High-voltage underground cables have
been installed in response to adverse public response to the visually
offensive high-rise transmission towers in or close to populated
communities. Underground cables rated for voltages as high as 500 kV
have been developed.
They were first placed in service in
the United States in 1976. Traditionally, underground cable systems
have been installed in cities and other heavily populated areas,
where open high-voltage lines present a safety hazard.
They have also been installed where
overhead lines were not practical, in locations such as air port
approaches because of aircraft safety issues, or water crossings
where overhead lines are not feasible because of interference with
water traffic.
For crossing large bodies of water,
trenches are dug or dredged to depths related directly to the voltage
being carried by the cable, and the crossings are marked near the
shore lines. Extruded dielectric cables have become the U.S. standard
for voltages to 161 kV.
Low-pressure cables have hollow cores
for the circulation of oil under low pressure. The oil provides
temporary protection of the enclosed wires from water damage should
the cable sheath develop a leak.
High-pressure oil-filled pipe-type
cables are commonly installed for 230- and 345-kV applications in the
United States. Oil is circulated in the pipe under high pressure (14
kg/cm2 or 200 psi).
Most new cable installations make use
of extruded dielectric, but pipe-type cables account for 75 percent
of the approximately 2400 circuit miles now in service.
From 15 to 20 percent of the cable is
extruded dielectric, and most of the remainder is self-contained
fluid filled cable.
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