Cryogenic power cable

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a cryogenic power cable which permits large capacity power transmission with the electric resistance and the joule loss of the conductor of the cable reduced by cooling the conductor with a cryogenic cooling medium and more particularly to a system wherein a layer of a thermal insulating material is provided on the cryogenic conductor before providing an electric insulating arrangement on the conductor so that the ordinary electric insulating materials which have conventionally been used for power cables can be used as they are for the cryogenic conductor.
With oil impregnated paper used for an electric insulation layer, for example, the temperature of the layer can be maintained within a range of -40.
degree.
to -100.
degree.
C.
and this provides for designing for high working stress.
At present, cryogenic power cables are being developed in various countries of the world.
However, all of such developments have been confined to the methods of effecting electrical insulation under cryogenic temperature.
For example, a cooling medium such as liquid nitrogen, helium, etc.
is used either directly for electrical insulation or for impregnating a plastic tape therewith.
The latter method gives a higher performance in terms of withstand voltage, and is therefore considered more promising than the former.
Hence such research and development work is being conducted in the latter method in most cases.
The electric insulating paper which is prepared by impregnating plastic tape with a cryogenic cooling medium, for example polyethylene paper impregnated with liquid nitrogen, gives the characteristic which is close to that of the conventional oil impregnated kraft paper.
However, as compared with the oil impregnated kraft paper, it is inferior in terms of impulse breakdown strength.
It also has many unknown points that require further studies with respect to stability over a long period of time.
On the other hand, polyethylene paper impregnated with liquid helium has the breakdown characteristic which is only about a half of that of the oil impregnated insulation material