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Thread: A Seminar on Gas Insulated Sub-Station (GIS)

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    Fuchcha FaaDoO Engineer Navneet lucky's Avatar
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    Ppt 32 A Seminar on Gas Insulated Sub-Station (GIS)

    Gas Insulated Sub-Station (SF6 Gas)

    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
    equipment experiences.

    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

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    Last edited by Navneet lucky; 13th October 2013 at 02:13 PM.

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