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.
Sulfur hexaflouride is an inert, non-toxic, colorless,
odorless, tasteless, and non-flammable 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 equipment experiences.
SF6 has two to three times the insulating ability of air at
the same pressure. SF6 is about one hundred 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 available. Best practices and the personnel safety
aspects of SF6 gas handling are covered in international standards (IEC, 1995).
The SF6 in the equipment must be dry enough to avoid
condensation of moisture as a liquid on the surfaces of the solid epoxy support
insulators because liquid water on the surface can cause a dielectric
breakdown. However, if the moisture condenses as ice, the breakdown voltage is
not affected. So dew points in the gas in the equipment need to be below about
–10°C. For additional margin, levels of less than 1000 ppmv of moisture are
usually specified and easy to obtain with careful gas handling.
Absorbants inside the GIS enclosure help keep the moisture
level in the gas low, even though over time, moisture will evolve from the
internal surfaces and out of the solid dielectric materials (IEEE Std.
1125-1993). Small conducting particles of mm size significantly reduce the
dielectric strength of SF6 gas.
This effect becomes greater as the pressure is raised past
about 600 kPa absolute (Cookson and Farish, 1973). The particles are moved by
the electric field, possibly to the higher field regions inside the equipment
or deposited along the surface of the solid epoxy support insulators, leading
to dielectric breakdown at operating voltage levels.
Cleanliness in assembly is therefore very important for GIS.
Fortunately, during the factory and field power frequency high voltage tests,
contaminating particles can be detected as they move and cause small electric
discharges (partial discharge) and acoustic signals, so they can be removed by
opening the equipment.
Some GIS equipment is provided with internal “particle
traps” that capture the particles before they move to a location where they
might cause breakdown. Most GIS assemblies are of a shape that provides some
“natural” low electric field regions where particles can rest without causing
problems.
SF6 is a strong greenhouse gas that could contribute to
global warming. At an international treaty conference in Kyoto in 1997, SF6 was
listed as one of the six greenhouse gases whose emissions should be reduced.
SF6 is a very minor contributor to the total amount of greenhouse gases due to
human activity, but it has a very long life in the atmosphere (half-life is
estimated at 3200 years), so the effect of SF6 released to the atmosphere is
effectively cumulative and permanent.
The major use of SF6 is in electrical power equipment.
Fortunately, in GIS the SF6 is contained and can be recycled. By following the
present international guidelines for use of SF6 in electrical equipment (Mauthe
et al., 1997), the contribution of SF6 to global warming can be kept to less
than 0.1% over a 100-year horizon.
The emission rate from use in electrical equipment has been
reduced over the last three years. Most of this effect has been due to simply
adopting better handling and recycling practices. Standards now require GIS to
leak less than 1% per year. The leakage rate is normally much lower. Field
checks of GIS in service for many years indicate that the leak rate objective
can be as low as 0.1% per year when GIS standards are revised.
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