Electricity: conductors and insulators – Conduits – cables or conductors – Conductor structure
Reexamination Certificate
1999-02-09
2002-04-23
Nguyen, Chau N. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Conductor structure
Reexamination Certificate
active
06376775
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to conductors for high-voltage windings, preferably for high-voltage windings of a stator in a high-voltage generator, which conductors are stranded with two or more layers of strands electrically insulated from each other, around a central core, which core may be one of the strands of the conductor, air or other material.
The invention also relates to rotating electric machine of the type described herein.
The rotating electrical machines referred to in this context are e.g. synchronous machines, but also double-fed machines, applications in asynchronous static current converter cascades, outerpole machines and synchronous flux machines as well as alternating current machines intended primarily as generators in a power station for generating electric power.
Magnetic circuits referred to in this context, include a magnetic core of laminated, non-oriented or oriented, sheet or other material, for example amorphous or powder-based, or any other arrangement for the purpose of allowing an alternating magnetic flux, a winding, a cooling system, etc., and which may be arranged in the stator of the machine, in the rotor or in both.
2. Discussion of the Background
The stator winding in known generators traditionally consists of an electric conductor having a number of insulated rectangular wires, also known as strands, of copper, aluminium or other suitable metal. These strands are transposed (i.e. change place with each other) and are surrounded by a common insulation in such a way that the bundle of conductors acquires a rectangular cross section. The copper conductors are rectangular in order to reduce the eddy-current losses accomplished by keeping (the linear dimension in the direction of the magnetic field small). However, a conductor with rectangular cross section in a high-voltage insulation (i.e. in a conductor) has the disadvantage in that it provides a much greater field strength at the corners of the conductor, the corners thus becoming dimensioning for the thickness of the insulation. Therefore, from the point of view of optimal insulation thickness, circular conductors would be preferable.
Circular conductors can be constructed in a great number of different ways. The conductor may, for instance, consist of the following:
1) a solid rod of copper or other metal with circular cross section,
2) a conductor stranded from circular wires having the same or different diameters,
3) a conductor stranded from sectioned wires or,
4) a conductor compressed from a number of segments, each of which is in turn stranded from circular wires and then formed into a segment.
In order to reduce the conductor dimension it is sometimes compressed or compacted after stranding, thereby altering the shape of the strands in the outermost layer or in the whole conductor, which may costitute a disadvantage.
To ensure high energy transfer in voltage-transmission lines with a conductor for a given voltage, the current must be increased, which is only possible if the conductor area is increased. When the current becomes large this entails the drawback that the current distribution in the conductor becomes uneven (the current endeavours to reach the outer surface of the conductor) and what is known as a “skin effect”, current pinch effect, is obtained. To counteract this effect, conductors , according to prior art, having a large cross section (>1200 mm
2
Cu) are produced, usually called Millikan conductors, i.e. conductors built up of a number of concentrically arranged wires which have subsequently been compressed and shaped. Such a conductor is often composed of 5 or 7 segments which are in turn insulated from each other. Such a construction is effective in reducing the current pinch effect in transmission and distribution cables for high-voltage.
It is previously known that, in distribution systems for high-voltage power transmission, all the strands in the cable have been insulated with varnish, for instance, in order to reduce the current pinch effect, see the publication Hitachi Cable Review, No. 11, August 1992, pages 3-6: “An EHV Bulk Power Transmission Line Made with Low Loss XLPE Cable”. This publication also describes how a few of the strands in the outermost layer are left uninsulated in order to prevent differences in potential between the wire strands and the inner semi-conductor layer. No application of this technology on generator windings, however, is described.
With generators having conventionally designed stator windings as described above, the upper limit for generated voltage has been deemed to be 30 kV. This usually means that a generator must be connected to the power network via a transformer which steps up the voltage to the level of the power network, —in the range of 130-400 kV.
During the last decades, there have been increasing demands for rotating electric machines for higher voltages than 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. It is also commonly known that the connection of a synchronous machine/generator to a power network must take place via a &Dgr;/Y-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. Thus, this transformer, and the synchronous machine, constitute integral parts of an installation. The transformer constitutes an extra cost and also has the disadvantage that the total efficiency of the system is reduced. If it were possible to manufacture machines for considerably higher voltages, the step-up transformer could thus be omitted.
Attempts to develop the generator for higher voltages have, however, been in progress for a long time. This is clear, for instance from “Electrical World”, Oct. 15, 1932, pages 524-525, which describes how a generator designed by Parson in 1929 was arranged for 33 kV. It also describes a generator in Langerbrugge, Belgium, which produced a voltage of 36 kV. Although the article also speculates on the possibility of increasing voltage levels still further, the development was curtailed by the concepts upon which these generators were based. This was primarily because of the shortcomings of the insulation system where varnish-impregnated layers of mica oil and paper were used in several separate layers.
Certain attempts to find 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 completely in oil. The oil can then be used as a coolant while at the same time using it as insulation. To prevent oil in the stator from leaking out towards the rotor, a dielectric oil-separating ring is provided at the internal surface of the core. The stator winding is made from conductors with an oval hollow shape provided with oil and paper insulation. The coil sides with their insulation are secured to the slots, made with rectangular cross section, by way of wedges. As coolant, oil is used both in the hollow conductors and in holes in the stator walls. Such cooling systems, however, entail a large number of connections for both oil and electricity at the coil ends. The thick insulation also entails an increased radius of curvature of the conductors, which in turn results in an increased size of the winding overhang.
The above-mentioned U.S. patent relates to the stator part of a synchronous machine which comprises a magnetic core of laminated sheet with trapezoidal slots for the sta
Carstensen Peter
Johannesson Kenneth
Leijon Mats
Milton Stefan
Rydholm Bengt
ABB AB
Nguyen Chau N.
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