Solid DC cable

Details Solid DC cable Solid DC cable

1998-04-29

2001-03-13

Kincaid, Kristine (Department: 2831)

Electricity: conductors and insulators

Conduits, cables or conductors

Insulated

C174S12000C, C174S1200FP

Reexamination Certificate

active

06201191

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an electrical power cable optimum for long-distance and large-capacity transmission, and particularly relates to a structure of a DC submarine power transmission cable.
DESCRIPTION OF THE RELATED ART
Conventionally, as a long-distance and large-capacity DC cable, there has been used a solid cable (Mass-Impregnated Cable, Non-Draining Cable, or the like) which uses kraft paper as insulating tape material and which is impregnated with high-viscosity insulating oil (for example, 25 to 100 cst at 120° C., and 500 to 2,000 cst at the maximum service temperature (50 to 60° C.) of the cable). The thickness of this insulating tape is, generally, about 70 to 200 &mgr;m because a thin insulating tape is low in mechanical strength, and a large-sized winding machine is required as the number of wound sheets increases.
Unlike an OF cable, an insulating oil is not supplied to a solid DC cable from the opposite ends of the cable. Accordingly, a void is generated because of shortage of the insulating oil in an insulating layer, and the void is apt to be a start point of discharge when it grows up to a harmful size. Such a void is apt to be generated first in an oil gap which is inevitably appears when the insulting tape is wound spirally, and apt to be generated next in porous substances of natural fibers in the insulating tape. The thicker the insulating tape, the larger the oil gap. In a conventional solid DC cable, for example, the voltage was comparatively low to be not higher than 400 kV, and the transmission current was comparatively small to be smaller than 1,000 A. Accordingly, voids apt to be generated in oil gaps just above a conductor, or just above the inner semiconductive layer in case that there applies the inner conductive layer have not been regarded as a problem particularly.
However, plans to transmit large electric power at a long distance through a solid DC cable have come out in succession recently. For example, lines for a transmission voltage of 450 kV or 500 kV or more, and a transmission current larger than 1,000 A have been planned. Under such a high voltage and such a large current, harmful voids formed in an insulating layer particularly just above a conductor could not be ignored.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a solid DC cable in which even if voids are generated when load is cut off, harmful discharge in the voids can be restrained.
In the present invention, (1) a carbon paper layer having volume resistivity which is one or more figures lower than the volume resistivity of an insulating tape constituting a main insulating layer, (2) a kraft paper layer having volume resistivity which is 70% or less of the volume resistivity of the insulating tape, or (3) the carbon layer of (1) and the kraft paper layer of (2) (which are successively provided from a conductor to the main insulating layer) is provided just above the conductor or just above the inner semi-conductive layer within a region in which the pressure of insulating oil becomes negative when a load is cut off. Preferably, this low-resistivity paper layer is provided also in the outer circumference of the main insulating layer.


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