30-11-2012, 12:35 PM
A Turbo Generator
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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.[citation needed]
Turbo generators were also used on steam locomotives as a power source for coach lighting and heating systems.
since the 1901 invention of the cylindrical rotor of
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.
In the early 1930ies two European manufacturers were introducing 36 kV stator windings, thus eliminating the machine transformer. All such designs were suffering of continuous heavy electrical discharges, and were soon discontinued. After a 60-year time-out, a manufacturer surprised the world in 1998 with a cable-based high-voltage
generator up to 400 kV. However again, the cable technology was not ready for turbo generator requirements, and a breakthrough for commercial application was not achieved. In the 1930 US manufacturers were introducing hydrogen as coolant. When combined with direct conductor hydrogen cooling in the rotor, and later in the stator, this allowed a
considerable increase in specific utilization and efficiency. By early 1960 the unit ratings were achieving 500 MVA. At that time deionized water cooling in the stator winding was introduced. Around 1960 all major manufacturers changed their insulation system to mica tape with synthetic resin impregnation, a technology for thermal qualification at 155°C, and which has been lasting into these days. By end of the 1960, with the power semiconductors becoming mature, the dc machine excitation (Fig.2) was superseded by the static excitation, and by an ac exciter machine with rotating diodes.
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.
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.
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.