Electricity: conductors and insulators – With fluids or vacuum – With fluid maintenance or conditioning means
Patent
1979-10-02
1982-08-31
Askin, Laramie E.
Electricity: conductors and insulators
With fluids or vacuum
With fluid maintenance or conditioning means
174 27, 174106R, 174126CP, H01B 906, H02G 506
Patent
active
043474012
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a gas-filled cable intended for the transmission of high-voltage electric current and consisting of one or more conductors, these being held axially in position by means of supporting insulators inside an enclosing conduit filled with an insulating gaseous medium.
According to known practice, gas-filled cables may be used for both underground and surface transmission of electric current up to about 1,000 kV. By comparison with overhead power-lines carrying the same voltage, gas-filled surface cables have proved to have a greater current-carrying capacity and at the same time to be less liable to operational disturbances. However, a major disadvantage of such cables has, until now, been the high cost of manufacture, the chief contributor to this being the outer conduit of the cable itself which must be so constructed as to keep power losses at a reasonable level. Laboratory tests have previously indicated that carbon steel is an unsuitable material for cable conduits, one reason for this being that its use is associated with large losses of power.
In cables buried underground a further difficulty is met with in the heat emitted from the cable, which must be carried off through the ground. This causes the ground in the vicinity of the cable to dry out, leading in turn to reduced thermal dissipation capacity in the ground itself and a possible deterioration of the natural environment in which the cable is buried-plant-life, for instance, may be harmed. In order to avoid the ground drying-out and the side-effects which this may involve, the temperature of the conduit of a gas-filled cable should not rise above approx. 40.degree. C.; and in view of the danger of thermal collapse of the insulating medium present within the cable itself, the maximum operational temperature inside the ducts should not be in excess of 105.degree. C. It is therefore essential that the heat developed in the cable be kept as low as possible and that the conditions in the vicinity of the cable be kept constantly favourable to thermal conduction. In previous types of gas-filled cables using carbon steel in the conduit, induced currents in the conduit cause heat releases greater than in cables having a conduit of e.g. aluminium, copper or other non-magnetic material.
A method known previously in connection with cooling systems for superconducting cryogenic cables uses a cable conduit comprising an outer sheath of high-alloy steel consisting of an iron
ickel alloy containing 30-45% nickel, and an inner lining or shield of e.g. aluminium. By this means, losses in the outer conduit can be kept down, while the conduit itself functions as a transport system for a cooling medium which may be liquid helium, He, or nitrogen gas, N.sub.2.
The main objective of the invention is to reduce heat emission from gas-filled cables at the same time as power losses are kept low and the total manufacturing costs are such that the cable type will be able to compete on the market. In gas-filled cables of the type described in the introductory paragraph this is rendered possible in that the conduit comprises a cylindrical metal sheath of magnetic material enclosing a metal shield of non-magnetic material having low resistivity. Here it has proved feasible to construct an outer metal sheath of plain or low-alloy merchant steel with a carbon content of less than 0.6%, preferably approx 0.2%. Like the sheath, the shield may also be cylindrical; a suitable thickness for this has been found to be approx. 8-30% of the total thickness of the conduit, although 12-20% is to be preferred.
Another objective of the invention is to achieve a cable of the type under discussion in which the insulation between the conduit and the conductors running inside the conduit is maintained at a constant high level of efficiency throughout the life of the cable, for it has been found that in gas-filled cables of the conventional type small particles, of e.g. metal, often remain inside the cable after it has been brought into service. These particles may
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Parker, G. H., Wire , "Clad Metals in Cable Construction", Mar. 1966, pp. 405, 406, 408, 455 and 456.
Afzelius Raoul
Bergqvist Hakan
Knudsen Niels
Orpana Veikko
Warulf Olov
Askin Laramie E.
SPACAB AB
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