Function of Excitation System:
Static Excitation of an Alternator is one of the Excitation method.The basic function of any excitation system is to provide DC power to a field winding. The functionality is extended to a large scope to improve the performance of the Generator.
In the event of sudden changes in demand on the generator, the excitation system must respond rapidly, making changes in the voltage that drives current through the field winding. So a reliable excitation is needed for every power plant to run the turbo-generators more efficiently.
The two types of excitation systems are
- Brushless excitation systems, with rotating exciter machines and Automatic Voltage Regulator (AVR), or
- Static Excitation Systems [SES], feeding rotor directly from thyristor bridges via brushes
Static Excitation of an Alternator:
Advantages of Static Excitation over Brushless Excitation:
- Brushless excitation system occupies more floor space for keeping shaft driven exciters.
- Most of the maintenance and operating problems with the rotating rectifiers.
- The exciter shaft and its coupling are sensitive to torques produced by system induced shaft-torsional vibrations.
- Static excitation system eliminates shaft connected separate exciter and all the excitation power required taken from the generator windings only.
Working of Static Excitation System:
In the static excitation system the rotating rectifier is not being used; the supply to the rotor field is being given through the brushes which are static.
In this system, the AC power is tapped off from the generator terminal stepped down and rectified by fully controlled thyristor Bridges and then directly fed to the field of the main generator through slip rings thereby controlling the generator voltage output.Here Alternator’s Stator and Rectifier are stationary.
A high control speed is achieved by using an internal free control and power electronic system. Any deviation in the generator terminal voltage is sensed by an error detector and causes the voltage regulator to advance or retard the firing angle of the thyristors thereby controlling the field excitation of the alternator.
Components of Static Excitaion System:
Excitation transformer is the power source for static excitation system. The outputs of a voltage source and a current source are combined in an excitation transformer system, providing three phase ac output.
Excitation transformer consists of both Voltage and Current sources. The voltage source is formed by three phase winding located in the generator stator winding slots. Its voltage is proportional to the generator air-gap flux and provides excitation to the generator field winding under no load conditions.
The current source is formed by using generator neutral leads of stator winding. This source provides additional field excitation needed under load conditions. It also provides excitation during system faults when the system voltage and machine air-gap flux are at low levels.
AC power from the excitation transformer is converted to DC power for providing excitation to the generator filed windings. A three-phase, full wave rectifier consists of silicon diodes and silicon rectifiers mounted alongside the generator.
Generator field excitation is closely regulated by the voltage regulator through control of the trigger circuits to SCR’s in the rectifier bridges.
The AC regulator performs the basic duty of controlling generator terminal voltage. The three phase current and voltages stepped down from generator terminals using three phase PTs and CTs for providing inputs to the voltage regulator.
The DC regulator provides a means for dc field voltage regulation when the ac regulator is not in service.
The generator filed collector and brush-holder rigging assembly are located in a small housing. The controlled power output from SCR rectifier unit is delivered to the field winding of alternator through brushes and slip rings as shown in above figure.
The other type of excitation system is
Brushless excitation systems, with rotating exciter machines and Automatic Voltage Regulator (AVR)