Turbo Generator-Seminar Report

turbo generator


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.


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. 

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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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..


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.


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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