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The choice of the method of installation may vary widely with
the type of cable system, and will depend on the thermal considerations,
economics and features that the end-user considers to be of most
importance.
Self-Contained Liquid-Filled (SCLF) Cables
For relatively short substation or power station leads, SCLF Cables
may be laid in unfilled concrete trenches, or racked in tunnels
and shafts supported by steelwork provided with clamping devices.
Such cables, supported in air, will require that thermal-mechanical
effects be considered, and controlled expansion movements must
be accommodated.
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Two 138 kV copper SCLF cable circuits in concrete chases, as they
exit from either side of a powerhouse access tunnel.
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The installation of longer underground transmission lines, for
instance between substations where city streets must be traversed,
is generally of two types:
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1) Duct and Manhole
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2) Direct Burial with Joint Bay
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The duct and manhole system is built in sections. Each section
of duct between manholes must be relatively straight and its length
compatible with the safe cable-pulling tension and sidewall pressure.
The duct system is occasionally used by some utilities in heavily
congested downtown areas where the system can be built while keeping
trench excavations to relatively short sections being open at
one time.
Direct burial is the most economical, and in many respects, the
simplest and preferred method of installing the cables. The lengths
of cable can be easily routed around obstacles and threaded between
other underground services, in a manner virtually impossible with
the other systems. The lengths are generally limited only by the
maximum sheath voltage imposed by the user, manufacturing and
transportation limitations, the amount of trench that can be kept
open at one time, and the safe cable-pulling tension.
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Installing 230 kV copper SCLF cables on wall brackets in a transformer
gallery.
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The installation procedure for direct burial is to provide a compacted
bed of sand, or other approved material of low thermal resistance,
at the bottom of the trench, and then pull-in the cables, on rollers,
one at a time. After the cables have been pulled-in, they are
properly spaced and covered with sand. Protective blocks are then
placed over the cables and the trench backfilled.
Another method, which provides additional mechanical protection,
is to pour concrete sidewalls on either side of the cables, see
Figure 5. Then, after sand filling, the concrete enclosure is
completed by pouring a concrete cap bridging the sidewalls.
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After the cables have been properly spaced they are carefully
covered and the fill compacted.
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At the ends of a cable system are the terminations (or potheads).
The purpose of the terminations is to seal the cable hydraulically,
to prevent the ingress of moisture, and to connect the cable electrically
to other parts of the transmission system.
The hydraulic system for the cable installation must be carefully
designed, in order that, under the most onerous operating conditions,
positive liquid pressure within the prescribed values will be
maintained along the entire length of the cable route.
High Pressure Liquid-Filled Pipe-Type Cables
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Installing two 8" (203 mm) cable pipes underground. The third,
smaller diameter pipe, is for liquid supply.
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In the installation of High Pressure Liquid-Filled Pipe-Type Cables,
the steel pipe is installed in the trench, and a second pipe may
be installed at the same time to provide for future requirements.
This saves considerable cost, time and disruption, and avoids
re-excavating when additional power transmission services are
required.
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Three joined copper cables of a pipe-type system, prior to reinstating
the cable insulation.
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Once the installation has begun, no moisture is allowed to enter
a pipe during its installation. After completion, the cable pipe
is given a vacuum dryness test, then is pressurized with dry nitrogen
at about 300 psi for 72 hours, and is kept under positive pressure
awaiting the installation of the cables. The pipe cables are shipped
to the site under dry nitrogen atmosphere on sealed cable reels.
Immediately before installation, the seal is broken, three cables
are yoked together, and they are pulled into the pipe simultaneously.
The three basic types of terminations (potheads) may be used.
Pipe-type cable systems are usually designed to operate under
a nominal liquid pressure of about 200 psi. It is the practice
of several major utilities in North America, to use forced cooling
in anticipation of the increase in power requirements. This involves
the continuous circulation of liquid through the pipe instead
of it being maintained under static pressure.
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Installing pipe-type cables. Three cables yoked together in preparation
for being pulled into the pipe simultaneously.
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Solid Dielectric Cables
The installation of Solid Dielectric Cables is usually carried
out in the same manner as described for SCLF cables. Unfilled
concrete trenches, tunnels, and shafts may be used for short leads,
and either the Duct and Manhole, or the Direct Burial with Joint
Bay method, is used for longer transmission lines.
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Solid dielectric cables, 138 kV, installed on special steel racks
at a substation.
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