Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
1998-11-27
2002-07-09
Nguyen, Chau N. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
Reexamination Certificate
active
06417456
ABSTRACT:
TECHNICAL FIELD
The present invention relates in a first aspect to an insulated conductor for high-voltage windings in rotating electric machines.
A second aspect of the present invention relates to a method of adapting an insulated conductor for high-voltage windings in rotating electric machines.
A third aspect of the present invention relates to a rotating electric machine comprising an insulated conductor of the type described above.
The machine is intended primarily as generator in a power station for generating electric power.
The invention is applicable in rotating electric machines such as synchronous machines. The invention is also applicable in other electric machines such as dual-fed machines, and applications in asynchronous static current cascades, outer pole machines and synchronous flow machines, provided their windings consist of insulated electric conductors of the type described in the introduction, and preferably at high voltages. “High voltages” here refer to electric voltages exceeding 10 kV. A typical working range for an insulated conductor for high-voltage windings according to the invention may be 36-800 kV.
BACKGROUND ART
In order to be able to explain and describe the machine, a brief description of a rotating electric machine will first be given, exemplified on the basis of a synchronous machine. The first part of the description substantially relates to the magnetic circuit of such a machine and how it is constructed according to classical technique. Since the magnetic circuit referred to in most cases is located 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.
The stator winding is located in slots in the sheet iron core, the slots normally having a rectangular or trapezoidal cross section as that of a rectangle or a trapezoid. Each winding phase comprises a number of series-connected coil groups connected in series and each coil group comprises a number of series-connected coils connected in series. The different parts of the coil are designated coil side for the part which is placed in the stator and end winding end for that part which is located 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 from the slot with a coil insulation, that is, an insulation intended to withstand the rated voltage of the machine to earth. 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 graphite-based paint to improve the contact with the surrounding stator which is connected to earth 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 connected in parallel, 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 operating temperature, copper, for example, has a maximum value of 3-3.5 A/mm2.
The maximum flux density (magnetic flux) in the stator and rotor material.
The maximum electric field strength in the insulating material, the so-called dielectric strength.
Polyphase ac windings are designed either as single-layer or two-layer windings. In the case of single-layer windings, there is only one coil side per slot, and in the case of two-layer windings there are two coil sides per slot. Two-layer windings are usually designed as diamond windings, whereas the single-layer windings which are relevant in this connection may be designed as a diamond winding or as a concentric winding. In the case of a diamond winding, only one coil span (or possibly two coil spans) occurs, whereas flat windings are designed as concentric windings, that is, with a greatly varying coil span. By coil span is meant the distance in circular measure between two coil sides belonging to the same coil, either in relation to the relevant pole pitch or in the number of intermediate slot pitches. Usually, different variants of chording are used, for example short-pitching pitch, to give the winding the desired properties.
The type of winding substantially describes how the coils in the slots, that is, the coil sides, are connected together outside the stator, that is, at the end windings ends.
Outside the stacked sheets of the stator, the coil is not provided with a painted semiconducting earth-potential layer. The end winding end is normally provided with an E-field control in the form of so-called corona protection varnish intended to convert a radial field into an axial field, which means that the insulation on the end windings ends occurs at a high potential relative to earth. This sometimes gives rise to corona in the end-winding-end region, which may be destructive. The so-called field-controlling points at the end windings ends entail problems for a rotating electric machine.
Normally, all large machines are designed with a two-layer winding and equally large coils. Each coil is placed with one side in one of the layers and the other side in the other layer. This means that all the coils cross each other in the end winding end. If more than two layers are used, these crossings render the winding work difficult and deteriorate the end winding end.
It is generally known that the connection of a synchronous machine/generator to a power network must be made via a AE/YD-connected so-called step-up transformer, since the voltage of the power network normally lies at a higher level than the voltage of the rotating electric machine. Together with the synchronous machine, this transformer thus constitutes integrated parts of a plant. The transformer constitutes an extra cost and also has the disadvantage the advantage that the total efficiency of the system is lowered. If it were possible to manufacture machines for considerably higher voltages, the step-up transformer could thus be omitted. During the last few decades, there have been increasing requirements for rotating electric machines for higher voltages than for what has previously been possible to design. The maximum voltage level which, according to the state of the art, has been possible to achieve for synchronous machines with a good yield in the coil production is around 25-30 kV.
Certain attempts to a new approach as regards the design of synchronous machines are described, inter alia, in an article entitled “Water-and-oil-cooled Turbogenerator TVM-300” in J. Elektrotechnika, No. 1, 1970, pp. 6-8, in U.S. Pat. No. 4,429,244 “Stator of Generator” and in Russian patent document CCCP Patent 955369.
The water- and oil-cooled synchronous machine described in J. Elektrotechnika is intended for voltages up to 20 kV. The article describes a new insulation system consisting of oil/paper insulation, which makes it possible to immerse the stator
Carstensen Peter
Leijon Mats
ABB AB
Dykema Gossett PLLC
Nguyen Chau N.
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