Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-04-09
2004-11-23
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S179000, C310S180000, C310S195000, C174SDIG001, C174SDIG002, C174SDIG002
Reexamination Certificate
active
06822363
ABSTRACT:
TECHNICAL FIELD
This invention is related to an electromagnetic device for electric power purposes, comprising a magnetic field generating electric circuit including at least one electric conductor having an insulation system. This electromagnetic device may be used in any electrotechnical connection. The power range may be from VA up to the 1000-MVA range. High voltage applications are primarily intended, up to the highest transmission voltages used today.
According to a first aspect of the invention a rotating electric machine is contemplated. Such electric machines comprise synchronous machines which are mainly used as generators for connection to distribution and transmission networks, commonly referred to below as power networks. The synchronous machines are also used as motors and for phase compensation and voltage control, in that case as mechanically idling machines. The technical field also comprises double-fed machines, asynchronous converter cascades, external pole machines, synchronous flux machines and asynchronous machines.
According to another aspect of the invention, said electromagnetic device is formed by a power transformer or reactor. For all transmission and distribution of electric energy, transformers are used and their task is to allow exchange of electric energy between two or more electric systems and for this, electromagnetic induction is utilized in a well-known manner. The transformers primarily intended with the present invention belong to the so-called power transformers with a rated power of from a few hundred kVA up to more than 1000 MVA with a rated voltage of from 3-4 kV and up to very high transmission voltages, 400 kV to 800 kV or higher.
Although the following description of the prior art with respect to the second aspect mainly refers to power transformers, the present invention is also applicable to reactors, which, as is well-known, may be designed as single-phase and three-phase reactors. As regards insulation and cooling there are, in principle, the same embodiments as for transformers. Thus, air-insulated and oil-insulated, self-cooled, pressure-oil cooled, etc., reactors are available. Although reactors have one winding (per phase) and may be designed both with and without a magnetic core, the description of the background art is to a large extent relevant also to reactors.
The magnetic field inducing electric circuit may in some embodiments be air-wound but comprises as a rule a magnetic core of laminated, normal or oriented, sheet or other, for example amorphous or powder-based, material, or any other action for the purpose of allowing an alternating flux, and a winding. The circuit often comprises some kind of cooling system etc. In the case of a rotating electric machine, the winding may be disposed in the stator or the rotor of the machine, or in both.
The invention also comprises a method for electric field control in an electromagnetic device and a method for production of a magnetic circuit.
PRIOR ART
In order to be able to explain and describe the invention, the prior art will be discussed hereinafter both in respect of a rotating electric machine and a power transformer.
Rotating Electric Machine
Such a rotating electric machine will be exemplified based upon a synchronous machine. The first part of the description substantially relates to the magnetic circuit of such a machine and how it is composed according to classic technique. Since the magnetic circuit referred to in most cases is disposed in the stator, the magnetic circuit below will normally be described as a stator with a laminated core, the winding of which will be referred to as a stator winding, and the slots in the laminated core for the winding will be referred to as stator slots or simply slots.
Most synchronous machines have a field winding in the rotor, where the main flux is generated by direct current, and an ac winding in the stator. The synchronous machines are normally of three-phase design. Sometimes, the synchronous machines are designed with salient poles. The latter have an ac winding in the rotor.
The stator body for large synchronous machines are often made of sheet steel with a welded construction. The laminated core is normally, made from varnished 0.35 or 0.5 mm electric sheet. For larger machines, the sheet is punched into segments which are attached to the stator body by means of wedges/dovetails. The laminated core is retained by pressure fingers and pressure plates.
For cooling of the windings of the synchronous machine, three different cooling systems are available.
In case of air cooling, both the stator winding and the rotor winding are cooled by cooling air flowing through. The cooling air channels are to be found both in the stator laminations and in the rotor. For radial ventilation and cooling by means of air, the sheet iron core at least for medium-sized and large machines is divided into stacks with radial and axial ventilation ducts disposed in the core. The cooling air may consist of ambient air but at high power a closed cooling system with heat exchangers is substantially used. Hydrogen cooling is used in turbogenerators and in large synchronous compensators. The cooling method functions in the same way as in air cooling with heat exchangers, but instead of air as coolant there is used hydrogen gas. The hydrogen gas has better cooling capacity than air, but difficulties arise at seals and in monitoring leakage. For turbogenerators in the higher power range it is known to apply water cooling of both the stator winding and the rotor winding. The cooling channels are in the form of tubes which are placed inside conductors in the stator winding. One problem with large machines is that the cooling tends to become non-uniform and that, therefore, temperature differences arise across the machine.
The stator winding is disposed in slots in the sheet iron core, the slots normally having a cross section as that of a rectangle or a trapezoid. Each winding phase comprises a number of series-connected coil groups and each coil group comprises a number of series-connected coils. The different parts of the coil are designated coil side for that part which is placed in the stator and coil end for that part which is disposed outside the stator. A coil comprises one or more conductors brought together in height and/or width. Between each conductor there is a thin insulation, for example epoxy/glass fibre.
The coil is insulated against the slot with a coil insulation, that is, an insulation intended to withstand the rated voltage of the machine to ground. As insulating material, various plastic, varnish and glass fibre materials may be used. Usually, so-called mica tape is used, which is a mixture of mica and hard plastic, especially produced to provide resistance to partial discharges, which can rapidly break down the insulation. The insulation is applied to the coil by winding the mica tape around the coil in several layers. The insulation is impregnated, and then the coil side is painted with a coal-based paint to improve the contact with the surrounding stator which is connected to ground potential.
The conductor area of the windings is determined by the current intensity in question and by the cooling method used. The conductor and the coil are usually formed with a rectangular shape to maximize the amount of conductor material in the slot. A typical coil is formed of so-called Roebel bars, in which certain of the bars may be made hollow for a coolant. A Roebel bar comprises a plurality of rectangular, parallel-connected copper conductors, which are transposed 360 degrees along the slot. Ringland bars with transpositions of 540 degrees and other transpositions also occur. The transposition is made to avoid the occurrence of circulating currents which are generated in a cross section of the conductor material, as viewed from the magnetic field.
For mechanical and electrical reasons, a machine cannot be made in just any size. The machine power is determined substantially by three factors:
The conductor area of the windings. At normal opera
ABB AB
Dykema Gossett PLLC
Mullins Burton
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