GIS is assembled of standard equipment
modules (circuit breaker, current transformers, voltage transformers,
disconnect and ground switches, interconnecting bus, surge arresters,
and connections to the rest of the electric power system) to match
the electrical one-line diagram of the substation.
A cross section view of a 242-kV GIS
shows the construction and typical dimensions (Figure 2.1). The
modules are joined using bolted flanges with an “O” ring seal
system for the enclosure and a sliding plug-in contact for the
conductor.
Internal parts of the GIS are supported
by cast epoxy insulators. These support
insulators provide a gas barrier
between parts of the GIS, or are cast with holes in the epoxy to
allow gas
to pass from one side to the other.
Up to about 170 kV system voltage, all
three phases are often in one enclosure (Figure 2.2). Above 170 kV,
the size of the enclosure for “three-phase enclosure,” GIS
becomes too large to be practical. So a “single-phase enclosure”
design (Figure 2.1) is used.
There are no established performance
differences between three-phase enclosure and single-phase enclosure
GIS. Some manufacturers use the single phase enclosure type for all
voltage levels.
Enclosures today are mostly cast or
welded aluminum, but steel is also used. Steel enclosures are painted
inside and outside to prevent rusting. Aluminum enclosures do not
need to be painted, but may be painted for ease of cleaning and a
better appearance. The pressure vessel requirements for GIS
enclosures are set by GIS standards (IEEE Std. C37.122-1993; IEC,
1990), with the actual design, manufacture, and test following an
established pressure vessel standard of the country of manufacture.
Because of the moderate pressures
involved, and the classification of GIS as electrical equipment,
third-party inspection and code stamping of the GIS enclosures are
not required.
Conductors today are mostly aluminum.
Copper is sometimes used. It is usual to silver plate surfaces that
transfer current. Bolted joints and sliding electrical contacts are
used to join conductor sections. There are many designs for the
sliding contact element. In general, sliding contacts have many
individually sprung copper contact fingers working in parallel.
Usually the contact fingers are silver plated.
A contact lubricant is used to ensure
that the sliding contact surfaces do not generate particles or wear
out over time. The sliding conductor contacts make assembly of the
modules easy and also allow for conductor movement to accommodate the
differential thermal expansion of the conductor relative to the
enclosure.
Sliding contact assemblies are also
used in circuit breakers and switches to transfer current from the
moving contact to the stationary contacts. Support insulators are
made of a highly filled epoxy resin cast very carefully to prevent
formation of voids and/or cracks during curing.
Each GIS manufacturer’s material
formulation and insulator shape has been developed to optimize the
support insulator in terms of electric field distribution, mechanical
strength, resistance to surface electric discharges, and convenience
of manufacture and assembly. Post, disc, and cone type support
insulators are used.
Quality assurance programs for support
insulators include a high voltage power frequency withstand test with
sensitive partial discharge monitoring. Experience has shown that the
electric field stress inside the cast epoxy insulator should be below
a certain level to avoid aging of the solid dielectric material.
The electrical stress limit for the
cast epoxy support insulator is not a severe design constraint
because the dimensions of the GIS are mainly set by the lightning
impulse withstand level and the need for the conductor to have a
fairly large diameter to carry to load current of several thousand
amperes. The result is space between the conductor and enclosure for
support insulators having low electrical stress.
Service life of GIS using the
construction described above has been shown by experience to be more
than 30 years. The condition of GIS examined after many years in
service does not indicate any approaching limit in service life.
Experience also shows no need for
periodic internal inspection or maintenance. Inside the enclosure is
a dry, inert gas that is itself not subject to aging. There is no
exposure of any of the internal materials to sunlight. Even the “O”
ring seals are found to be in excellent condition because there is
almost always a “double seal” system. The lack of aging has been
found for GIS, whether installed indoors or outdoors.
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