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For high power demands, large conductors and high voltages are
necessary. The size and type of conductor is determined mainly
on the basis of the power requirements of the system, the heat
losses of the component parts, and the thermal properties of the
environment surrounding the cable.
When a cable is operating, heat is generated by the conductor,
the total amount depending on the electrical load that the cable
is carrying. Cables are designed to achieve nominal maximum conductor
temperature (usually 85°C maximum continuous) when at full load,
such that no deterioration occurs in the cable insulation. The
greatest single factor in designing underground systems to carry
heavy electrical loads is to be able to predict, within close
limits, the heat dissipation properties of the cable's surrounding
environment.
The ability of the earth to conduct heat is dependent on the type
of soil and its moisture content, and therefore, it is influenced
to a large degree by seasonal climatic conditions.
In designing an underground transmission system, the cable engineer
has various options available in order to optimize the design.
They include:
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Selection of the type of cable … Self-Contained Liquid-Filled,
High Pressure
Liquid-Filled Pipe-Type, or Solid Dielectric Cable.
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Selection of the type of conductor … Copper segmented, copper
Type 'M', or copper compacted strand.
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Method of sheath bonding.
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Whether to replace the original soil with a special low thermal
resistant backfill.
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Selection of the spacing between the cables to allow optimum dissipation
of heat.
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Use of assisted cooling.
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Use of forced cooling.
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An extensive underground cable system being installed using the
direct burial method.
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