This invention relates to limiting electrical degradation of all-dielectric self-supporting (ADSS) cables.
When ADSS cables are suspended from towers from which power transmission lines are also suspended, they can suffer from electrical degradation caused by dry band arcing.
Dry band arcing arises since there is an induced voltage gradient along the length of the ADSS cable. This voltage gradient does not present a problem if the cable is dry. When the cable surface becomes electrically conductive due to moisture, however, a current is drawn along the length of the cable. This current is greatest at the tower where the cable is suspended and at earth potential and will dry the cable surface leading to a break in the previously continuously electrically conductive surface. This break can extend around the cable to form a dry band and arcing across the dry band can degrade the cable.
An object of this invention is to limit such dry band arcing adjacent the earthed suspension regions of ADSS cables.
Briefly this is achieved by providing a current path between a location on the cable remote from the earthed suspension region thereof and the earthed region which current path includes an arc gap (or an equivalent thereto) such that current will flow along this path rather than along the surface of the cable between the location and the earthed suspension region when dry band arcing at a current above a predetermined value would otherwise occur.
In one aspect, the invention provides an assembly for use adjacent an earthed suspension region of an all-dielectric self-supporting cable suspended in an electric field at said region from a tower or the like for limiting dry band arcing adjacent said region, said assembly comprising an electrically conductive device connectable to said cable remote from said region and means for defining a current path having an arc gap or equivalent solid state device connectable between said device and said earthed region.
The current path defining means may include an electrically conductive clamp means clampable to said earthed region.
The device may comprise a clamping means clampable to said cable.
The arc gap is advantageously defined between an electrically conductive annular first member and an electrically conductive second member positioned at the axis thereof
Preferably insulating means locate said second member relative to said first member.
The annular first member may be located adjacent the clamp means or the clamping means, or anywhere between the two.
In the embodiment of the invention described hereinafter the annular first member is located adjacent the clamping means and the second member comprises an elongate member extending from the clamp means to the annular first member.
If, however, the annular first member is located adjacent the clamp means the second member may comprise an elongate member extending from the clamping means to the annular first member.
The invention also includes an installation comprising an all-dielectric self-supporting cable suspended at an earthed suspension region thereof in an electric field from a tower or the like, wherein an electrically conductive device is connected to said cable remote from said region and means defining a current path having an arc gap or equivalent solid state device is provided between said device and said earthed region.
The invention also includes a method of limiting dry band arcing adjacent an earthed suspension region of an all-dielectric self-supporting cable suspended at said region from a tower or the like comprising directing current along a current path including an arc gap or an equivalent solid state device rather than along the surface of the cable when dry band arcing at a current above a predetermined value would otherwise occur.
The invention also includes a device comprising an electrically conductive annular first member and an electrically conductive second member positioned at the axis thereof such that an arc gap is defined between the internal surface of the first member and the external surface of the second member.