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General Cable have been manufacturing medium voltage paper
lead cables for 30 yrs. Applicable manufacturing standards are BS6480,
AS1026, and AS4026.
Paper lead is an established design used by
General Cable for voltages from 600/1000V to 19000/33000V.
The conductor insulation consists of layers of paper impregnated with
an oil wax mixture to form a dielectric material to exclude moisture.
The oil wax mixture compound is called MIND, or mass impregnated non
draining. Mass impregnated refers to the process of soaking the paper
layers, and the material must remain in a waxlike state at all operating
conditions., ie non draining. General Cable
use only the highest quality MIND compound.
CONDUCTORS:
Conductors can consist of plain annealed copper or aluminium. Usually
conductors in multicore cables are made to the following pattern to limit electrical stress to an acceptable level: 11kV are circular for 16mm2 and 25mm2, sectored above; 22kV circular for 25mm2 to 50mm2 inclusive, sectored above; 33kV circular for 50mm2, oval above.
SCREENING:
Multicore cables are assembled in two ways.
Firstly, belted construction which is used up to 11kV. The individual cores are paper insulated, laid up, and a paper wrapping called a belt applied over the laid up cores. The electric field is contained within the combination of insulation and belting, and the thermo-mechanical movement between these layers is restricted by the cable operating temperature of 65�C.
Secondly, screened construction, patented by Hochstadter and used for 11kV and above, and for single core cables. The individual cores are paper insulated with a metallised paper screen, and laid up with a conductive wrap to tie the screens electrically together. The electric field is now uniformly contained within the insulating papers and the design is capable of higher voltage levels than the belted design, as well as an increased operating temperature of 70�C.
MIND APPLICATION:
Impregnation is preceded by heating to 120�C and evacuation to a pressure of � 25N/m� so as to remove both air and moisture from the paper web and ensure that the whole of the matrix is filled with MIND compound. Quality control procedures ensure that this vital process is correctly performed to the highest standard.
LEAD LAYER:
This layer is impervious to moisture and petroleum fluids and gases. Problems associated with lead such as resistance to fatigue cracking, extrusion defects, fractures associated with internal pressure and corrosion have been eliminated by choice of materials and manufacturing techniques: for example cables manufactured by General Cable use alloy E for increased fatigue resistance.
The lead layer is also used for earth fault current, and this rating can be increased by the use of steel tape or steel wire armour.
MECHANICAL PROTECTION:
Both a wire or a tape armour are available. Steel wire armour (SWA) or aluminium wire armour (AWA) for single core cables is added for increased tensile rating. Steel tape armour (STA) is added for impact resistance and can be replaced by high density polyethylene (HDPE) if not required for fault current (this option is used in AS4026). For submarine installation, two steel wire armour layers can be used to minimise the twisting effect of one layer and this is called torque balancing. Protection from the toredo worm can also be incorporated with the application of copper tape under the armour layer.
CABLE FINISH:
The final cable layer can consist of polyvinyl-chloride (PVC), HDPE, or bitumised hessian and the choice of this layer is determined by the cable environment. e.g. HDPE should be used if mechanical and chemical protection is required; hessian is more suitable for submarine use if no abrasion or movement of the cable occurs.
INSTALLATION:
Cables should never be bent to a small radius. The prescribed minimums on the following datasheets should be considered the exception rather than the rule and the actual bending radius the largest which circumstances will permit. A cable must also not be laid or otherwise bent when it is at such a low temperature that damage might be caused to the insulation or serving. Where environmental conditions are at or below 0�C, cable drums will require heating, or drums stored on a site with a higher ambient temperature, and moved and installed as quickly as possible before significant cooling can take place.
Cables can be pulled into position by either cable stocking or conductor hauling eye, but for high pulling tensions a hauling eye is preferred to avoid stretching of the outer layer.
APPLICATIONS:
1MVA 11kV(60A) or 3MVA 33kV supplies to isolated areas.
Submarine applications with a single or double layer of armour.
Low temperature applications such as ski slopes.
Disadvantages: high unit weight and a despatch weight limit of 10 tonnes means multiple joints for larger conductor cable.
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Copyright 2004 General Cable Australia Pty Limited
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