A
gas-insulated substation (GIS) uses a superior dielectric gas, sulfur
hexafluoride (SF6),
at a moderate pressure for phase to phase and phase to ground insulation. The
high-voltage conductors, circuit breaker interrupters, switches, current
transformers, and voltage transformers are encapsulated in SF6 gas
inside grounded metal enclosures. The atmospheric air insulation used in a
conventional, air-insulated substation (AIS) requires meters of air insulation
to do what SF6 can
do in centimeters. GIS can therefore be smaller than AIS by up to a factor of
ten. A GIS is mostly used where space is expensive or not available. In a GIS,
the active parts are protected from deterioration from exposure to atmospheric
air, moisture, contamination, etc. As a result, GIS is more reliable, requires
less maintenance, and will have a longer service life (more than 50 years) than
AIS. GIS was first developed in various
countries between 1968 and 1972. After about 5 years of experience, the user
rate increased to about 20% of new substations in countries where space was
limited. In other countries with space easily available, the higher cost of GIS
relative to AIS has limited its use to special cases.
