static exciter.
A. DC exciter:
In this type of exciter a separately or self excited DC generator driven by a motor or connected to the same shaft as that of the main generator rotor is used.
In case of separately excited DC generator the field winding of the DC generator is energized through a permanent magnet AC generator, the three-phase out of which is converted to DC through rectifiers.
This type of excitation was widely used up to 1960 but now-a-days AC excitation or static excitation is being used. For older generating stations where DC excitation is still used the voltage regulator alone is replaced with electronic regulators.
B. AC exciter:
In this type of exciter the AC generator whose armature is mounted on the same shaft as that of the main generator, with its field stationary, is used for supplying field current to the main generator field winding.
The output of AC exciter generator is converted to DC through rotating rectifiers, and as the armature is rotating at the main generator rotor speed, the output is directly connected to the main field winding.
The filed of the AC exciter generator itself is energized through a pilot permanent magnet AC generator whose three-phase output is converted to DC through rectification.
C. Static exciter:
In this exciter the output of the main synchronous generator is converted from AC to DC through static rectification and then the output is supplied to the main generator field winding through slip rings.
2. Regulator:
The voltage regulator amplifies the error signal generated by voltage transducers and the output of the voltage regulator is sent to exciter which there by controlling the main generator field winding.
Voltage regulation is accomplished by control of thyristor power amplifiers. The excitation may be controlled either by a dc regulator or automatically by the ac voltage regulator in response to the generator terminal voltage.
The system may be configured to provide VAr or power factor control instead of terminal voltage regulation. It includes
1. AC regulator: to maintain generator stator voltage.
2. DC regulator: to holds constant generator field voltage when ac regulator is faulty. Also used for testing and startup.
3. Stabilizing circuit: to stabilize itself from abnormality.
3. Terminal voltage transducer:
Terminal voltage transducers measure the three-phase terminal voltage through potential transformers and convert them and filter them to DC which is compared with the reference voltage and the error signal is used to control the main exciter field winding current through the exciter.
4. Load compensator:
It may be provided if desired to hold constant voltage at a remote point from generator terminal.
5. Power system stabilizer:
It provides additional input signal to the regulator to damp out power system oscillations.
6. Limiters and protective circuits:
Limiting and protection circuits are used to limit main generator field winding current, over excitation, under excitation, terminal voltage, Volts-per-Hertz etc.