Excitation System
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.
The excitation system is the single unit in which the
each alternator has its exciter in the form of generator. The centralised
excitation system has two or more exciter which feeds the bus-bar. The
centralised system is very cheap, but the fault in the system adversely affects
the alternators in the power plant.
Types of Excitation System
The excitation system is mainly classified into three
types. They are
1. DC Excitation
System
2. AC Excitation
System
o
Rotor Excitation System
o
Brushless Excitation System
3. Static Excitation
System
Their types are explained below in details.
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.
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