Gas Insulated Substations are high voltage Substations that are compact, requiring little
maintenance when compared to air-insulated conventional Substations. Compressed Gas
Insulated Substations (CGIS) consist basically a conductor supported on insulators inside an
enclosure which is filled with sulfur hexafluoride gas (SF6). The compactness is with the use of
SF6 gas, which has high dielectric strength. The voltage withstand capability of SF6 Busduct is
strongly dependent on field perturbations, such as those caused by conductor surface
imperfections and by conducting particle contaminants. The contaminants can be produced by
abrasion between components during assembly or operations.
A gas-insulated substation (GIS) uses a superior dielectric gas, SF6, at 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 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 10. A GIS is mostly used where space is
expensive or not available. In a GIS the active parts are protected from the deterioration from
exposure to atmospheric air, moisture, contamination, etc.
As a result, GIS is more reliable and requires less maintenance than AIS.
GIS was first developed in various countries between 1968 and 1972. After about 5 years of
experience, the use rate increased to about 20% of new substations in countries where space is
limited. In other countries with space easily available, the higher cost of GIS relative to AIS has
limited use to special cases.
Sulfur hexaflouride is an inert, nontoxic, colorless, odorless, tasteless, and nonflammable gas
consisting of a sulfur atom surrounded by and tightly bonded to six flourine atoms. It is about
five times as dense as air. SF6 is used in GIS at pressures from 400 to 600 kPa absolute. The
pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperatures the
SF6 has two to three times the insulating ability of air at the same pressure. SF6 is about 100
times better than air for interrupting arcs. It is the universally used interrupting medium for high
voltage circuit breakers, replacing the older mediums of oil and air. SF6 decomposes in the high
temperature of an electric arc, but the decomposed gas recombines back into SF6 so well that it
is not necessary to replenish the SF6 in GIS.
There are some reactive decomposition byproducts formed because of the trace presence of
moisture, air, and other contaminants. The quantities formed are very small. Molecular sieve
absorbants inside the GIS enclosure eliminate these reactive byproducts. SF6 is supplied in 50-kg
gas cylinders in a liquid state at a pressure of about 6000 kPa for convenient storage and
transport. Gas handling systems with filters, compressors, and vacuum pumps are commercially