Inductor devices – With coil winding and/or unwinding
Reexamination Certificate
2000-05-25
2002-02-26
Mai, Anh (Department: 2832)
Inductor devices
With coil winding and/or unwinding
C336S206000, C336S181000, C336S195000
Reexamination Certificate
active
06351202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a stationary induction apparatus such as a transformer or a reactor, and more particularly to an insulating structure for a winding thereof.
2. Description of the Background Art
A transformer is a stationary induction apparatus including two windings. The transformer has come into widespread use in power systems for transmission of electric power, and is for transforming a current value and a voltage value through the use of electromagnetic induction between identical frequency circuits. Furthermore, a reactor is composed of one or more windings, and is for adding an inductance to an electric circuit or a power system. In general, among internal cooling modes, there are oil cooling, gas cooling, liquid cooling, and air cooling, while among external cooling modes, there are air cooling, water cooling, and other types of cooling. Additionally, among magnetic circuits, there are an iron core formed by stacking silicon steel plates and an air core having no iron core.
In any case, parts of an electric conductor forming a winding carry different voltages. For insulation between electric conductors, there have been employed insulating methods, such as an oil filled mode using an insulating oil and a fibrous insulating material, a sulfur hexafluoride (SF
6
) gas filled mode, a resin mold mode, and others. In general, these insulation means comprise a combination of a plurality of materials or layers differing from each other in insulating ability. In one concept, in a conventional transformer or reactor, to enhance the overall insulating ability, an insulating medium such as oil, gas, or air, having a lower insulating ability than a solid, is subdivided by solid insulating layers to shorten the distance in the insulating medium. For example, as mentioned in the “Electrical Engineering Handbook” of the Institute of Electrical Engineers of Japan, 1988, pp. 673-674, the conventional transformer is constructed such that each of the oil layers among a high-voltage winding, a low-voltage winding, an iron core and a tank is divided by insulating partitions made from inter-layer insulating paper. It has been known that the insulating ability of this oil layer per unit distance improves by distance-reduction of the width of the oil layer. That is, the shortening of the width distance of the oil layer, divided by the insulating partitions made from the inter-layer insulating paper, not only enhances dielectric strength but also improves the insulating ability between the windings in addition to suppressing discharge phenomena, provided by the insulating portions, which causes dielectric breakdown. Likewise, in a gas insulating device, as is known from Paschen's law, the gas section is divided for the distance-reduction of the width of each gas layer, thereby boosting the dielectric breakdown electric field.
Furthermore, a problem in insulation arising in the conventional oil filled transformer relates to an electrification phenomenon called “flow electrification”. In general, in an oil filled transformer, an oil is circulated from a lower portion of a winding to an upper portion thereof for cooling. When this oil comes into contact with an insulating material surface, charge transfer takes place in the vicinity of the boundary between the oil and the insulating material. In addition, the charge transferred due to the oil flow is carried away to cause charge separation. Accordingly, the oil and the insulating material are electrified in reverse polarity. If the electrified charge is accumulated on the surface of the insulation material, a strong electric field appears partially in a surface along an oil layer or an insulating member, which results in abnormal insulation. To prevent this flow electrification, there are countermeasures: (1) restraining the oil flow rate to below some limit by utilizing the characteristic in which the charge quantity decreases with a decrease of the oil flow velocity, and (2) adding an additive to the oil to suppress charge-transfer.
Japanese Unexamined Utility Model Application Publication No. (SHO) 58-175618 discloses a technique in which, to achieve the insulation distance reduction, insulating paper is placed to run among cylindrical windings so that the filling is made with the insulating paper. This utilizes the fact that an oil immersed solid insulating layer has a higher insulating ability than an oil layer. However, irregularities on the winding surfaces are unavoidable due to its structure and manufacturing precision, which indicates that difficulty will be encountered in practice in bringing the insulating paper into close contact with the windings. If an oil layer exists on the winding surfaces, an electric field is concentrated in the oil layer due to the difference in dielectric constant between the insulating paper and the oil. Additionally, as mentioned above, since the oil layer is inferior to the insulating paper layer in insulating ability, a poor insulation structure is formed. Accordingly, it is difficult for the insulating paper to display the entirety of its insulating ability.
So far, a method of preventing such a drawback in the insulation structure has been employed for high-voltage rotating machines. For a stator winding of a high-voltage rotating machine above 1 kV, a winding called “formed-coil” has been put to use. This has a construction in which a plurality of insulation-coated conductors are bundled and covered with a composite solid insulation of a synthetic resin and mica, and further is inserted into a slot made in an iron core. Gaps develop between these conductors and the solid insulation and between the solid insulation and the iron core due to stress or deterioration occurring, for example, in the manufacturing process, at the start/stop or during the operation, which constitutes a weak point on insulation and causes partial discharge. A way to prevent this is described in “Manufacturing and Maintenance of Electric Coil” 1990, p.133, translated by Hisayasu Mitsui, et al. published by Kaihatsusha (from the original by H. Seuentz, “Herstellung der Wicklungen electrischer Maschinen”). That is, a semi-conductive layer called an internal corona shield is placed inside a solid insulation and between the solid insulation and a conductor so that the same electric potential is maintained with respect to the conductor, while a semi-conductive layer called an external corona shield is placed outside the solid insulation and between the solid insulation and an iron core to maintain the identical electric potential with respect to the iron core. At this time, the semi-conductive layer is constructed to have a high adhesive strength with respect to the solid insulation so that a gap more easily occurs between the semi-conductive layer and the conductor or between the semi-conductive layer and the iron core. Accordingly, even if a gap develops between the semi-conductive layer and the conductor or between the semi-conductive layer and the iron core, the electric field in the interior of the gap is relieved, thereby suppressing the occurrence of partial discharge and preventing the occurrence of weak points in the insulation structure.
If an insulating structure, in which both surfaces of a solid insulation extending perpendicularly to the electric field applying direction are covered with a semi-conductive material as mentioned above, is applied to a transformer or a reactor, then the lowering of the insulating ability may be prevented and the reliability may be improved. Such an insulating structure for a transformer or a reactor is disclosed, for example, in Japanese Unexamined Patent Application Publication No. (HEI) 10-6350 or in PCT International Publication No. W097/45847. However, a conventional high-voltage rotating machine is constructed such that, in the entire winding, the insulation has the same thickness, and the electrical fields in the insulation differs greatly at a high-voltage section of the winding and at a low-voltage section th
Hasegawa Taketoshi
Hosokawa Noboru
Ishikawa Kiyoyuki
Ito Keiichi
Toba Yasuo
Leydig , Voit & Mayer, Ltd.
Mai Anh
Mitsubishi Denki & Kabushiki Kaisha
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