Generator Construction 210 MW
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Transcript of Generator Construction 210 MW
1. Introduction :
An electrical generator is a machine, which converts mechanical energy (or power) into
electrical energy (or power). Energy conversion is based on the principal of the production of
dynamically (or motionally) induced emf. Whenever conductor cuts magnetic flux, dynamically
induced emf is produced in it according to Faraday’s Laws of electromagnetic induction. This
emf causes a current to flow if the conductor circuit is closed.
Hence two basic essential parts of an electrical generator are 1) A Magnetic field & 2)
A Conductor or Conductors (armature) which can so move as to cut the flux. Generators are
A.C. or D.C. the device in which electricity is generated by keeping the magnetic field stationary
and armature rotating is called D.C. generator and the device in which electricity is generated
by keeping the armature (conductor) stationary and magnetic field rotating is called A.C.
generator. In the case of A.C. generators standard construction consists of armature winding
mounted on stationary element called stator and field windings on a rotating element called
rotor. The details of construction are as elaborated ahead.
1) Stator : consists of Body/Frame, Core, Winding, Distillate Header, Terminal Bushing,
End shield, gas coolers etc.
2) Rotor : consists of shaft, winding, wedges, retaining ring, fans, field leads, slip ring &
bush gear.
2. STATOR
2.1 STATOR BODY :
The stator body with core and stator winding form the heaviest component of the
entire Turbogenerator. The active parts to be accommodated and the forces and torque
arising during operation call for a rigid and strong stator shell. Moreover, it is designed to
withstand high internal pressure, which may arise due to unlikely event of explosion of hydrogen
air mixture without any residual deformations.
Stator body is a totally enclosed gas tight fabricated structure made up of high quality
mild steel and austenitic steel. It is suitably ribbed with annular rings called inner walls to
ensure high rigidity and strength. The arrangement, location and shape of inner walls is
determined by the cooling circuit for the flow of gas and the required mechanical strength and
stiffness and side walls are suitably blanked to house four longitudinal hydrogen gas coolers in-
side the stator body.
PIPE CONNECTIONS :
The water connection to gas coolers is done by routing stainless steel pipes inside the
stator body which emanate from bottom and emerge out at side walls. These stainless steel
pipes serve as inlet and outlet for gas coolers. From side wall these are connected to gas
coolers by means of eight U-Tubes outside the stator body. For filling the generator with H2, a
perforated manifold is provided at the top inside the stator body. The feed and vent terminating
flanges for Hydrogen, carbon dioxide and air are provided at the bottom of stator body.
Manhole is provided at the bottom to inspect inside of the generator if required.
GENERATOR CONSTRUCTION DETAILS (210 MW)
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TERMINAL BOX :
The beginnings and ends of three phases of stator winding are brought out to the slip
ring end of the stator body and brought out through 9 terminal bushings in the terminal box.
The terminal box is a welded construction of (non-magnetic) austenitic steel plates. The
material eliminates stray losses due to eddy currents. Which may result in excessive heating.
The terminal box is a welded construction of (non-magnetic) austenitic steel plates.
This material eliminates stray losses due to eddy currents, which may result in excessive
heating.
JACKING & DRAGGING PROVISION :
Suitable jacking points are provided on stator body. Provision is also made for dragging
the completely wound stator. The dragging provision is optional and achieved by welding a
cradle plate on the sole of stator body.
TESTING OF STATOR BODY :
On completion of manufacture of stator body, it is subjected to a hydraulic pressure of
8 kg/cm2 for 30 minutes for ensuring its withstanding capability to all explosion pressure that
might arise on account of hydrogen air mixture explosion.
Complete stator body is then subjected to gas tightness test by filling in compressed air.
2.2 STATOR CORE :
A rotating magnetic flux threads with the core. In order to minimize the magnetizing
and eddy current losses in the active portion of the stator, the entire core is built up of thin
laminations. Each lamination layer is made up of a number of individual segments. The segments
are stamped out from sheets of cold rolled high quality silicon steel. Before insulating with
varnish each segment is carefully deburred. The core is stacked with lamination segments in
individual layers. The segments are assembled in an interleaved manner from layer to layer so
that a monolithic core of high mechanical strength and uniform permeability to magnetic flux is
obtained. The stampings are held in position by twenty core bars having dovetail section.
Insulating paper pressboards are also put between the layer of stampings to provide additional
insulation and to localize short circuit that may occur due to failure of varnish insulation of
sheet stamping. To ensure tight monolithic core the stampings are hydraulically compressed
during the stacking procedure at different stages when a certain heights of stack are reached
forming different pockets. Between two packets one layer of ventilation segments is provided.
The steel spacers are spot welded on stamping. These spacers form ventilating ducts from
where the cold hydrogen from gas coolers enters the core radially inwards there-by taking
away the heat. The pressed core is held in pressed condition by means of two massive non-
magnetic steel castings of press ring. The pressure of press ring is transmitted to stator core
stampings through press fingers of non-magnetic steel and duralumin placed adjacent to press
rings. The non magnetic steel press fingers extend up to the tip of stamping teeth so as to
ensure the firm compression of the teeth part of the core portion too. The stepped arrangement
of the stampings towards the bore at the two ends provides an efficient support of tooth -
Portion and contributes to a reduction of eddy current losses and local heating in this range in
addition to the provision of more area of cross section for gas flow. To avoid heating of press
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rings due to end leakage flux two rings made of copper sheet are used as flux shield. To
monitor the formation of hot spots, resistance temperature detectors are placed along the
bottom of slots.
CORE SUSPENSION :
The revolving magnetic field exerts pull on core, resulting in a revolving and nearly
elliptical deformation of the core, which sets up a stator vibration at twice the system
frequency known as double frequency vibrations. Generator core, is spring mounted in the
stator frame to dampen the transmission of double frequency vibrations to the foundation.
The elastic suspension of core consists of, longitudinal bar type springs called core
bars. Twenty core bars are welded to inner walls of stator body with the help of brackets.
These core bars provide radial and tangential isolation from magnetic vibrations of stator core.
These are made up of spring steel having a rectangular cross section and dove tail cut at top.
Similar type of dovetail is also stamped on to stampings and fit into that of core bar dovetail,
thus offering a hold point for stampings. Core bars have longitudinal slits that act as inertial
slots and help in damping the vibrations. Apart from this uniform distribution of forces is also
achieved by putting a spring steel tape all around the core bars.
2.3 STATOR WINDING :
GENERAL :
The stator has a three phase, double layer, short pitched and bar type of windings
having two parallel paths. Each slot accommodates two bars. The slot lower bars and the slot
upper bars are displaced from each other by one winding pitch and connected at their ends so
as to form coil groups.
CONDUCTOR CONSTRUCTION :
Each bar consist of solid as well as hollow conductors with cooling water passing
through the latter. An alternate arrangement of hollow and solid conductors ensures an optimum
solution for increasing current and to reduce losses.
The conductors of small rectangular cross section are provided with glass lapped
strand insulation. These are arranged side by side in two layers. The individual layers are
insulated from each other by a separator in the straight slot portion the strands are transposed
by 3600 to reduce the eddy losses.
The transposition provides for a mutual neutralization of voltages induced in the individual
strands due to the slot cross field and end winding field and ensures that no circulating
currents will arise.
To ensure that strands are firmly bonded together and to give dimensional stability in
slot portion, a layer of glass tape is wrapped over the complete stack. After that the stack is
pressed and cured in steam heated hydraulic press.
Prior to applying the bar insulation, overhang on both ends of bar is formed as an
involute in hydraulic press. Coil lugs for electrical and water connections are brazed at both ends.
Bar insulation is done with epoxy mica thermosetting insulation. This insulation is void
free and possesses better mechanical properties.
The bar insulation is cured in a electrically heated press and thus epoxy resin fill all
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voids and eliminate air inclusions. The insulation is highly resistant to high temperatures and
temperature changes. The composition of the insulation and synthetic resin permits the machine
to be operated continuously under conditions corresponding to these for insulation class ‘B’.
CORONA PREVENTION :
To prevent corona discharges between insulation and the wall of the slot, the insulation
in slot portion is coated with semiconducting varnish. At the transition from slot to over hang
winding a stress grading varnish is coated to ensure a uniform control of the electric field and
to prevent the formation of creepage sparks during operation & during high voltage test.
In the course of manufacture the bar is subjected to a number of tests to ensure proper
quality. The various test which are performed are –
a) Inter turn insulation test on stack after consolidations to ensure absence of interturn
short.
b) Each bar is subjected to hydraulic test to ensure the strength of all joints.
c) Flow test is performed on each bar to ensure that there is no reduction in cross section
area of the ducts of the hollow conductor.
d) Leakage test by means of air pressure is performed to ensure gas tightness of all joints.
e) High voltage test to prove soundness of insulation.
f) Dielectric loss factor measurement to establish void free insulation.
LAYING OF STATOR WINDING :
The stator winding is placed in open rectangular slots of the stator core which are
uniformly distributed on the circumference. After laying top bar, slot wedges are inserted.
Below slot wedges, high strength glass textolite spacers are put to have proper tightness. In
between top and bottom bars, spacers are also put. These measures prevent vibrations that
may be set up by the bar currents.
2.4 END WINDING :
In the end winding, the bars are arranged close to each other. Lower layers of bars are
braced with terelyne cord with binding ring as well as with adjacent bars. Upper layer is also
braced in a similar manner. These are fixed with epoxy glass ring made in segment and flexible
spacer put in between the two layers. After laying, varnish is added on terelyne cord. After
that varnish is cured to have solid bracing.
Bus bars are connected to bring out the three phases & six neutrals. These bus bars
are connected with terminal bushings. Both are water cooled, connection is made by brazing
the two lugs properly.
2.5 ELECTRICAL & WATER CONNECTION :
Putting copper ferrule over the two limbs of coil lug makes electrical connection between
top and bottom bar. In between, copper wedges are inserted and then soldering is done. After
that joint is subjected to ultrasonic testing.
Water connection on Exciter side is done by simply connecting copper tube in two lugs.
On turbine side, each lug is connected through a teflon hose to inlet/outlet header. Bus bars
and the terminal bushings are also provided with water connections by copper tubes.
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2.6 DISTILLATE HEADER :
Ring type water headers, made of copper are provided separately for distillate inlet and
outlet in the stator on turbine side. The headers are supported on insulators and isolated from
stator body. The end connection of top and bottom bar is done by putting copper ferrule over
the two coil lugs and then soldered to have solid joint. Connecting a copper tube to the two
opening of lugs at exciter ends does the water connection. At turbine side, each individual bar
is connected with inlet/outlet headers by P.T.F.E. hoses.
Fibermoulded covers filled with putty insulate the bar heads.
The complete water path is subjected to rigorous hydraulic and pneumatic tests at
various stages to ensure water tightness and to detect blocking of the flow paths.
The vent pipe connections are provided at the top of both inlet and outlet header to
expel air during filling these headers with distillate. These vent pipes can be connected to gas
trap device if provided, to measure the extent of hydrogen leaking into water circuit.
2.7 TERMINAL BUSHING :
Three phases and six neutral terminals are brought out from the stator frame through
bushings, which are capable of withstanding high voltage, and provided with gastight joints.
The bushings are bolted to the bottom plate of the terminal box, with their mounting flanges.
The terminal box that is welded underneath the stator frame at exciter end is made of non-
magnetic steel to avoid admissible temperature rise.
The conductor of the bushing is made of high conductivity copper tube on which silver
plated terminal plates are brazed at both ends. A copper pipe is connected to circulate water
for cooling. The terminal bar conductor is housed in porcelain insulator which can be mounted
on the terminal box by means of ring.
The bushing is connected to terminal bus bar by means of flexible copper leads for
making the electrical connections conveniently.
2.8 END SHIELD :
To make the stator body gas tight at the two ends, two end shields are fitted with the
help of bolts. Gas tightness is achieved by putting a rubber sealing cord.
The end shields are made up in two halves for convenience during erection and
inspection. To avoid leakage of gas through the split surface rubber sealing is put between
two halves of end shields.
A chamber is provided near the internal diameter to collect oil which might enter from
shaft seal. This chamber is connected to liquid leakage Detector which gives an alarm for
presence of any liquid.
Aluminum alloy casting of fan shield is supported on end shields to direct the gas flow
from the propeller fan. Shaft seal and oil catcher are also mounted on end shields.All end
shields of Turbogenerators are tested hydraulically for checking the strength of weld seams.
3. ROTOR :
The rotor comprises of following components
1. Rotor shaft
2. Rotor winding
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3. Rotor wedges & other locating parts for winding
4. Retaining ring
5. Fans
6. Field lead connections
3.1 ROTOR SHAFT
The rotor shaft is long forging measuring more than 9 meters in length and slightly more
than one meter in diameter. The main constituents of the steel are chromium, molybdenum.
nickel, and vanadium. The shaft and body are forged integral to each other by drop forging
process. Following tests warrant adherence to the specified mechanical and magnetic properties
as well as a homogeneous forging.
1) Mechanical tests
2) Chemical Analysis
3) Magnetic permeability test
4) Micro structure analysis
5) Ultrasonic Examination
6) Boroscopic Examination
On 2/3 of its circumference approximately, the rotor body is provided with longitudinal
slots to accommodate field windings. The slots pitch is selected in such a way that two solid
poles displaced by 180 are obtained.
The rotor with all its sub assemblies mounted over it is dynamically balanced to a high
degree of accuracy and subjected to 20% over speeding for two minutes.
3.2 ROTOR WINDING :
The field winding consists of several coils inserted into the longitudinal slots of the
rotor body. The coils are wound around the poles so that one north magnetic pole and one
south magnetic pole are obtained on shaft.
COPPER CONDUCTOR :
The conductors are made of hard drawn silver bearing copper. Apart from low electrical
resistance this grade exhibits high creep resistance so that coil deformations due to thermal
cycling due to start and stop operation are minimum.
INSULATION :
Layer of glass laminates insulates the Individual turns from each other. This laminate is
built by glass prepeg strips on the turn of copper and baked under pressure and temperature
to give a monolithic interturn insulation. The coils are insulated from rotor body by U- shaped
glass laminate moulded slot troughs made from glass cloth impregnated with epoxy varnish. At
the bottom of slot D Shaped liners are put to provide a plane seating surface for conductors
and to facilitate easy flow of gas from one side to another.
The overhang windings are insulated from retaining ring by two layers of retaining ring
segments having L shape and made of glass cloth impregnated by epoxy resin.
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COOLING OF WINDING
The rotor winding are cooled by means of direct cooling method of gap pickup method.
In this machine there are zones which result in multijet flow of hydrogen exposing large
amount of rotor winding copper to the cooling medium thus creating very effective cooling and
enabling a very low ratio of maximum to average copper temperature.
The overhang portion of the winding is cooled by axial two-flow system and sectionalized
into small parallel paths to minimize the temperature rise. Cold gas enters the overhang from
under the retaining rings through special chamber in the end shields and ducts under the fan
hub and gets released into the air gap at the rotor barrel ends.
3.3 ROTOR WEDGES
The slot wedges are made from duralumin an alloy of copper Magnesium and Aluminum
having high good electrical conductivity and high mechanical strength. The slot wedges behave
as damper winding bars also under unbalanced operation of generator.
The end wedges are made from an alloy of chromium and copper having high electrical
conductivity. These wedges are connected with damper segments under the retaining ring for
short circuiting induced shaft current. The end wedges are insulated from retaining rings by
glass textolite liner.
The field lead wedges are used to protect the field lead bar against centrifugal forces.
Ventilation slot wedges are used to cover the ventilation canals in the rotor so that hydrogen
for over hang portion flows in a closed channel.
3.4 RETAINING RING
The overhang portion of field winding is held by retaining ring against centrifugal forces.
They are shrink fitted to the ends of the rotor body barrel at one end, while the other side of
the retaining ring does not make contact with the shaft thus ensuring an unobstructed shaft
deflection at the end winding and eliminating the chances of fretling corrosion.
The centering rings are shrinks fitted at the free end of the retaining ring that serves
to rein-force the retaining ring, securing end winding in axial direction at the same time. A
spring ring is used to prevent any relative movement between the retaining ring and centering ring.
The nut for retaining ring is screwed on the retaining ring at fixed end.
To reduce stray losses, the retaining rings are made of non magnetic, austenitic steel
and cold worked, resulting in high mechanical strength.
3.5 FANS
Two single stage axial flow propeller type fans circulate the generator cooling gas.
Fitted on either sides of rotor body. Fan hubs are made of alloy steel forging with three
peripheral grooves milled on it. Fan blades which are a precision casting with special alloy are
machine in the tail portion so that they fit into the groove of fan hub. To check the fan blades
from coming out of hub, ground tapered pins are used by reaming the two components
together. Split pins are used alongwith slotted nuts to prevent the pins coming out during
operation.
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3.6 FIELD LEAD CONNECTIONS
Slip Rings
The slip rings consist of helically grooved alloy steel rings shrunk on the rotor body
shaft and insulated from it. For convenience in assembly both the rings are mounted on a
single common steel bush which has an insulating jacket pre moulded on it. The complete bush
with slip rings is shrunk on the rotor shaft.
The slip rings are provided with inclined holes for self-ventilation. The helical grooves
cut on the outer surface of the slip rings improve brush performance.
Field Lead
The slip rings are connected to the field winding through semiflexible copper leads and
current carrying bolts placed radially in the shaft. Leads are made up insulated by glass cloth
impregnated with epoxy resin for low resistance and ease of assembly. Two semi-circular hard
copper bars insulated from each other and from rotor shaft are placed in central bore of rotor
joining two sets of current carrying bolts with special profiled precision conical threads.
The radial holes with current carrying bolts in the rotor shaft are effectively sealed to prevent
the escape of hydrogen. A field lead bar does the connection between current carrying bolt
and field winding.
3.7 BEARING :
The rotor shaft is supported on pedestal type of bearings which has spherical seating
to allow self alignment. On the top of bearings pedestal a vent pipe emerges connecting
bearing chamber to the atmosphere for venting out oil vapour or traces of Hydrogen. A current
collector located just above the rotor shaft and touching it is also mounted on the bearing
body to give, shaft voltage for rotor earth fault protection. To prevent the flow of shaft
currents slip ring and bearing and connecting pipes are insulated from earth.
For visual checking of oil flow check windows are provided in the drain oil pipe. The oil
is supplied through inlet pipe and flow is adjusted by means of proper selection of diaphragm
orifice.
The generator bearings are provided with a hydraulic shaft lifting device to reduce
bearing friction during start up and barring gear operation of the turboset. For this purpose, oil
at high pressure is forced between the bearing surface and the shaft journal, lifting the rotor
shaft to allow the formation of a lubricating oil film.
3.8 BRUSH GEAR
The rotor winding is solidly connected to the slip rings by means of field lead bars,
current carrying bolts, field lead core bar and flexible leads. The field current to the rotor
winding is provided through the Brush gear.
The current carrying brush gear assembly is rigidly fixed on the extended part of the
bearing pedestal on the exciter side. There are two brush gear stands, each made up of two
symmetrical silicon brass casting half rings. Which are bolted at the top to make one stand
assembly, kept vertically. These ring stands are designed as helical from one end to the other
to achieve uniform wear of slip rings as well as carbon brushes and smooth removal of carbon dusts
all along the width of slip rings. Provision is made on these ring stands for connecting cables.
286
Three insulated straight pipes are rigidly placed horizontally in between the stand at
three different positions to avoid any axial displacements to increase vibration rigidity of ring
under normal running conditions. Brush holders are fixed on both sides of the brass rings and
provides regular staggering of carbon brushes along the width of slip rings. The brushes are
spring loaded to maintain required contact pressure of 0.2 kg/cm2 and the brush pressure can
be adjusted individually. The design of brush gear permits replacement of the brushes during
normal operating condition.
This complete brush gear stand assembly is rigidly fitted in position on brush gear
support, which as a whole unit is to be fixed on to the bearing pedestal. A glass textolite
packing is provided in between the brush gear support and stand to insulate the latter.
3.9 BRUSHES AND BRUSH HOLDERS :
Brushes have a low co-efficient of friction and are self lubricating. The brushes are
provided with double flexible copper leads. Before filling the brushes are rubbed with medium or
fine sand paper in the direction of rotation. This is to obtain the most favorable condition for
equal current distribution.
Moving the spring fulcrum either up or down on the brush holder adjusts the brush
pressure. Excessive pressure tends to induce chattering or bouncing of the brushes, insufficient
pressure tends to cause sparking.
3.10 SHAFT SEAL (RING TYPE)
The locations where the rotor shaft passes through the stator casing, are provided
with radial seal rings. The seal ring is guided in the seal body, which is bolted on to the end
shield and insulated to prevent the flow of shaft currents. The seal ring is lined with babbit on
the shaft journal side. The gap between the seal ring and the shaft is sealed with seal oil. The
seal oil is supplied to the sealing gap from the seal body via radial holes and an annular groove
in the seal ring.
To ensure effective sealing, the seal oil pressure in the annular gap is maintained at a
higher level than the gas pressure within the generator casing. The oil drained on the hydrogen
and air sides of the seal rings is returned to the seal oil system through drains in the seal body
and seal cover respectively. In the seal oil system, the oil is regenerated by a vacuum
treatment and then returned to the shaft seals.
On the air side, pressure oil also called ring relief oil is supplied laterally to the seal ring
via an annular groove. This ensues free movement of the seal ring in radial direction. Gas
coolers consist of cooling tubes made out of admiralty brass with coiled copper wire wound on
them to increase the surface area of cooling. Cooling water flows through the tubes while
hydrogen flowing across the cooler comes into contact with the external surface of the
cooling tubes. Water chambers are bolted to the tube plates on either end through rubber
gaskets. The out side flange of water chamber on slipring side is elastically fixed to the stator
body with the help of moulded rubber gasket to allow free expansion of cooler where as on the
Turbine side it is fixed rigidly to the stator. End covers of water chambers are removable
without purging the hydrogen from the generator. This enables cleaning of the tubes of coolers
while the generator is running at partial load. Four gas coolers mounted longitudinally inside
the generator stator body cool hot hydrogen.
287
In order to remove air from gas coolers while filling them with water, vent pipes are
provided on slip ring side. For alignment of the coolers in the stator while insertion, the bolts
are provided at each end. The rollers in gas cooler facilitate easy insertion of cooler into the
stator frame.
3.11 VENTILATION CIRCUIT :
Two axial fans mounted on the rotor on both sides circulate the gas in two independent
and symmetrical closed circuits with respect to center line of the generator. The schemes of
stator core cooling divide itself into 3 paths.
FLOW PATH – 1
The cold gas after gas cooler enters chamber at the back of core and enters radially
the ventilation ducts and flows to air gap after removing heat of the core. To remove higher
losses in the end part, some of the cold gas is diverted towards press ring and press fingers.
FLOW PATH – 2
The direct cooling of rotor winding is accomplished by Gap Pick up method. The barrel
portion is divided into a number of inlet and outlet zones. The gas picked up by the wedge
scoops in the inlet zones due to the pressure created under the scoops by rotation, passes
inwards, pass the lateral ducts on one side of the rotor coil stack and joins the corresponding
duct on the other side and flows outwards and thrown into the gap in the outlet zones. Such
a multi jet flow of hydrogen exposes large amount of rotor winding copper to the cooling
medium thus creating very effective cooling and enabling a very low ratio of maximum to
average copper temperature.
FLOW PATH –3
The overhang portion of the winding is cooled by axial flow system and sectionalised
into small parallel paths to minimize the temperature rise. Cold gas enters the overhang from
under the retaining rings, through special chamber in the stator end shields and ducts under
the fan hub and gets released into the air gap at the rotor barrel ends.
4. TEMPERATURE MONITORING :
Temperature of various active parts of the generator are monitored and continuously
recorded. For this purpose, resistance temperature detectors are embedded inside the stator
slots for measurement of core iron and winding copper temperatures.
Apart from RTD, mercury in steel dial type thermometers are also locally mounted to indicate
hot and cold hydrogen, lub oil outlet from bearings, inlet and outlet water to generator gas
cooler temperatures.
Types of Resistance Temperature Detectors :
Depending upon the medium where temperature is to be measured the following types
are used:
CRT-01-Suitable for temperature measurement of air & Hydrogen.
CRT-02&04- Suitable for Temperature measurement of Liquid media.
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CRT-03- Suitable for temperature measurement of bearing and seal babbit metals.
CRT-05- For stator winding /core temperature.
For temperature measurement of various active parts in the generator, copper element
resistance thermometers are used.
The basic principle is the change in electrical resistance of a conductor due to
temperature. The resistance (Rt) at any temperature T is found by applying the equation.
Rt
= Ro (1 + !T)
Where Ro = Reference resistance at 00C.
!""= Temperature coefficient in Ohms per 0C.
T = Temperature in 0C.
The standard resistance of the copper resistance elements used is 53 Ohms at 00C.
The temperature coefficient amount to alfa (!) equal to 0.00426 Ohms/0C.
Resistance characteristic for copper resistance thermometer with 53 ohms at 00C.
✦ ✦ ✦
289
290
In all A.C. machines heat is generated due to windage cu-loss and core-loss. The rise
in temperature must be controlled to protect the insulating material. The insulating materials
can withstand temperatures from 900C to 1300C depending upon their grade. In earlier, smaller
machines, air-cooling was sufficient. As the machine sizes increased, it was necessary to
develop better ways of cooling to restrict the maximum temperature.
In principle, Generators and motors are similar machines we are already familiar with
various methods adopted for motor cooling i.e. direct cooled, T.E.F.C. T.E.W.C. etc.
HYDROGEN COOLING :
The turbo generator runs at 3000 rpm. With available material, the rotor diameter is
limited to about 1.2 M. and length between bearings is restricted to 5 to 7 times diameter. As
the air gap must also be limited, the size of stator gets limited. Thus the size of a Generator
can not be increased in proportion to its size. Using higher current densities can only increase
capacity. As the heat generated goes up, superior cooling methods are necessary to restrict
the temperature rise.
For a 60 MW, AIR COOLED Generator the break up of losses is as below :
Total losses - 1320 KW
Windage loss 40% i.e. 528.0 KW
Rotor loss 12% i.e. 158.0 KW
Stator I2 R loss 12% i.e. 158.0 KW
Stator iron loss 26% i.e. 344.0 KW
Stray losses 10% i.e. 132.0 KW
The quantity of cooling air required is 133 tons/hr. the total weight of stator and rotor
is about 140 Tons.
It can be seen that windage loss is the biggest loss. With hydrogen at only 5 lb/in2 this
loss came down to 1/10th i.e.only 52.8 KW. This illustrates very dramatically, the advantages
of Hydrogen cooling.
TABLE – RELATIVE QUALITIES
HYDROGEN
AIR Pressure in Psi.
0.5 30 45
1 0.07 0.14 0.22
1 6.7 6.7 6.7
1 1.55 2.7 3.6
Density Thermal Conductivity Heat transfer co-eff.
HYDRGEN COOLING OF GENERATOR& D. M. WATER COOLING OF STATOR
Above table shows the superiority of Hydrogen over air. Further, hydrogen is non-toxic,
and does not support combustion. In case of any internal fault (electrical), Hydrogen will not
react in any way. However, with Hydrogen, a reliable sealing system is necessary. Monitoring
of H2 purity is also essential as more than 20% oxygen can form an explosive mixture. Purity is
generally maintained above 98.5% to derive full benefit of Hydrogen cooling.
As the loss is reduced, generator loading and hence capacity is increased. It should be
noted that heat is produced in
1) Rotor slots - Cu-loss
2) Stator slots - Cu-loss
3) Stator core - Iron loss.
Hence, to effectively carry away this heat special construction features are adopted.
Ventilation ducts and passages are provided in stator core the hot gas is cooled by D.M. water
coolers, housed in the stator. Fans on the rotor circulate the gas to coolers and then stator
core to the air gap where it cools the rotor and travels towards the fans.
As generator capacities increased, may novel ways were developed to cool the conductors
directly by providing gas passes along and below the slots. Even hollow conductors are used
through which cooling gas is circulated.
However Hydrogen cooling also reached its limit and for 210 MW generators, it became
necessary to introduce cooling of stator conductors by water D.M. water has high resistivity
and it is a very good cooling medium. Hydrogen cooling reduced windage loss to 1/10th and
water cooling needs only 1/8th pumping power compared to fan power necessary for hydrogen
circulation for stator cooling. Resistivity of D.M. water should be around 200 Kn/cm.
Equipment for Hydrogen cooling :
1) H2 and CO2 cylinders.
2) H2 and CO2 manifolds and suitable piping, valves for filling and purfing out CO2.
3) H2 gas drier.
4) Purity meter for continuous monitoring of purity.
5) Shaft sealing arranement.
Shaft sealing arrangement :
To prevent escape of H2 from generator casing, two types of Hydrogen oil seals are
used -
1) Radial
2) Axial thrust type.
Generally in both cases, the oil used is same as turbine lub. Oil. In some cases, a
separate circuit is provided for seal oil and normally it does not mix with turbine oil. The axial
thrust type seal is found more reliable and is used on existing 210 MW BHEL machines.
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Stator water cooling :
The stator conductors are made up of small cross section conductors to minimize eddy
current losses. In a typical hollow bar construction, there are 14 hollow conductors sandwitched
between solid conductors. The hollow cross-section is very small and hence good filters are
provided in the circuit to avoid choking.
D.M. Cooling water is admitted to upper bar and it returns through lower bar of next
slot. The hot water then goes to expansion tank which is maintained under a vacuum of about
300 mm of Hg. This helps to remove gases from the water and thus corrosion is minimsed. From
expansion tank, water is pumped by primary stator water pumps through DM/DM Coolers,
filters back to stator conductors.
Flow meters, purity meters (conductivity) and magnetic filters are included in the
circuit. Water pressure must be kept below the hydrogen gas pressure in the casing. Cooling
water flow of 27 M3/Hr is necessary for 210 MW unit.
1) Stator Water :
Highest Water temp. - 60-700C
Cold water temp. – 400C
2) Gas Vs. Water cooling of hollow conductors.
Heat transfer is better in turbulent flow so velocity should be high and Reynold’s
number must be exceeded.
For gases, R.N. is high, so high velocities are necessary. As ducts are small, pressure
drop becomes high and too much fan power is needed.
Water has high density and low R.N. Also, its heat transfer coeff. is 50 times more than
hydrogen. Therefore, very low water velocities are needed for efficient cooling. Pumping
power becomes only 1/8th of fan power necessary for gas circulation to obtain the same
cooling effect.
TYPICAL PROBLEMS :
HYDROGEN COOLING SYSTEM :
1) Low H2 Purity : Even when all other things are normal, the purity meter shows a gradual
drop in purity. This is due to reduced flow through purity meter. The drier contains calcium
chloride which may block the flow after absorbing enough moisture. The CaCl2 should be
replaced.
2) Excessive H2 leakage that is not tracable. The leakage may be at slip rings or through
generator output leads. Shutdown is necessary for checking the leakage and attending
the same.
3) Breaking of Cu-connectors and damage to brass nuts. This can be prevented by using
flexible pipes suitably wirebraided.
STATOR WATER SYSTEM :
1) Inadequate flow of cooling water – even when filters are clean. This indicates blocking of
stator conductors. The reason may be inadequate cleaning of stator water system initially
292
or after a maintenance job. Provision of back wash helps greatly. Cearing any debris stack
up in stator bars is very difficult and so enough care must be taken for proper cleaning initially.
2) Drop in purity of D.M. Water : One important source is the injector which maintains
vacuum in expansion tank.
3) If stator water pump has excessive gland leakage and a hose of raw-water is directed at
the gland, D.M. water will get contaminated. This is a common practice and operators
must be warned against this.
Quantities of Gases for Puging
CO2 :
To remove air 90 M3 (To get 98% CO2 in Air)
To remove H2 120 M3 (To get 98% CO2 in H2)
Hydrogen :
For initial filling 300 M3 (above 98% purity)
Average make up (daily) 15 M3
When air is admitted to purge out CO2, the CO2 percentage should come down below 5%
Temperature limits :
Operational Limit due to Temperatures and Temperature Measurement :
i) Stator winding copper - 750C
ii) Stator core iron - 950C
iii) Cold Hydrogen - 440C
iv) Hot Hydrogen - 750C
v) Distillate at inlet of stator winding - 450C
vi) Distillate of outlet of stator winding - 850C
vii) Bearing & seal babbit - 750C
viii) Rotor winding temperature - 1150C
HEAT ABSORPTIN CAPACITY OF COOLANTS :
COOLANT VELOCITY FT/SEC HEAT ABSORPTION
CAPACITY W/0C/SEC
AIR 100 3.5
HYDROGEN (0.5 PSIG) 130 4.5
HYDROGEN (30 PSIG) 130 13
OIL 6.5 360
WATER 6.5 840
✦ ✦ ✦
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In order to prevent the escape of Hydrogen from the generator casing along the rotor
shaft, shaft seals with oil under pressure are used. The shaft seals are radial thrust type and
are mounted between the end shield and the bearing at either end of the stator. The shaft
seal consists of seal liner, lined with babbit surface that contacts a collar of the rotor shaft.
The seal liner is enclosed in a sealed body. Sealing oil is fed through the seal body to form an
oil film between babbit and the shaft collar. The liner is kept pressed to the shaft collar due to
oil pressure in a chamber between the seal body and the liner and is free to move axially along
with the shaft inside the seal body.
Rubber sealing rings inserted between the seal body and the liner prevent oil leakage,
from the high pressure oil zones. The oil catcher mounted on the shaft does not permit
hydrogen side drain oil to go inside the machine. Drain oil in the air side is allowed to mix with
the bearing oil but cannot leak due to the sealing ring assembly mounted on the sealing body
fixed with the bearing.
The seal oil supply system consists of an oil injector, two seal oil pumps, one cooler,
two oil filters, differential pressure regulator, pressure oil regulator, damper tank, hydraulic
seal, visual window, oil check pipe etc. The schematic diagram gives the complete details of
the system.
1. SUPPLY OF OIL TO SHAFT SEAL :
The oil to the shaft seal is supplied from two different sources. During the operating
condition of the turbo-unit, the oil supply is taken from the governing system at 20 kg/cm2
and lubrication system at 1.5 kg/cm2. These two oils are diffused through the oil injector to
get the supply of oil to the design pressure of oil in the supply pipe line.
2. OIL PUMPS :
In case of failure of oil injector or during a standstill condition of the generator, the oil
is supplied to the seals by means of two 100% oil pumps – one as standby and the other as
emergency. The standby pump is driven by 415 Volts A.C. motor and the emergency by 220 V.
D.C. motor. The pumps are electrically interlocked.
The standby oil pump automatically starts in case the pressure at the supply pipeline,
detected by the electrical contact pressure gauge, drops by 1.5 kg/cm2 from the rated value.
3. OIL COOLER :
In order to cool the seal oil, one oil cooler, vertical type, is used. The quantity of
process water at 330C for cooling the seal oil is 95 M3/hr. at maximum 5 kg/cm2g.
4. OIL FILTERS :
Two 50% oil filters are provided in the pipeline for filtering the seal oil.
GENERATOR SEAL OIL SYSTEM FOR BHEL210 MW UNITS (LMW Design)
5. DIFFERENTIAL PRESSURE REGULATOR :
In order to maintain a constant pressure difference between oil and H2, a differential
pressure regulator is adopted. This differential pressure maintains the oil at a pressure higher
than the hydrogen pressure by 0.8 kg/cm2.
6. THRUST OIL REGULATOR :
One thrust oil regulator in the circuit provides for holding the shaft sealing ring against
a collar on the rotor. The outlet pressure of the regulator is regulated in the range 1 to 2.2 Kg/
cm2.
7. DAMPER TANK :
The oil after the differential pressure regulator enters the damper tank and then to the
shaft seals.
The damper tank, provided in the system, supplies the oil to the shaft seal during
transient conditions at changeover from injector supply to the pumps supply and vice-versa.
The damper tank is situated at a height of 6 metres from the centre line of the generator, in
order to keep the constant pressure difference between oil and hydrogen. To allow for constant
checks on the level of oil in this tank, magnetic float indicators are installed which give the
signals during low and emergency oil level in this tank.
A greater part of the oil is drained towards the air side into oil of generation bearing. A
small part of the oil is drained towards hydrogen side.
To limit the zone of hydrogen dissipation throughout the turbine oil system pipeline, one
300 mm oil pipeline has a U-shaped oil seal at its end connected to turbine oil tank.
8. HYDRAULIC SEAL TANK :
The oil discharged towards the hydrogen side is drained into the dydraulic seal. A 500
mm high loop in front of the hydraulic seal inlet is provided on the pipeline for oil discharge from
the seal at the exciter end. This U-shaped pipeline prevents the gas circulation through the
hydraulic seal, which is caused by the difference in the values of vacuum produced by the
fans at both the generator rotor sides. The hydraulic seal ensures oil drainage from the
hydrogen side of shaft seals and, at the same time, prevents escape of hydrogen from the
generator casing through oil drain pipeline.
The hydraulic seal has a level float regulator built into it for maintaining a set level of oil
in the tank. The gas separated out assemblies at the top of the hydraulic seal and this is
connected to the machine. Gas samples are also taken from the hydraulic seal for testing the
purity of H2. Warning signals for high and low oil level in the hydraulic seal are provided by
means of magnetic level indicator.
9. INSTRUMENTATION :
The following pressure gauges are provided in the oil supply system for visual checks.
• Pressure gauges at the inlet and outlet of oil injector and centrifugal pump system.
• Pressure gauges at the outlet of cooler.
• Pressure gauges at the inlet and outlet of the differential pressure regulator.
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• Electrical contact pressure gauges at the oil supply header.
• Electrical contact pressure gauges at the inlet of seal oil to shaft seal.
• Electrical contact pressure gauges at the inlet of thrust oil to shaft seal.
• Pressure gauge at the outlet of thrust oil regulator.
In order to check the differential pressure between seal oil and H2, a differential
pressure recording and indicating instrument with two position contact devices, having scale
0-1 kg/cm2 is used. The differential pressure transducer is mounted near the pipe line and the
indicating and the recording instrument is mounted on the unit control board.
9. ANNUNCIATIONS PROVIDED :
The following annunciations are provided on the signalling panel.
1. Pressure of seal oil ... Low
2. Pressure of pressure oil ... Low
3. Oil level in Damper tank ... Low
4. Oil level in Damper tank ... Emergency
5. Oil level in Hydraulic seal ... Low
6. Oil level in Hydraulic seal ... High
7. Automatic switching on of AC standby oil pump
8. Automatic switching on of DC Emergency oil pump.
9. Differential pressure between oil and H2
... Low
10. Differential pressure between oil and H2
... High
10. TECHNICAL DATA :
1. Quantity of oil for both the shaft seals. ... 160 L/min.
2. Rated pressure of shaft seal oil 4.1 to 4.5 kg/cm2
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Generator seal oil system for BHEL 210 MW Units (Russian Design)
298
✦ ✦ ✦
Introduction :
An electrical generator is a machine, which converts mechanical energy (or power) into
electrical energy (or power). Energy conversion is based on the principal of the production of
dynamically induced emf. Whenever conductor cuts magnetic flux dynamically an induced emf
is produced in it according to Faraday’s Laws of electromagnetic induction. This emf causes a
current to flow if the conductor circuit is closed.
Hence two basic essential parts of an electrical generator are 1) A Magnetic field & 2)
A Conductor or Conductors (armature) which can so move as to cut the flux. Generators are
A.C. or D.C. the device in which electricity is generated by keeping the magnetic field stationary
and armature rotating is called D.C. generator and the device in which electricity is generated
by keeping the armature (conductor) stationary and magnetic field rotating is called A.C.
generator. In the case of A.C. generators standard construction consists of armature winding
mounted on stationary element called stator and field windings on a rotating element called
rotor.
The stator has a balanced, distributed three phase windings. The rotor is a cylindrical
one and excited by the D. C source. The rotor winding is so arranged on rotor periphery that
the field excitation produce nearly sinusoidal distributed flux/ pole in the air gap. As the rotor
rotates, three phase emfs are produced in stator windings.
Natural choice for excitation source was a shaft driven D. C. Generator whose output
was supplied to the generator field through brushes and slip rings. Large capacity generators
require very high excitation currents limiting thereby use of directly coupled conventional D. C.
machines due to commutation and brushgear problem. Therefore mainly two types of excitation
systems are in service namely a) AC excitation system & b) Static excitation system
CONSTRUCTION OF EXCITER
Prime mover Generator
D.C.
Generator
as exciter
Excitation
to the Gen.
(A) AC excitation system : A typical AC excitation system contains shaft mounted main
exciter and pilot exciter and there is another AC excitation system known as Brushless excitation
system.
Description of typical excitation system that contains shaft mounted main exciter and
pilot exciter.
Main Exciter : Directly coupled higher frequency generators working at 400-500 Hz frequency
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For 210 MW TG set maximum continuous ratings of the three phase main exciter are as
per the following :
1500/1350 KVA/KW, 360 volts, 2400 A, 500 Cycles/Second, 3000 RPM
The stator core and windings are air cooled, the ventilation circuit being formed by the
end cover and ducting in the stator casing. The main exciter has induction type generator in
which rotor has only empty slots on the surface of rotor without any windings inserted in
them. The main as well as excitation D. D. windings are mounted on stator.
Pilot Exciter : Pilot exciter is a permanent magnet type A. C. Generator having 400Hz frequency
which provides osurce of supply for the automatic voltage regulator (AVR) circuit of the T. G.
set. Stator has a frame of three phase winding which is brought out to the terminals at the
terminal box. Permanent magnets are mounted on the rotor supported on tow journal type
bearings.
Brushless Excitation : Brushless excitation system is alternative to the conventional slipring
excitation system and this system eliminates the need for brushgear maintenance and reduces
the overall unit size. This system is used in 5oo MW units of CSTPS. Revolving armature AC
exciter is the main component of this system. Revolving armature AC exciter is connected to
the rotating rectifier mounted on same shaft, which itself is directly coupled to the main
generator shaft. Field windings are on the rotor of the generator and they are directly connected
to the output of the rotating rectifier. Excitation to the main exciter is obtained from pilot
exciter. The three phase pilot exciter has a revolving field with permanent magnet poles. The
three phase ac generated by the permanent magnet pilot exciter is rectified and controlled by
the AVR (Automatic Voltage Regulator) to provide a variable DC current for exciting the main
exciter.
are also utilized as main exciter. The main exciter output is rectified through static silicon
rectifier unit and fed to the generator. The armature is designed for low voltage operation,
with comparatively high current levels. A permanent magnet type generator also known as
pilot exciter that provides the excitation required for the main exciter and is mounted on the
same shaft.
Pilot Exciter
PMGGenerator
Field excitation
to the Gen.
Rectifier
300
(B) Static Excitation : As the technology is advanced static excitation is used for controlling
the AVR circuit. Static excitation system is used in most of the 210 MW units of the MSPGCL.
With large alternators in the power system, excitation control plays a vital role. With
ever growing size of alternators various characteristic parameters also change and the effect
of these on system performance has to be taken care of to a certain extent by the excitation
system. Today more stringent specifications are required than ever before.
In order to maintain system stability it is necessary to have very fast response excitation
systems for large synchronous machines operating with the grid. This means that the field
current of a synchronous machine must be matched extremely fast to changing operational
conditions. It is because of this reason that the static excitation system is preferred.
A high control speed is achieved by using an inertia free control and power electronic
system. Fully controlled thyristor bridges are used to feed the generator field for controlling
the generator output parameters. Any deviation in the generator terminal voltage is sensed by
an error detector and causes the firing angle of the thyristors, which in turn controls, the field
supply of the alternator.
For synchronous machines, normally only positive excitation current i.e. current in only
one direction in the rotor is required for the purpose of voltage control.
STATIC EXCITATION EQUIPMENT :
The static excitation equipment consists of the following main components or assemblies.
1. Rectifier transformer :
The power transformer gets input supply from the generator output terminals. The
secondary is connected to the thyristor bridge which delivers a variable DC output to the
generator field. The transformer is housed in a ventilated cubicle. Normally dry type transformer
is provided with protective devices.
2. Thyristor bridges :
The thyristor bridge is assembled in one or more number of cubicles depending on the
number of thyristor bridges connected in parallel. The number of bridges is so designed that in
case one bridge fails during operation, the remaining bridges will have adequate capacity to
Main ExciterPilot Exciter
Diode Wheel
AVR
Generator
301
feed the generator field for full load output.In such (n-1) operation, where ‘n’ specifies the
total number of thyristor bridges, the converter is capable of delivering full power for field
forcing conditions. Fans mounted on the top of the cubicle cool the thyristor bridges. Adequate
protection and monitoring is provided for the thyristors and cooling fans.
3. Control electronics :
The control circuits contain various electronic sub-assemblies in modular form mounted
in various racks in regulation cubicle. The racks are mounted in a swing frame in the cubicle.
Other items pertaining to the control scheme like auxiliary transformer, relays, filters, MCBs
contactors, etc., are mounted in the cubicle on channels. Various features and working of the
control scheme is explained separately.
4. Field flashing unit :
Since it is not possible to start the excitation system with the residual voltage at
nominal speed, a field flashing circuit is provided to overcome this problem. Initially the station
auxiliary supply of 415V AC is stepped down by a small transformer, then rectified in a rectifier
bridge and supplied to the generator field through a breaker. As soon as the generator output
builds upto about 30%, the excitation system starts working smoothly and the field flashing
circuit is then cut off at 70% of the generator voltage.
Field flashing can also be done by feeding the generator field from station battery
supply. The battery will be required to deliver approximately 50% of the no load current for 30
secs. Blocking diodes are provided to prevent any back feed from the field to the battery when
the generator voltage rises under excitation control by thyristors. Field flashing cubicle contains
the field flashing contactor, diode bridge, dropping resistors, etc.
5. Field suppression :
For rapid de-excitation of the synchronous machine and complete isolation of the field
from the thyristor bridge a field breaker is provided. In case of severe internal faults or a three
phase short circuit at the generator terminals or a short circuit on the slip rings, the field
breaker provides protection by isolating the DC source from the field. The field energy is
dissipated through a field discharge resistance, which gets connected across the field under
such operation. Normally non-linear discharge resistance is provided for rapid action.
302
✦ ✦ ✦
# = power factor angle
$ = angle by which Ef leads vt called as load angle or torque angle.
Magnitude of Ef determines the VARs delivered by the generator. Because of the
assumed linearity of the magnetic circuit, voltage phasors Ef, Ea and Vt are proportional to flux
GENERATOR EXCITATION SYSTEM
I THEORY :
The synchronous machine is the most important element of a power system. The
synchronous generator converts mechanical power into electrical form and feeds into the
power network. Normally, a synchronous generator operates in parallel with other generator
connected to an infinite bus. An infinite bus means a large system whose voltage and frequency
remain constant independent of the power exchange between the synchronous generator and
the bus, and independent of the excitation of the synchronous generator. The generator
excitation which is controllable determines the flow of VARs into or out of the generator.
The stator has a balanced, distributed three phase windings. The rotor is a cylindrical
one and excited by the D. C source. The rotor winding is so arranged on rotor periphery that
the field excitation produces nearly sinusoidal distributed flux/ pole (fr) in the air gap. As the
rotor rotates, three phase emfs are produced in stator windings.
On no load the voltage Ef induced in the reference phase a lags 900 behind fr which produces
it and is proportional to %f.
%f
Ef =Vf
Obviously the terminal voltage Vt = Ef. As balanced steady load is drawn from the
generator the stator currents produce synchronously rotating flux fa/pole. This flux, called
armature reaction flux, is stationary with respect to field flux ff because the direction of
armature flux is the direction of rotation of the rotor.
&"% r = %"f + %"a
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phasors f f, f a and f r respectively; further the voltage phasors lag 900 behind flux phasors.
Vt = Ef – j Ia x a
Xa is the synchronous reactance of the generator.
Ef = voltage induced by field flux f f alone.
= no load voltage.
The field induced emf Ef leads the terminal voltage by the torque angle d. This, in fact,
is the condition for active power to flow out of the generatro. The magnitude of power
delivered depends upon sin d.
The flow of reactive power and terminal voltage of a synchronous generator is mainly
controlled by means of its excitation. AVR [Auto Voltage Regulator] of the generator controls
the terminal voltage and reactive power flow of the generator.
From the above phasor diagram we can conclude.
' Ef ' = 'Ia' Xs
Sin (90+#) Sin$
&'Ia' Cos # = 'Ef' Sin $ Xs
& P = 'Ef''Vt' Sin $ Xs
i.e for Sin d = 900 maximum power can be drawn from the generator.
The power factor or flow of reactive power can be changed by the excitation system
by keeping active power constant.
From the above equation we can device that
Ef sin $ = Ia Xs cos % = PXs___
'V+'
= constant for constant delivery of power to the system.
304
Figure shows the phasor diagram for a generator delivering constant power to infinite
bus but with varying excitation. As |Ef | sin $ remains constant f the tip of phasor Ef moves
along a line parallel to V+ as excitation is varied. AVR does the job of controlling the excitation.
Objectives of excitation control :
Besides maintaining the field current and steady state operating point, the excitation
system is required to improve the natural damping behavior and to extend the stability limits.
The operating conditions to be taken into consideration are :
• Good response in Voltage and reactive power control.
• Satisfactory steady state stability i.e. sufficient damping of electromagnetic and
electromechanical transients.
• Transient stability for all stated conditions.
• Quick voltage recovery after fault clearance.
In steady state operation, voltage response is of secondary importance. It is stability,
i.e. damping behaviour of small oscillations which is more important. Under fault conditions the
transient stability i.e., ability to return to the normal operating point is the main objective and
voltage response again plays a secondary role.
Natural choice for excitation source was a shaft driven D. C. Generator whose output
was supplied to the generator field through brushes and slip rings. Large capacity generators
require very high excitation currents limiting thereby use of directly coupled conventional D. C.
machines due to commutation and brushgear problem. Therefore mainly two types of excitation
systems are in service namely a) AC excitation system & b) Static excitation system
(A) AC excitation system : A typical AC excitation system contains shaft mounted main
exciter and pilot exciter and there is another AC excitation system known as Brushless excitation
system. The details of this type of excitation system are covered in construction of exciter.
(B) Static excitation system :
With large alternators in the power system, excitation control plays a vital role. With
ever growing size of alternators various characteristic parameters also change and the effect
of these on system performance has to be taken care of to a certain extent y the excitation
system. Today it is required to meet more stringent specifications than ever before.
In order to maintain system stability it is necessary to have very fast response excitation
305
systems for large synchronous machines operating with the grid. This means that the field
current of a synchronous machine must be matched extremely fast to changing operational
conditions. It is because of this reason that the static excitation system is preferred.
A high control speed is achieved by using an inertia free control and power electronic
system. Fully controlled thyristor bridges are used to feed the generator field for controlling
the generator output parameters. Any deviation in the generator terminal voltage is sensed by
an error detector and causes the firing angle of the thyristors, which in turn controls the field
supply of the alternator.
For synchronous machines, normally only positive excitation current i.e. current in only
one direction in the rotor is required for the purpose of voltage control.
I. STATIC EXCITATION EQUIPMENT :
The static excitation equipment consists of the following main components or assemblies.
1. Rectifier transformer :
The power transformer gets input supply from the generator output terminals. The
secondary is connected to the thyristor bridge which delivers a variable DC output to the
generator field. The transformer is housed in a ventilated cubicle. Normally dry type transformer
is provided with protective devices.
2. Thyristor bridges :
The thyristor bridge is assembled in one or more number of cubicles depending on the
number of thyristor bridges connected in parallel. The number of bridges is so designed that in
case one bridge falls during operation, the remaining bridges will have adequate capacity to
feed the generator field for full load output, under such (n-1) operation, where ‘n’ specifies the
total number of thyristor bridges, the converter is capable of delivering full power for field
forcing conditions. The thyristor bridges are cooled by fans mounted on the top of the cubicle.
Adequate protection and monitoring is provided for the thyristors and cooling fans.
3. Control electronics :
The control circuits contain various electronic sub-assemblies in modular form mounted
in various racks in regulation cubicle. The racks are mounted in a swing frame in the cubicle.
Other items pertaining to the control scheme like auxiliary transformer, relays, filters, MCBs
contactors, etc., are mounted in the cubicle on channels. Various features and working of the
control scheme is explained separately.
4. Field flashing unit :
Since it is not possible to start the excitation system with the residual voltage at
nominal speed, a field flashing circuit is provided to overcome this problem. Initially the station
auxiliary supply of 415V AC is stepped down by a small transformer, then rectified in a rectifier
bridge and supplied to the generator field through a breaker. As soon as the generator output
builds upto about 30%, the excitation system starts working smoothly and the field flashing
circuit is then cut off at 70% of the generator voltage.
Field flashing can also be done by feeding the generator field from station battery
306
supply. The battery will be required to deliver approximately 50% of the no load current for 30
secs. Blocking diodes are provided to prevent any back feed from the field to the battery when
the generator voltage rises under excitation control by thyristors. Field flashing cubicle contains
the field flashing contactor, diode bridge, dropping resistors, etc.
5. Field suppression :
For rapid de-excitation of the synchronous machine and complete isolation of the field
from the thyristor bridge a field breaker is provided. In case of severe internal faults or a three
phase short circuit at the generator terminals or a short circuit on the slip rings, the field
breaker provides protection by isolating the DC source from the field. The field energy is
dissipated through a field discharge resistance, which gets connected across the field under
such operation. Normally non-linear discharge resistance is provided for rapid action.
Over voltage protection :
An additional over voltage protection circuit is provided across the field so that during
faults on the stator side, the induced over-voltages on rotor side are limited to remain below
the insulation level of the field winding.
Installation :
The static excitation equipment should be normally mounted in dust free atmosphere
on a floor free from vibration and heat source.
The scheme can be understood easily by referring to the block diagram.
II. DESCRIPTION :
1. Error detector and amplifier :
The generator terminal voltage is stepped down by a three phase P.T. and fed to the
Automatic Voltage Regulator (A.V.R.). The AC input thus obtained is rectified, filtered and
compared against a highly stabilized reference value and any difference is amplified in different
stages of amplification. The AVR is designed with highly stable elements so that variation in
ambient temperature does not cause any drift or change in the output level. For parallel
running of generators, compounding feature is provided. Three CTs sensing the current in the
generator terminals feed proportional current across variable resistors in the AVR. The voltage
thus obtained across the resistors can be added vectorially either for compounding purpose or
for transformer drop compensation. The percentage of compensation can be adjusted as the
resistors are of the variable type.
The AVR also has a built-in frequency dependent circuit so that when the machine is
running below the rated frequency fn the regulated voltage should be proportionally reduced
with frequency. With the help of a potentiometer provided in the AVR the circuit can be made
to respond proportionally to voltage above a certain frequency and proportionally to frequency
below a certain frequency. The range of adjustment of this cut off frequency lies between 40
and 60 Hz.
Various negative feed backs are provided to damp oscillations of the control variable
and thus make the amplifier stable.
307
2. Gate control unit :
The output of the AVR is fed to a gate control unit. It gets its synchronous AC
reference through a filter circuit and generates a row of pulses whose position depends on the
DC input from the AVR i.e., the pulse position varies continuously as a function of the control
voltage. The pulse limits for rectifier and inverter (de-excitation) operation can be adjusted
independent of each other by potentiometers provided on the front side of the module. Six
double pulses displaced by 600 from one another are generated at the output.
Two relays are provided, by exciting which, the pulses can be either blocked completely
or shifted to inverter mode of operation.
3. Pulse amplifier :
The pulse output of the gate control unit is amplified further at an intermediate stage
of amplification. This is also known as pulse coupling stage. The unit besides pulse amplification
of the preceding stage also enables direct parallel coupling of two fault independent pulse
output channels to excitation system. It also has a DC power supply unit which operates from
a three phase 380 V supply & delivers +15V, -15 V, +5V and a coarse stabilised voltage UL.
A built-in relay is provided which can be used for blocking the 6 pulse channels. In a
two channel system (like auto and hand) the changeover is effected by energizing/ de-
energizing the relay.
4. Pulse final stage :
This unit receives input pulses from the previous stage i.e. pulse amplifier (intermediate
state) and transmits them through pulse transformers to the gates of the thyristors. The
steep pulses at the output ensure simultaneous firing of several thyristors in parallel. A built-in
power supply provides the required DC supply (15 V, + 5V &UL) to the final amplifiers.
Each thyristor bridge has its own final pulse stage. Therefore, even if a thyristor bridge
fails with its final pulse stage, the remaining thyristor bridges can continue to provide full load
output and thereby ensure (n-1) operation.
Pulses can be blocked with an internal relay provided in this unit. Pulses are blocked in case
of :
• Failure of one or more thyristor fuses.
• Failure of the power supply of the final stage.
• Failure of the converter cooling fan.
5. Manual control channel :
A separate manual control channel is provided where the controlling DC voltage through
a motor operated potentiometer. The DC signal is fed to a separate grid control unit whose
output pulses, after being amplified at an intermediate stage, can be fed to the final pulse
stage. When one channel is working generating the required pulses, the other between ‘Auto’
and ‘Manual’ control is effected by blocking or releasing the pulses of the corresponding
intermediate stage.
A pulse supervision unit detects spurious pulses or loss of pulses on the pulses busbar
and transfers control from ‘ Automatic’ channel to ‘Manual’ channel. However during manual
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channel operation the pulse supervision is locked. Hence no supervision exists for the manual
channel pulses.
6. Follow-up unit :
To ensure a smooth changeover form ‘Automatic’ to ‘Manual’ control it is necessary
that the position of the pulses in both channels should be identical. A pulse comparison unit
detects any difference in the position of the pulses and with the help of a follow-up unit,
actuates the motor operated potentiometer on the ‘manual’ channel to turn in a direction so
as to eliminate the difference.
However, while transfering control from ‘Manual’ to ‘Auto’ mode, any difference in the
two control levels can be visually checked on the balance meter provided on the swing frame
in regulation cubicle.
7. Limit controllers in excitation system :
When a generator is running in parallel with the power net-work, it is essential to
maintain its synchronism without exceeding the max. permissible load on the machine and also
without tripping by the protection system. The automatic voltage regulator alone cannot
ensure this.
It is necessary to influence the voltage regulator by suitable means to limit the over
excitation and under excitation. This not only improves the security of parallel operation but
makes operation of the system easier. However it must be made clear that the limiters do not
replace the protection system but only prevent the protection system from tripping unnecessarily
under extreme transient condition.
Following limiters are normally used in the static excitation system :
1. Stator current limiter in over excited and under excited operation.
2. Rotor current limiter
3. Rotor angle limiter
Rotor and stator current limiters reduce excitation in over excited operation while rotor
angle and stator current limiters increase excitation in under excited operation.
In the first case i.e., over-excited operation, the limiter intervenes only after a certain
time i.e., over-excitation only for a short time is made available to facilitate field forcing to
overcome short lived faults in the network. However, if the over-excitation persists, say, due
to a remote three phase short circuit, the rotor current or stator current limiter after a time
delay will be in action and protect the rotor current or stator current limiter after a time delay
will be in action and protect the field winding & exciter equipment against over heating.
In the second case i.e., under-excited operation, the rotor angle or stator current
intervenes immediately, thereby, increasing the excitation and prevent a further increase in
rotor angle which otherwise would cause the generator to fall out of step.
The rotor angle limiter becomes operative as the load becomes increasingly capacitive.
For instance, with long, lightly loaded high voltage lines capacitive load is found.
Also due to failure of the voltage regulator or mal-operation of the system or tap-
change in unit transformer the machine can become under excited.
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7.1 Stator current limiter :
This module functions in conjunction with an integrator which provides the necessary
dead time and the gradient, that can be adjusted by potentiometers. The regulator consists
essentially of a measuring converter, two comparators, two PID regulators and DC powe pack.
A discriminator in the ckt. Differentiates between inductive & capacitive current. The positive
& negative signals processed by two separate amplifiers are brought to the output stage &
only that output which has to take care of the limitation is made effective.
7.2 Rotor current limiter :
The unit basically comprises an actual value converter, a limiter with adjustable PID
characteristics, a reference value, dv/dt sensor and a signalisation unit.
The field current is measured on AC input side of the thyristor converter and is converted
into a proportional DC voltage. The signal is compared with an adjustable reference value,
amplified, and with necessary time lapse fed to the voltage regulator output.
For operation DC power supply is given from a separate DC power supply unit.
7.3 Rotor angle limiter :
This unit limits the angle between the voltages or it limits the angle between generator
voltage and the rotor voltage. It comprises an actual value converter, a limiting amplifier with
adjustable PID characteristics and a reference value unit. The limiting regulator operates as
soon as the DC value exceeds the reference value. For its operation the unit is given separate
power supply from a DC power pack.
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