The field current is supplied to
the rotor through multi contact system arranged at the exciter side shaft end.
3.3.1 BEARINGS:
The generator rotor is
supported in two sleeve bearings. To eliminate shaft current the exciter and
bearing is insulated from foundation plate and oil piping.
The
temperature of each bearing is maintained with two RTD’s (Resistance
Temperature Detector) embedded in the lower bearing sleeve so that the ensuring
point is located directly below the Babbitt.
All bearings have provisions for fitting vibration pick up to monitor
shaft vibrations.
The oil supply of bearings is
obtained from the turbine oil system.
3.4 EXCITATION SYSTEM:
In all industrial applications,
the electrical power demand is ever increasing. This automatically demands for
the design, development and construction of increasingly large capacity
Synchronous generators. These generators should be highly reliable in operation
to meet the demand. This calls for a reliable and sophisticated mode of
excitation system.
When the first a.c generators were
introducing a natural choice for the supply of field systems was the DC
exciter. DC exciter has the capability for equal voltage output of either polarity,
which helps in improving the generator transient performance. DC exciters, how
ever, could not be adopted for large ratings because of the problems in the
design commutator and brush gear, which is economically unattractive. Of
–course, the problems are not uncommon in power stations but Of the environment
with sulphur vapours, acidic fumes as in the cases of petrochemical and
fertilizer industries, exposure of DC exciter. This adds to the problem of
design.
Types of a.c exciters
are:
(1) High frequency excitation
(2) Brush less excitation
(3) Static excitation
The
high frequency D.C exciter is a specially designed “inductor type alternator”
with no winding on its rotor. It is designed to operate at high frequency to
reduce the size of the rotor; the a.c exciter was very reliable in operation.
Though this system eliminates all problems associated with commutator, it is
not free from problems attributable to slip rings and its brush gear. Thus
brushless excitation system was introduced.
The BL exciter consists of field
winding on the stator. This system proved to be highly reliable and required
less maintenance. Absence of power cables and external ac power supplies males
the system extremely reliable. The problem associated with brushes like fast
wear out of brush, sparkling etc, are eliminated.
This suffers from the disadvantage
of lack of facility for field suppression in the case of an internal fault in
generator.
The system comprises shaft driven
AC exciter with rotating diodes.
3.5 PERMANENT MAGNET GENERATOR AND AVR:
This system is highly reliable
with least maintenance and is ideally suitable for gas driven generators.
The static excitation system was developed
contemporarily as an alternative to brush less excitation system. This system
was successfully adapted to medium and large capacity Turbo generators. Though
the system offers very good transient performance, the problems associated with
slip rings and brush gear system are still present.
This
system consists of rectifier transformer, thyristor converts, field breaker and
AVR. This system is ideally suitable where fast response is called for. The
system is flexible in operation and needs very little maintenance.
Thus,
each excitation system has its own advantages and disadvantages. The selection
of system is influenced by the transient response required, nature of pollution
and pollution level in the power plant and cost of equipment.
Exciters
are those components, which are used for giving high voltage to the generator
during the start up conditions. The main parts that are included in the exciter
assembly are:
(1) Rectifier wheels
(2) Three phase main exciter
(3) Three phase pilot exciter
(4) Metering and supervisory
equipment
3.5.1 RECTIFIER WHEELS:
The main components of the rectifier
wheels are Silicon Diodes, which are arranged in the rectifier wheels in a
three-phase bridge circuit. The internal arrangement of diode is such that the
contact pressure is increased by centrifugal force during rotation.
There are some additional components
contained in the rectified wheels. One diode each is mounted in each light
metal heat sink and then connected in parallel. For the suppression of
momentary voltage peaks arising from commutation, RC blocks are provided in
each bridge in parallel with one set of diodes. The rings from the positive
shrunk on to the shaft. This makes the circuit connections minimum and ensures
accessibility of all the elements.
3.5.2 THREE PHASE PILOT EXCITER:
The three phase pilot exciter is a
six-pole revolving field unit; the frame accommodates the laminated core with
the three-phase winding. The rotor consists of a hub with poles mounted on it.
Each pole consists of separate permanent magnets, which are housed, in
non-metallic enclosures. The magnets are placed between the hub and the
external pole shoe with bolts. The rotor hub is shrunk on to the free shaft
end.
3.5.3 THREE PHASE MAIN EXCITER:
Three phases main exciter is a
six-pole armature unit; the poles are arranged in the frame with the field and
damper winding. The field winding is arranged on laminated magnetic poles. At
the pole shoe, bars are provided which are connected to form a damper winding.
The rotor consists of stacked laminations,
which are compressed through bolts over compression rings. The three- phase
winding is inserted in the slots of the laminated rotor. The winding conductors
are transposed with in the core length and end turns of the rotor windings are
secure with the steel bands. The connections are made on the side facing of the
rectifier wheels. After full impregnation with the synthetic resin and curing,
the complete rotor is shrunk on to the shaft.
3.5.4 AUTOMATIC VOLTAGE REGULATOR:
The general automatic voltage
regulator is fast working solid thyristor controlled equipment. It has two
channels, one is auto channel and the other is manual. The auto channel is used
for the voltage regulation and manual channel is used for the current
regulation. Each channel will have its own firing for reliable operation.
The main features of
AVR are:
(1) It has an automatic circuit to
control outputs of auto channel and manual channel and reduces disturbances at
the generator terminals during transfer from auto regulation to manual
regulation.
(2) It is also having limiters for
the stator current for the optimum utilization of lagging and leading reactive
capabilities of turbo generator.
(3) There will be automatic transfer
from auto regulation to manual regulation
in case do measuring PT fuse failure or some internal faults in the auto channel.
(4) The generator voltage in both
channels that is in the auto channel and the manual channel can be controlled
automatically.
3.5.5 COOLING SYSTEM:
Cooling is one of the basic
requirements of any generator. The effective working of generator considerably
depends on the cooling system. The insulation used and cooling employed is
inter-related.
The losses in the generator
dissipates as the heat, it raises the temperature of the generator. Due to high
temperature, the insulation will be affected greatly. So the heat developed
should be cooled to avoid excessive temperature raise. So the class of
insulation used depends mainly on cooling system installed.
There are various methods of cooling, they
are:
a. Air cooling- 60MW
b. Hydrogen cooling-100MW
c. Water cooling –500MW
d. H 2 & Water cooling – 1000MW
Hydrogen cooling has the
following advantages over Air-cooling:
1. Hydrogen has 7 times more heat
dissipating capacity.
2. Higher specific heat
3. Since Hydrogen is 1/14th of air
weight. It has higher compressibility
4. It does not support combustion.
DISADVANTAGES:
1. It is an explosive when mixes
with oxygen.
2. Cost of running is higher.
Higher
capacity generators need better cooling system.
3.6 VARIOUS LOSSES IN A GENERATOR
In generators, as in most electrical devices, certain forces act
to decrease the efficiency. These forces, as they affect the generator, are
considered as losses and may be defined as follows:
3.6.1 Copper loss in the winding.
3.6.2 Magnetic Losses.
3.6.3 Mechanical Losses
3.6.1 Copper
loss:
The
power lost in the form of heat in the armature winding of a generator is known
as Copper loss. Heat is generated any time current flows in a conductor.
I2R
loss is the Copper loss, which increases as current increases. The amount of
heat generated is also proportional to the resistance of the conductor. The
resistance of the conductor varies directly with its length and inversely with
its cross- sectional area. Copper loss is minimized in armature windings by
using large diameter wire. These includes rotor copper losses and Stator copper
losses
3.6.2 Magnetic
Losses (also known
as iron or core losses)
(i) Hysteresis loss (Wh)
Hysteresis loss is a heat loss caused by the magnetic properties of the
armature. When an armature core is in a magnetic field the magnetic particles
of the core tend to line up with the magnetic field. When the armature core is
rotating, its magnetic field keeps changing direction. The continuous movement
of the magnetic particles, as they try to align themselves with the magnetic
field, produces molecular friction. This, in turn, produces heat. This heat is
transmitted to the armature windings. The heat causes armature resistances to
increase. To compensate for hysteresis losses, heat-treated Silicon steel
laminations are used in most dc generator armatures. After the steel has been
formed to the proper shape, the laminations are heated and allowed to cool.
This annealing process reduces the hysteresis loss to a low value. 
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