Electricity: conductors and insulators – With fluids or vacuum – With cooling or fluid feeding – circulating or distributing
Patent
1995-05-17
1998-12-15
Sough, Hyung S.
Electricity: conductors and insulators
With fluids or vacuum
With cooling or fluid feeding, circulating or distributing
336216, H01F 3600
Patent
active
058500545
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
In the application of the superconducting effect, the object whose electrical conductors in this application consist of a number of strands of a winding, is normally disposed in a so-called cryotank. In the lower part of the cryotank the refrigerant is arranged in the form of a cryogenic liquid surrounding the object. The space of the cryotank above the liquid level is occupied by the refrigerant in gaseous state. The electric current connection to the object is performed via current leads in bushings which via fixing flanges are connected to the lid of the cryotank. The invention relates to a connection arrangement between the strands of the winding and the current leads in a.c. applications which ensures a good division of current between the strands.
BACKGROUND ART, THE PROBLEMS
The conductors in reactor or transformer windings are often divided into a plurality of strands insulated from each other to minimize the unfavourable effects of the skin effect. Even if the strands are well transposed, there will always be a certain variation in the induced voltage since the different strands do not surround an exactly equally great magnetic flux. This, in turn, leads to the current distribution between the different strands becoming uneven, whereby the so-called copper losses increase. The resistance of the strands, however, has a stabilizing effect on the current distribution since the strands in which the induced voltage is greatest will have the largest currents and hence also the greatest resistive voltage drops.
Now, if such a winding consists of a plurality of superconducting strands, the stabilizing resistive voltage drops will be negligible. Since the prior art--see, for example, an article entitled "Development of a Large-Capacity Superconducting Cable for 1000 kVA-Class Power Transformers", IEEE TRANSACTIONS ON MAGNETS, VOL. 28, NO. 1, January 1992, pages 394-397 (especially page 397 and FIG. 7)--comprises electrically connecting the strands of the winding to each other at the terminals of the winding, the variation in the induced voltage may give rise to a great variation in the current distribution. The strand which has to carry the largest current may then risk arriving at a state in which it loses its superconducting ability because the critical current density is exceeded. This leads to an unwanted local heating.
However, a corresponding problem does not arise in d.c. applications with superconducting strands, for example in connection with magnets. The reason for this is that, in steady state, no voltage is induced which may give rise to variation in the current distribution and that current changes take place with a very low time rate of change.
A problem which arises in connection with superconducting applications is the heat influx to the cryogenic liquid which takes place because of the temperature difference between the surroundings and the object. This is due to the fact that the current leads of the bushing, besides being good electric conductors, are also good thermal conductors. In addition, at least at high currents, heat is developed in the current leads of the bushing due to the current which flows through the current leads. The electric heat generation takes place as a result of the ohmic resistance in the current leads. In case of alternating current, there is also the generation of heat because of the occurrence of eddy currents. The increased resistance arising because of the skin effect must also be taken into account. The gas developed because of the heat influx to the cryotank is allowed, via an opening on that part of the bushing which is located outside the cryotank, to flow freely out into the surroundings.
The above means that a gas flow, which at the interface between liquid and gas largely maintains the temperature of the liquid, on its way up to the lid and the discharge into the surroundings, where it assumes the temperature of the surrounding air, flows around the current leads and hence can be used for cooling thereof. Since the direction of the
REFERENCES:
patent: 4447670 (1984-05-01), Eckels
K. Funaki et al., Development of a Large-Capacity . . . Transformers, IEEE Transactions on Magnetics, vol. 28, No. 1, Jan. 1992, pp. 394-397.
M.N. Wilson, Superconducting Magnets, 1983, pp. 256-273.
Bonmann Dietrich
Hornfeldt Sven
Asea Brown Boveri AB
Sough Hyung S.
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