The gas-insulated
transmission line (GIL) is a system for the transmission of electricity at high
power ratings over long distances. In cases where overhead lines are not
possible, the GIL is a viable technical solution to bring the power transmitted
by an overhead line underground without a reduction of power transmission
capacity.
As a
gas-insulated system, the GIL has the advantage of electrical behavior similar
to that of an overhead line, which is important to the operation of the
complete network. Because of the large cross section of the conductor, the GIL
has low electrical losses compared with other transmission systems (overhead lines
and cables). This reduces the operating and transmission costs, and it
contributes to reduction of global warming because less power needs to be
generated.
Safety of
personnel in the vicinity of a GIL is very high because the solid metallic
enclosure provides reliable protection. Even in the rare case of an internal
failure, the metallic enclosure is strong enough to withstand damage.
This allows
the use of GILs in street and railway tunnels and under bridges with public traffic.
No flammable materials are used to build a GIL. The use of GILs in traffic
tunnels makes the tunnels more economical and can solve some environmental
problems. If GIL is added to a traffic tunnel, the cost can be shared between
the electric power supply company and the owner of the traffic part (train,
vehicles).
The
environmental advantage is that no additional overhead line needs to be built
parallel to the tunnel. Because of the low capacitive load of the GIL, long
lengths of 100 km and more can be built.
Where
overhead lines are not suitable due to environmental factors or where they
would spoil a particular landscape, the GIL is a viable alternative because it
is invisible and does not disturb the landscape. The GIL consists of three
single-phase encapsulated aluminum tubes that can be directly buried in the
ground or laid in a tunnel. The outer aluminum enclosure is at ground
potential.
The interior,
the annular space between the conductor pipe and the enclosure, is filled with
a mixture of gas, mainly nitrogen (80%) with some SF6 (20%) to provide
electrical insulation. A reverse current, more than 99% of the conductor
current value, is induced in the enclosure. Because of this reverse current, the
outer magnetic field is very low.
GIL combines
reliability with high transmission capacity, low losses, and low emission of
magnetic fields. Because it is laid in the ground, GIL also satisfies the
requirements for power transmission lines without any visual impact on the
environment or the landscape. Of course, the system can also be used to supply
power to meet the high energy demands of conurbations and their surroundings.
The directly
buried GIL combines the advantage of underground laying with a transmission
capacity equivalent to that of an overhead power line.
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