TECHNICAL FIELD

One important condition for transformers to be provided with superconducting windings while at the same time becoming a competitive product in relation to transformers with conven¬ tional windings is that the alternating-current (ac) losses are minimized. Because the currently available high- temperature superconducting materials are designed as a tape, superconducting windings will resemble conventional sheet-wound windings. Since the ac losses in the supercon¬ ducting tape are to a great extent dependent on the direction of an alternating magnetic field relative to the plane of the tape, special measures must be taken to be able to maintain the ac losses at an acceptable level. The present invention relates to a design which contributes to minimize these losses.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 schematically shows the magnetic field configura¬ tion of two concentric transformer windings around a limb of a transformer core,

Figure 2 shows how, according to the state of the art, conventional sheet-wound transformer windings are designed to keep the losses as low as possible.

Figure 3 shows how to proceed, according to the state of the art, to obtain low losses when using superconducting windings.

Figure 4 shows a design according to the invention to obtain low losses.

BACKGROUND ART, PROBLEMS

Characteristic of superconductors is that the possible current density is considerably higher than for ordinary conductors. As mentioned above, a superconductor which is to be used in a transformer winding usually has the shape of a tape. The currently available tape contains about 30% superconductor and 70% silver matrix. The superconducting material has such an orientation in the tape that the ac losses are 3-5 times greater if the alternating magnetic field to which the conductor is subjected is directed perpendicular to the plane of the tape than if the field lies in the plane of the tape.

If no special measures are taken for a transformer, the magnetic field configuration for two concentric windings 1 and 2 around a limb 3 of the transformer core will have the appearance shown in Figure 1. The substantially axial magnetic leakage flux 4 extending between the windings deflects into a more or less radial direction at the two ends of the windings. This causes the windings to become traversed by a magnetic flux with a radial component which generates eddy currents in the conductor and causes losses. This is, of course, true irrespectively of whether the winding consists of a conventional sheet winding or if it is designed as a superconducting sheet (tape) winding. Because of the loss properties of the superconducting tape depending on the direction of the field relative to the plane of the tape, this problem will become still more accentuated in superconducting windings than in conventional windings.

A typical example of the technique used in conventional sheet windings to reduce the losses arising because of the radial field component is disclosed in US 4 323 870. Instead of attempting to influence the field configuration, the conductor material is allowed to follow the field. This is done in such a way that the tape is formed such that the magnetic field vector at each point is tangent to the conductor surface. In this way, the eddy currents can be

eliminated. The accompanying Figure 2, which is identical with Figure 1 in the above US patent, shows how sheet-wound windings can be designed to achieve this desired effect. The inward rounding towards the core limb is brought about by rounding off the winding support body 5, which is really not necessary from a mechanical point of view, at the ends towards the core limb, and the outward rounding can be performed with the aid of wedges or linings of different kinds. This method, however, entails an increased space requirement, increased radius of the windings resulting in increased losses, and a relatively time-consuming and diffi¬ cult winding work.

An example of how to proceed to keep the losses low when superconducting windings are used is clear from SE 92024553, "Winding support body for transformers/reactors with super¬ conductors" and the accompanying Figure 3. The superconduc¬ ting tape 8 is wound onto a winding support body 6 consis¬ ting of a substantially straight, circular-cylindrical, tubular support body. Between the core limb 3 and the winding support body there is a so-called cryogenic wall 7. On that side where the winding is to be placed, the support body is provided outwards towards the two ends with a helical slot along the support body, the slot having a plane surface with a width equal to the width of the conductor. In this way, each turn of the slot around the support body outwards towards the ends forms a surface which practically constitutes the envelope surface of a straight frustum of a cone. The angle between a generatrix 9 of the envelope surface of the straight frustum of a cone and the axial centre line of the support body increases for each turn outwards towards the ends of the support body such that the envelope surface, at all points, coincides with the direction of the magnetic field. The mid-portion of the support body has a circular-cylindrical envelope surface without slots. Because of the helical slot along the ends of the support body, the superconducting tape can be wound continuously in one or more layers along the entire support body.

SUMMARY OF THE INVENTION, ADVANTAGES

According to the invention, a different approach is used for preventing the occurrence of eddy currents in the tape at the ends of the windings and ac losses associated therewith. Instead, the magnetic field is influenced such that, in the axial length of the whole windings, the magnetic field extends in parallel with the symmetry axis of the windings, that is, it does not, as in Figure 1, deflect at the ends of the windings.

Between the concentrically disposed secondary and primary windings of the transformer, there is a concentric tubular space, that so-called main channel, filled with insulating material for electrical insulation between the two windings. To control the field so as to extend in parallel with the symmetry axis of the windings also at the ends of the windings, a number of concentric cylindrical screens of magnetic material with a high relative permeability, suitably greater than 10 ³ , are introduced together with and in the insulation in the two ends of the main channel. If the screens are of a non-electrically conducting material, they may form closed turns. At a certain electrical conduc¬ tivity, the screens are to be slotted or, in case of over- lapping, have a satisfactory insulation at the overlap.

One embodiment of the invention is clear from Figure 4. The concentric windings are shown in Figure 1 at 1 and 2 and the transformer limb is shown at 3. The magnetic field lines 4 are indicated in the main channel 10 which is filled with insulation material. In the upper and lower parts of the main channel, a number of cylindrical screens 11 and 12 of a magnetic material according to the above are concentrically introduced. As is clear from the figure, the object of these screens is to concentrate the flux lines such that, also at the upper and lower parts of the windings, they extend in the main channel and do not more or less radially penetrate through the windings.

The scope of the invention allows for the number of screen rings to be different depending on the current space in the main channel, the thickness and the magnetic properties, in other respects, of the screen material. However, the -screen rings must be dimensioned such that, at rated current, they do not become magnetically saturated. At the same time, there must be sufficient space for the necessary insulation between the windings. Otherwise, the axial length of the screens should be adapted to the current transformer design. A suitable material for these screens is so-called amorphous transformer sheet or other material with negligible hysteresis losses.

In one embodiment the concentric rings may be replaced by a helical ring.