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Three types of cable are used for underground power transmission
installations.
• Self-Contained Liquid-Filled Cable
• High Pressure Liquid-Filled Pipe-Type Cable
• Solid Dielectric Cable
Submarine Cables for underwater installations may also be part of a power transmission
system.
Self Contained Liquid-Filled (SCLF) Cables
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Closing the segments of a two layer copper conductor, the production
of the conductor being the start of the cable manufacturing process. |
Impregnated paper insulation was first used by Ferranti in a 10
kV cable. Impregnated paper does not give the impression of being
a suitable insulation material. However, the combination of its
excellent electrical and mechanical characteristics has resulted
in a reliable and economic insulation unsurpassed for many decades,
and it now claims a history of almost 100 years. Many paper-insulated
circuits dating back to the early part of this century are still
in service. Others that have been replaced by larger cables after
decades of operation, have exhibited excellent insulation integrity
when examined in the laboratory.
The drying and impregnating process for solid and non-pressure
type paper cables, leads to the formation of dielectric (air)
voids which ionize under electrical stress. The ionic bombardment
of the liquid and paper can ultimately result in failure. This
led Emanueli to develop the self-contained oil-filled cable which
was actually implemented in 1924. The basic principle of a liquid-filled
cable is that all the spaces inside the cable sheath are completely
filled, thus preventing voids in the insulation.
Two typical self-contained, single-conductor, liquid-filled cables
are shown in Figures 1 and 2. The first has a copper segmented
conductor and a hollow core, while the second has a Milliken,
or Type 'M', copper conductor with a spiral steel core.
The manufacturing process starts with the production of the conductor.
Segmented copper conductors, as in Figure 1, may comprise one,
two or more layers of segments, applied one layer at a time from
the smallest diameter to the largest. Due to the shape of the
segments, this type of conductor, while having a hollow core,
is usually self-supporting and does not require a central helix
to prevent it from collapsing.
With very large conductors, as shown in Figure 2, the Type 'M'
construction is used. This conductor consists of six segments.
Each segment is made up of wires stranded and shaped in the stranding
machine, the segments being lightly insulated from one another
by paper tapes. The six segments are cabled over a spiral steel
core.
The paper insulation is stored and applied under constant temperature
and low relative humidity conditions in one pass. For a cable
of about 220 kV and over, this requires a machine with many taping
heads maintained under uniform temperature, constant relative
humidity and extreme cleanliness conditions.
The paper taping operation should be carried out in one pass,
and under uniform temperature, humidity and extreme cleanliness
conditions.
The insulated cable is placed in a horizontal pan, which in turn,
is placed inside the impregnating tank. The insulation is dried
under vacuum, then completely immersed and impregnated with insulating
liquid under pressure. After cooling, the metallic sheath is applied.
Metal sheaths are either a lead alloy or aluminum. They are seamless,
and their integrity is essential.
The sheathed and impregnated cable is provided with a polyethylene
(PE) or polyvinyl chloride (PVC) jacket. From the time the cable
is impregnated, it is always connected to a liquid supply under
positive pressure, and as a result, during shipment, installation,
and operation the cable is never permitted to develop voids.
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Applying copper reinforcing tapes over a lead sheath during the
manufacture of a cable. |
High Pressure Liquid-Filled Pipe-Type Cables (HPLF)
For bulk power transmission in liquid-filled pipe-type cables,
three single paper-insulated liquid-impregnated cores are arranged
in an equilateral configuration inside a buried steel pipe, as
shown in Figure 3.
The pipe is filled with insulating liquid, and the cables are
designed to operate without individual metallic sheaths while
under a nominal hydraulic pressure of about 200 psi. The maximum
electrical operating levels, are the same as those of the Self-Contained
Liquid-Filled (SCLF) cable.
In the case of SCLF cables, with a normal operating pressure of
over 10 psi, it is the impermeability of the metallic sheath that
retains the liquid under pressure. However, in the case of high
pressure liquid-filled pipe-type cable, with an operating pressure
of over 200 psi, the entire pipe is filled with liquid, including
the cables. All are under the same liquid pressure, and the steel
pipe provides the impermeability.
The cable manufacturing process begins with the production of
the copper conductors. Up to cross sections of 500 mm2, they are usually stranded compact round shapes, with compact
segmented Type 'M' conductors being used for larger sizes. The
advantage of the compact segmented design is its low AC resistance.
After application of the paper insulation, mass impregnation takes
place. A seal prevents excessive loss of impregnating liquid and
exposure of the impregnated paper to the moist atmosphere. The
cable is finally wound on to special reels which are sealed under
a blanket of nitrogen at low pressure until the time of installation.
The steel pipe is in 40- to 50-foot (12- to 15-metre) random lengths,
with a wall thickness of 1/4 inch (6mm).
Typical nominal pipe sizes for various systems, are as follows:
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Internal Diameter
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System
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5" (127 mm) ............
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72 kV
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6" (152 mm) ............
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138 kV
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8" (203 mm) ............
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230 kV
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10" (254 mm) ..........
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345 kV
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The pipe size chosen is usually the nearest standard size which
has an internal area 2.5 or 2.8 times that of the three cables.
This amounts to about a 50% fill, which allows ample installation
space without jamming and permits lateral movement of conductors
during electrical load cycles.
Solid Dielectric Cables
In Solid Dielectric Cables, the insulation is usually cross-linked
polythethylene (XLPE), or ethylene-propylene rubber (EPR). This
cable design, Figure 4, has a copper compacted strand single conductor.
These cables are normally rated for installations with a maximum
nominal electrical loading of 138 kV.
The basic manufacturing process starts with extrusion of the conductor
shield, the insulation and the insulation shield in a single operation.
This triple extrusion operation ensures the smoothness of the
interface between the three layers, which has a vital influence
on the electrical strength of the cable. The core then passes
into a curing tube where the temperature of the extrudate is raised
to initiate the chemical cross-linking reaction. It then passes
into a cooling zone where the temperature is reduced nearly to
ambient. Both the curing and the cooling, are conducted under
pressure to avoid the formation of micro-voids which could considerably
reduce the electrical strength and endurance of the cable. A copper
tape shielding is then wrapped on, usually followed by a sheath
and finally, a polyethylene (PE) or polyvinyl chloride (PVC) jacket.
For high-voltage cables (over 69 kV) such as are required in underground
transmission systems, a lead-alloy or an aluminum sheath is always
specified. Its purpose is to protect the insulated core against
the ingress of moisture. This sheath ensures the absence of moisture,
and enhances the reliability and service life of the transmission
circuits. When cables are intended for underground installations
in the vicinity of oil refineries or other petro-chemical complexes,
a lead-alloy sheath is often specified to protect against the
detrimental effects of petroleum by-products.
Submarine Cables
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Copper wire armouring being applied during the manufacture of
a submarine cable. |
Although all three types of high-voltage cable may be considered
for a submarine cable (fresh or sea water), either the Self-Contained
Liquid-Filled Cable or the Solid Dielectric Cable is usually chosen.
Most frequently it is the SCLF cable, which is capable of handling
higher voltages.
Because of the severe environmental demands placed on a submarine
cable, a lead-alloy sheath is often chosen, because of its compressibility,
flexibility and corrosion resistance. The sheath is usually covered
by a number of outer layers, comprising polyethylene jackets,
metal wire armouring, and bitumized jute.
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