Paper Lead Cables
General Cable NZ have been manufacturing medium voltage
paper lead cables for 30 yrs. Applicable manufacturing standards
are BS6480, AS/NZS 1026, and AS/NZS 4026.
Paper lead is an established design used by General Cable NZ
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
impregnating the insulating papers, and Non Draining to the
property of the compound to resist drainage from the cable under
normal operating conditions. General Cable NZ 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.
Insulation/Screening/Layup:
Multicore cables are assembled in two ways.
Belted construction – Suitable for use 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.
Screened construction - 11kV and above, and for single core
cables.
The individual cores are paper insulated with the inclusion of
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/m² 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.