INTRODUCTION
A turbo generator is a turbine directly connected to
an electric generator for the generation of electric power. Large steam powered
turbo generators (steam turbine generators) provide the majority of the world's
electricity and are also used by steam powered turbo-electric ships.
Smaller turbo-generators with gas turbines are often
used as auxiliary power units. For base loads diesel generators are usually
preferred, since they offer much better fuel efficiency and are also more
reliable, but on the other hand they are much heavier and need more space.
The efficiency of larger gas turbine plants can be
enhanced by using a combined cycle, where the hot exhaust gases are used to
generate steam which drives another turbo generator.
The Turbo generator was invented by a Hungarian engineer Ottó
Bláthy.
Turbo generators were also used on steam locomotives
as a power source for coach lighting and heating systems.
Charles Brown
for a high-speed generator, the turbo generator has been the unique solution
for converting steam turbine power into electrical power. The continuously transposed
stator bar, invented by Ludwig Roebel in 1912, opened the door for large scale
winding application. Up to the 1930ies the generators were designed in 2-, 4-
and even 6-pole, in accordance with the speed optimums of the steam turbines in
those days. The 1920ies ended with impressive power generation plants, having
generator units in the 100
MVA range (see
Fig.1). The stator winding insulation consisted in the beginning of plied-on
mica-paper, compounded by Shellac varnish, later substituted by asphalt. Voltages
were up to 12 kV.
A 500 MW TURBO GENRATOR
The generator
has for a long time been developed by repeating the cycle: design – test –
adjust design tools – extrapolate design. A tremendous breakthrough came with
the large computers in the 1960ies, immediately being used for the key competences,
such as magnetic field calculations, nonlinear coolant flow networks and
mechanical turbine generator
shaft
calculations. Some programs of that area are even in use in the today’s PC
environment. As an example, magnetic equivalent circuits were established to
determine excitation currents. Once these programs were calibrated on measured
data, they have been proven very accurate and still
today, for most
applications make obsolete any FEM method.
The two poles and four poles
differ considerably in construction. At 50c\s. the former run at 3000r.p.m and
the latter at 1500.The useful range of two pole machines has been extended to
300 MVA. , and in consequence the four-pole
Construction is obsolete.
STATOR OF TURBO GENRATOR
Generally, the stator of a turbo generator comprises:
a cylindrical core, which extends along a first longitudinal axis and comprises
a plurality of axial cavities and two opposite headers; connection terminals of
the turbo generator; a plurality of electrical windings, which are split into
groups and which extend along paths defined in part in the axial cavities and
in part at the headers; the electrical windings of each group being
isopotential and connected in parallel between a pair of terminals.
A known stator of a three-phase turbo generator
comprises six terminals (three of which are connected to earth and three of
which are connected to the electrical energy distribution main), nine
electrical windings which are split into three groups each comprising three
isopotential electrical windings connected in parallel between a pair of
terminals; seventy-two cavities, each of which is occupied at the same time by
two different portions of electrical windings. The electrical windings have
straight segments accommodated in the cavities and connection segments, which
are arranged at the headers and which have the function of connecting together
the straight segments arranged in different axial cavities and some straight
segments to the terminals.
Considering that, according to the wiring diagram of
the stator described above, each axial cavity is occupied at the same time by
two different electrical windings and that each electrical winding presents a
path essentially identical to the other electrical windings, each electrical
winding presents sixteen straight segments, which are arranged at corresponding
axial cavities, and a plurality of connection segments, which are adapted to
connect the straight segments to each other and to the terminals, and are
arranged at the headers.
The connection segments determine a considerable axial
dimension at the header of the stator, above all considering that the
electrical windings are generally defined by bars which must be maintained
spaced apart one from the other. Furthermore, the axial dimension of the stator
is increased by the wiring configuration followed by the electrical windings:
indeed, in an electrical winding it is often necessary to connect together two
straight segments arranged in diametrically opposite axial cavities.
The technical solution of forming isopotential
electrical windings between a pair of terminals, rather than a single
electrical winding between a pair of terminals, allows to decrease the current
value in the single isopotential electrical windings; to increase the cooling
surface; and to reach higher unitary powers with respect to traditional
electrical windings and for a given ventilating gas. The currently known
solutions envisage the formation of two or three isopotential electrical
windings connected in parallel between each pair of terminals.
The stators which adopt this type of solution, i.e. of
fractioning the electrical windings of the stator, in addition to the
aforementioned drawback of the axial dimensions due to the high number of
connection segments at the headers, present the
drawback of overloading with electrical current the connection zones of the
electrical windings to the terminal pair to which they lead.
STATOR
CORE
The active part of the stator consists of segmental
lamination of low loss alloy steel the slots, ventilation holes and dovetail
keyholes, are punched out in one operation the stampings are rather complicated
on account of the number of holes and slots that have to be produced .
The use of cold-rolled grain-oriented steel sheet has
possibilities in machines as well as in transformers, most particularly in two
pole machines where the major loss occurs in the annular part of the core
external to the slotting. Hear the flux direction is manly
Circumferential,
and by cutting the core-plate sectors in such a way that the preferred flux
direction is at right angles to their central radial axis, substantial
reduction in core-loss can be secured
It is of great important that the assembled stator
laminations are uniformly compressed during and after building, and that slot
are accurately located. The core plates are assembled between end plates with
fingers projecting between the slots to support the flanks of the teeth. The
end plates are almost invariably of non-magnetic material, for this stepped
reduces stray load loss. The end packets of core plates may be stepped to a larger
bore for the same reason.
STATOR
WINDING
The windings of two pole machines are comparatively
straightforward. The number of slots
must be a multiple of 3(or 6 if two parallel circuits are required ).single layer
concentric or two-layer short-pitched windings may be used.
The
single layer concentric winding is readily clamped in the overhang, but causes
a higher load loss because the end connections run parallel to the stator end plates.
chording is not possible so that flux harmonics have full effect.
The
two layer winding is more common ,chorded to about 5/6 pitch which practically
eliminates 5th and 7th harmonics from the open circuit
e.m.f wave . The end windings are packed ,and clamped or tied with glass cord.
It is invariable practice with two layer windings to
make the coils as half turns and to
joint the ends. The conductors must always be transposed to reduce eddy-current
losses. The conductors are insulated in many cases with bitumen-bonded micanite,
wrapped on as tape ,vaccume dried,then impregnated with bitumen under pressure
and compressed size. The process is illustrated in picture.each copper bar A
forming part of a conductor is insulated with mica tape ,B and C . A set of bars
forming one conductor is assembled and
pressed,D.the conductor is insulated with layers of mica tape,E;then the
conductors are assembled to form a slot bar,F,and pressed to the required
dimensions.synthetic resinsbhave now replaced bitumen.
Within the slots,the outer surface of the
conductor insulation is at earth potential: in the overhang it will approach
more nearly to the potential of the enclosed copper. Surface discharge will
take place if the potential gradient at the transition from slot to overhang is
excessive, and it is usually necessary to introduce voltage grading by means of
a semi conducting (e.g.graphitic) surface layer, extending a short distance
outward from the slot ends.
The slot
inductance is increased by setting the winding more deeply in to the slots.
This has the incidental advantage of spacing the overhang farther away from the
rotor end-rings.
VENTILATIONS
Forced
ventilation and total enclosure are necessary to deal with the large-scale losses
and rating per unit volume . the primary cooling medium is air or hydrogen ,
which is in turn passed through a water-cooled heat-exchanger.
AIR COOLING
The water
coolers are normally in two section, so that one can be cleared while the
machine is operating. Fans on the rotor,or separate fans,may be employed ,the
latter in large machines where bearing-spacing or limitation of the diameter
makes integral fans inadequate.
With integral fans mounted on the rotor
,the air is fed to the space surrounding the stator overhang,and pipes and
channels convey a proportion towards the centre of the stator core.thereform it
flowes readily inward to the airgap,then axiallynto the end outlet
compartments. With separate fans ,however,air can be fed directly to the middle
as well as to the ends.An improvement of the efficiency by reduction of the airflow
losses is in continuous progress using as support CFD programs. In the last
decades the improvement of the cooling, such as axial ventilation of the rotor
and indirect cooling of the stator winding, allowed huge capability
enhancement, a better utilisation of the materials as well as a better
efficiency.
This trend
continues especially for the hydrogen and the aircooled generators.
HYDROGEN COOLING
A
hydrogen-cooled turbo generator is a turbo generator with gaseous hydrogen as a
coolant. Hydrogen-cooled turbo generators are designed to provide a low-drag
atmosphere and cooling for single-shaft and combined-cycle applications in combination
with steam turbines. Because of the high thermal conductivity and other
favorable properties of hydrogen gas this is the most common type in its field
today. Based
on the air-cooled turbo generator, gaseous hydrogen went into service as a
coolant in the rotor and the stator in 1937 at Dayton, Ohio, in October by the
Dayton Power & Light Co allowing an increase in specific utilization and
a 99.0 % efficiency
The use of
gaseous hydrogen as a coolant is based on its properties, namely low density,
high specific heat, and highest thermal conductivity at 0.168 W/(m•K) of all
gases; it is 7-10 times better coolant than air. Other advantage of hydrogen is
its easy detection by hydrogen sensors. A hydrogen-cooled generator can be
significantly smaller, and therefore less expensive, than an air-cooled one.
For stator cooling, water can be used.
Helium with a
thermal-conductivity of 0.142 W/(m•K) was considered as coolant as well,
however its high cost hinders its adoption despite its non-flammability.
Generally, three
cooling approaches are used. For generators up to 300 MW, air cooling can be
used. Between 250-450 MW hydrogen cooling is employed. For the highest power
generators, up to 1800 MW, hydrogen and water cooling is used; the rotor is
hydrogen-cooled, the stator windings are made of hollow copper tubes cooled
with water circulating through them.
The generators
produce high voltage; the choice of voltage depends on the tradeoff between
demands to electrical insulation and demands to handling high electric current.
For generators up to 40 MVA, the voltage is 6.3 kV; large generators with power
above 1000 MW generate voltages up to 27 kV; voltages between 2.3-30 kV are
used depending on the size of the generator. The generated power is left to a
nearby station transformer, where it is converted to the electric power
transmission line voltage (typically between 115 and 1200 kV).
To control the
centrifugal forces at high rotational speeds, the rotor is mounted horizontally
and its diameter typically does not exceed 1.25 meter; the required large size
of the coils is achieved by their length. The generators operate typically at
3000 rpm for 50 Hz and 3600 rpm for 60 Hz systems for two-pole machines, half
of that for four-pole machines.
The
turbogenerator contains also a smaller generator producing direct current
excitation power for the rotor coil. Older generators used dynamos and slip
rings for DC injection to the rotor, but the moving mechanical contacts were
subject to wear. Modern generators have the excitation generator on the same
shaft as the turbine and main generator; the diodes needed are located directly
on the rotor. The excitation current on larger generators can reach 10 kA. The
amount of excitation power ranges between 0.5-3% of the generator output power.
The rotor
usually contains caps or cage made of nonmagnetic material; its role is to
provide a low-resistance path for eddy currents which occur when the three
phases of the generator are unevenly loaded. In such cases, eddy currents are
generated in the rotor, and the resulting Joule heating could in extreme cases
destroy the generator.
Hydrogen gas is
circulated in a closed loop to remove heat from the active parts then it is
cooled by gas-to-water heat exchangers on the stator frame. The working
pressure is up to 6 bar.
An on-line
thermal conductivity detector (TCD) analyzer is used with three measuring
ranges. The first range (80-100% H2) to monitor the hydrogen purity during
normal operation. The second (0-100% H2) and third (0-100% CO2) measuring
ranges allow safe opening of the turbines for maintenance.
Hydrogen has
very low viscosity, a favorable property for reducing drag losses in the rotor;
these losses can be significant, as the rotors have large diameter and high
rotational speed. Every reduction in the purity of the hydrogen coolant
increases windage losses in the turbine; as air is 14 times more dense than
hydrogen, each 1% of air corresponds to about 14% increase of density of the
coolant and the associated increase of viscosity and drag. A purity drop from
97 to 95% in a large generator can increase windage losses by 32%; this equals
to 685 kW for a 907 MW generator. The windage losses also increase heat
losses of the generator and the associated cooling problems.
The absence of
oxygen in the atmosphere within significantly reduces the damage of the
windings insulation by eventual corona discharges; these can be problematic as
the generators typically operate at high voltage, often 20 kV.
The bearings
have to be leak-tight. A hermetic seal, usually a liquid seal, is employed; a
turbine oil at pressure higher than the hydrogen inside is typically used. A
metal, e.g. brass, ring is pressed by springs onto the generator shaft, the oil
is forced under pressure between the ring and the shaft; part of the oil flows
into the hydrogen side of the generator, another part to the air side. The oil
entrains a small amount of air; as the oil is recirculated, some of the air is
carried over into the generator. This causes a gradual air contamination
buildup and requires maintaining hydrogen purity. Scavenging systems are used for
this purpose; gas (mixture of entrained air and hydrogen, released from the
oil) is collected in the holding tank for the sealing oil, and released into
the atmosphere; the hydrogen losses have to be replenished, either from gas
cylinders or from on-site hydrogen generators. Degradation of bearings leads to
higher oil leaks, which increases the amount of air transferred into the
generator; increased oil consumption can be detected by a flow meter associated
to each bearing.
Presence of
water in hydrogen has to be avoided, as it causes deterioration to hydrogen
cooling properties, corrosion of the generator parts, arcing in the high
voltage windings, and reduces the lifetime of the generator. A desiccant-based
dryer is usually included in the gas circulation loop, typically with a
moisture probe in the dryer's outlet, sometimes also in its inlet. Presence of
moisture is also an indirect evidence for air leaking into the generator
compartment. Another option is optimizing the hydrogen scavenging, so the
dew point is kept within the generator manufacturer specifications. The water
is usually introduced into the generator atmosphere as an impurity in the
turbine oil; another route is via leaks in water cooling systems.
The flammability
limits (4-75% of hydrogen in air at normal temperature, wider at high
temperatures), its autoignition temperature at 571°C, its very low minimum
ignition energy, and its tendency to form explosive mixtures with air, require
provisions to be made for maintaining the hydrogen content within the generator
above the upper or below the flammability limit at all times, and other
hydrogen safety measures. When filled with hydrogen, overpressure has to be
maintained as inlet of air into the generator could cause a dangerous explosion
in confined space. The generator enclosure is purged before opening it for
maintenance, and before refilling the generator with hydrogen. During shutdown,
hydrogen is purged by an inert gas, then the inert gas is replaced by air; the
opposite sequence is used before startup. Carbon dioxide or nitrogen can be
used for this purpose, as they do not form combustible mixtures with hydrogen
and are inexpensive. Gas purity sensors are used to indicate the end of the
purging cycle, which shortens the startup and shutdown times and reduces
consumption of the purging gas. Carbon dioxide is favored as due to very high
density difference it is easily displaced by hydrogen.
Hydrogen is
often produced on-site in electrolyzers, as this reduces the need for stored
amount of compressed hydrogen and allows storage in lower pressure tanks, with
associated safety benefits and lower costs. Some gaseous hydrogen has to be
kept for refilling the generator but it can be also generated on-site.
As technology
evolves no materials susceptible to hydrogen embrittlement are used in the
generator design. Not adhering to this can lead to equipment failure.
STATOR OF A HYDROGEN COOLED
TURBOGENERATOR
DIRECT COOLING
Direct cooling of stator winding is applied
at ratings rather higher than that which makes the method necessary for rotors .tubular
conductors can be used or thin walled metal ducts lightly insulated from normal
stator conductors. A similar design serves for water cooling a stator. Here
arrangements are required in the overhang for the parallel flow of coolant as
well as for the series connection of successive coil-sides. Insulating tubes
convey the liquid to and from the water “headers”, and the water itself must
have adequate resistivity to limit conduction loss. Water cooling has obvious
disadvantages for rotors.
ROTOR OF TURBOGENERATOR
The rotor accommodates the field winding
whose poles are made of steel laminations. A squirrel cage winding for
absorbing purposes compensates for parallel services and abnormal load operation.
The rotor is dynamically balanced and designed to withstand to the electrical
and mechanical effects of overspeed as required by the applicable standard and
of the triggering according to the design. Manufactured with non-salient poles,
the rotor has a constant air gap along the whole iron core periphery. The rotor
has a cylindrical shape in whose periphery slots is inserted the excitation
winding. The field coils are made of bars, wires or copper laminations
insulated with a class-H insulating material. The non-salient pole rotor of the
turbogenerator is practically a monobloc with no overhangs or recesses. As
a result, it becomes sturdier and more
resistant to overspeed and coil triggering.
Generator rotor, including an inner and an
outer concentric rotor part having a non-drive side and enclosing a high vacuum
space, a first and a second bearing disposed on the non-drive side, a hollow
shaft end of the outer rotor part being supported in the first bearing, a
journal of the inner rotor part being extended through the hollow shaft end and
separately supported in the second bearing, a high-vacuum contact less liquid
seal disposed between the hollow shaft end and the journal and having a sealing
gap formed there between, a co-rotating sealing-liquid reservoir connected to
the liquid seal, and magnetic field means for holding magnetic sealing liquid
in the sealing gap.
Rotors are most generally made from solid
forgings must be homogeneous and flawless. Test pieces are cut from the
circumference and the ends to provide information about the mechanical
qualities and the micro structure of the material. A chemical analysis of the
test pieces is subsequently made. One of the most important examinations is the
ultrasonic test, which will discover internal faults such as crackes and
fissures. This will usually render the older practice of trepanning along the
axis necessary.
The rotor forging is planed and milled to
form the teeth. About two-thirds of the rotor pole-pitch is slotted, leaving
one-third unslotted for the pole centre.
ROTOR WINDING
The normal rotor winding is of silver-bearing
copper. The heat developed in the conductors causes them to expands, while the
centrifugal force presses them heavily against the slot wedges, imposing a
strong frictional resistance to expension. Ordinary copper soften when hot, and
may be subject to plastic deformation. As a result, when the machine is stopped
and the copper cools,it contracts to a shorter length than originally. The
phenomenan of copper-shortening can be overcome by preheating the rotor before
starting up with new machines the use of silver-bearing copper, having a much
higher yield point,mitigates the trouble.
Concentric multi-turn coils accommodated in
a slot number that is a multiple of four are used,the slot-pitch being chosen
to avoid undesirable harmonics in the waveform of the gap density. The slots
are radial and the coils formed of flat strip with seprators between turns.the
coils may be performed. The insulation is usually micanite,but bonded asbestos
and glass fabric have both been used.As much copper as possible is accommodated
in the rotor slots,the depth and width of the slots being limited by the
stresses at the roots of the teeth,and by the hoop stresses in the end in
retaining rings. The allowable current depends on cooling and expension.
Comparatively high temperature-rises are allowed:the hot spot temperature may
reach 140 degree centigrade.
EXCITER
Installed at the non-drive end side of the
generator, the exciter is formed by fixed poles that accommodate the excitation
field coils, the armature and the rotating rectifier bridge. Its purpose is to
supply direct voltage to exciter rotor. It supplies direct current controlled
by the voltage regulator according to the load requirements, thus maintaining
constant voltage for the main generator.
Exciter
Stator The poles
accommodate the field coils which are series connected, their ends being
connected to the terminal block (I(+) and K(-)). Its purpose is to supply the
flux to the exciter rotor. It is supplied with a direct current controlled by
the voltage regulator according to the load requirements, thus keeping the main
generator voltage constant.
Exciter Rotor The
exciter rotor is mounted on the main shaft of the
machine. The rotor is formed by laminations
with slots that accommodate a star-connected
three-phase winding. The phases are connected to the rotating rectifying diode set.
SLIP RINGS
Slip rings are required for conveying the
exciting current to and from the rotor winding. Rings of steel ,shrunk over
micanite, may be placed one at each end of the rotor,or both at one end, inside
or outside the bearing.
INSULATION
Since its introduction at the end of the
1950ies the synthetic resin mica tape insulation technology has been in use.
Over the past years a worldwide re-evaluation of insulation technologies has
been observed. On the far horizon polymer insulations might become an option.
However when benchmarking with mica tape insulation, the required tight quality
control for the application in manufacturing and the non-existent inherent
fault tolerance for inner discharges become obstacles. Therefore it looks that
small steps in
today’s proven insulation technology will
be realized earlier. Such novelties close to introduction are: - Improved tape,
now commercially available: high
thermal conductivity using fillers (HTC),
higher mica content by denser roving carrier. Both technologies are in
verification tests. The maximum achievable thermal conductivity is at 0.5 W/mK.
- Improving the insulation system to a higher thermal class (class 180). Such a
technology is in final verification and will soon be available. - Increasing
the electrical field stress to a higher value, a 15% gain seems achievable.
This allows a better heat transfer and more copper in the slot. As specified by
standards, insulation verification tests are commonly based on comparative
tests in specific
characteristics. Any modified insulation
system must be at least as good in these characteristics as the established technology.
Other criteria are sensitivity to manufacturing variances, throughput time,
environmental compliance and second source availability for the components. All
these improvements for the stator winding insulation look likely to shift the
bottleneck into the rotor. Fortunately, the rotor material technology brings
along all prerequisites to be upgraded into class 180 technology. This is due
to the fact that many components are inherently class 180 and simply need a
tighter specification to become qualified. In the case of class 180, allowing
class 155 operation, and probably in a later stage class 180 peaking, it is of
utmost importance that both stator and rotor winding designs can accommodate
their elongation due to thermal expansion. A set of design measures has been
worked out to provide this safety. The materials used in laminates can be the
same or different. An example of the type of laminate using different materials
would be the application of a layer of plastic film — the "laminate"
— on either side of a sheet of glass — the laminated subject. Vehicle
windshields are commonly made by laminating a tough plastic film between two
layers of glass. Plywood is a common example of a laminate using the same material
in each layer. Glued and laminated dimensioned timber is used in the
construction industry to make wooden beams, Glulam, with sizes larger and
stronger than can be obtained from single pieces of wood. Another reason to
laminate wooden strips into beams is quality control, as with this method each
and every strip can be inspected before it becomes part of a highly stressed
component such as an aircraft undercarriage.
Examples of laminate materials include
Formica and plywood. Formica and similar plastic laminates (such as Pionite,
Wilsonart, Lamin-Art or Centuryply Mica) are often referred to as High Pressure
Decorative Laminate (HPDL) as they are created with heat and pressure of more
than 5 psi (34 kPa). A new type of HPDL is produced using real wood veneer or
multilaminar veneer as top surface. Alpikord produced by Alpi spa and
Veneer-Art, produced by Lamin-Art are examples of these types of laminate.
Laminating paper, such as photographs, can
prevent it from becoming creased, sun damaged, wrinkled, stained, smudged,
abraded and/or marked by grease, fingerprints and environmental concerns. Photo
identification cards and credit cards are almost always laminated with plastic
film. Boxes and other containers are also laminated using a UV coating. Lamination
is also used in sculpture using wood or resin. An example of an artist who used
lamination in his work is the American, Floyd Shaman.
Further, laminates can be used to add
properties to a surface, usually printed paper, that would not have them otherwise.
Sheets of vinyl impregnated with ferro-magnetic material can allow portable
printed images to bond to magnets, such as for a custom bulletin board or a
visual presentation. Specially surfaced plastic sheets can be laminated over a
printed image to allow them to be safely written upon, such as with dry erase
markers or chalk. Multiple translucent printed images may be laminated in
layers to achieve certain visual effects or to hold holographic images. Many
printing businesses that do commercial lamination keep a variety of laminates
on hand, as the process for bonding many types is generally similar when
working with arbitrarily thin material..
Sealing
After bearing maintenance, both halves of
seal labyrinth should be fixed together by a circlip ring. They must be
inserted into the ring seat, so the locking pin is fitted into the undercut of
the upper half part of housing. Poor installation damages the sealing. Before
seal assembling, clean carefully the contact surfaces of the ring and seating
and coat the contact area with soft sealing compound. Drain holes at bottom half
of the ring should be cleaned and cleared. When installing this halve of the
sealing ring, press it slightly against bottom shaft side.
CONCLUSION
Since more than 100 years turbogenerators
have been in use for steam turbine and gas turbine applications of any size.
The technical evolution has not stopped; new market requirements and new
material technologies ask for adaptations in design. The future market will be
characterized by a revitalized need for very large turbogenerators, both
two-pole and 4-pole. The future will also be characterized by an exciting
competition between well-established conventional solutions and new “high tech”
solutions. In any case highly skilled engineers paired with the best available
design tools will be required .
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