Definition: The system which is used for providing the necessary field current to
the rotor winding of the synchronous machine, such type of system is called an
excitation system. In other words, excitation system is defined as the system
which is used for the production of the flux by passing current in the field
winding. The main requirement of an excitation system is reliability under all
conditions of service, a simplicity of control, ease of maintenance, stability
and fast transient response.
The amount of excitation required depends on the load current, load
power factor and speed of the machine. The more excitation is needed in the
system when the load current is large, the speed is less, and the power factor
of the system becomes lagging.
Types of Excitation System
The excitation system is mainly classified into three types. They are
- DC Excitation System
- AC Excitation System
- Rotor Excitation System
- Brushless Excitation System
- Static Excitation System
1. DC Excitation System
The DC excitation system has two exciters – the
main exciter and a pilot exciter. The exciter output is adjusted by an
automatic voltage regulator (AVR) for controlling the output terminal voltage
of the alternator. The current transformer input to the AVR ensures limiting of
the alternator current during a fault.
When the field breaker is open, the field discharge resistor is
connected across the field winding so as to dissipate the stored energy in the
field winding which is highly inductive.
The main and the pilot exciters can be driven either by the main shaft
or separately driven by the motor. Direct driven exciters are usually preferred
as these preserve the unit system of operation, and the excitation is not
excited by external disturbances.
The voltage rating of the main exciter is about 400 V, and its
capacity is about 0.5% of the capacity of the alternator. Troubles in the
exciters of turbo alternator are quite frequent because of their high speed and
as such separate motor driven exciters are provided as standby exciter.
2. AC Excitation System
The AC excitation system consists of an alternator and thyristor
rectifier bridge directly connected to the main alternator shaft. The main
exciter may either be self-excited or separately excited. The AC excitation
system may be broadly classified into two categories which are explained below
in details.
a. Rotating Thyristor Excitation System
The rotor excitation system is shown in the figure below. The rotating
portion is being enclosed by the dashed line. This system consists an AC exciter,
stationary field and a rotating armature. The output of the exciter is
rectified by a full wave thyristor bridge rectifier circuit and is supplied to
the main alternator field winding.
The alternator field winding is also supplied through another
rectifier circuit. The exciter voltage can be built up by using it residual
flux. The power supply and rectifier control generate the controlled triggering
signal. The alternator voltage signal is averaged and compare directly with the
operator voltage adjustment in the auto mode of operation. In the manual mode
of operation, the excitation current of the alternator is compared with a
separate manual voltage adjustment.
b. Brushless Excitation System
This system is shown in the figure below. The rotating portion being
enclosed by a dashed line rectangle. The brushless excitation system consists
an alternator, rectifier, main exciter and a permanent magnet generator
alternator. The main and the pilot exciter are driven by the main shaft. The
main exciter has a stationary field and a rotating armature directly connected,
through the silicon rectifiers to the field of the main alternators.
The pilot exciter is the shaft driven permanent magnet generator
having rotating permanent magnets attached to the shaft and a three phase
stationary armature, which feeds the main exciter field through silicon
rectifiers, in the field of the main alternator. The pilot exciter is a shaft
driven permanent magnetic generator having rotating permanent magnets attached
to the shaft and a 3-phase stationary armature, which feeds the main’s exciter
through 3-phase full wave phase controlled thyristor bridges.
The system eliminates the use of a commutator, collector and brushes
have a short time constant and a response time of fewer than 0.1 seconds. The
short time constant has the advantage in improved small signal dynamic
performance and facilitates the application of supplementary power system
stabilising signals.
3. Static Excitation System
In this system, the supply is taken from the alternator itself through
a 3-phase star/delta connected step-down transformer. The primary of the
transformer is connected to the alternator bus and their secondary supplies
power to the rectifier and also feed power to the grid control circuit and
other electrical equipment.
This system has a very small response time and provides excellent
dynamic performance. This system reduced the operating cost by eliminating the
exciter windage loss and winding maintenance.
