Friday, February 19, 2010

G.I.S - Gas Insulated Substations #1

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. The IEEE [4, 5] and the IEC [6] have standards covering all aspects of the design, testing, and use of GIS. For the new user, there is a CIGRE application guide. IEEE has a guide for specifications for GIS.

What is Sulphur Hexafluoride SF6?

Sulfur hexafluoride is an inert, nontoxic, colorless, odorless, tasteless, and nonflammable gas consisting of a sulfur atom surrounded by and tightly bonded to six fluorine 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 or spark, 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 interaction of sulfur and fluorine ions with trace amounts of moisture, air, and other contaminants. The quantities formed are very small. Molecular sieve absorbents inside the GIS enclosure eliminate these reactive by products over time. SF6 is supplied in 50 kg gas cylinders in a liquid state at a pressure of about 6000kPA for convenient storage and transport.