What Are Gas Insulated Substations?

High-voltage gas-insulated substations have been in service since the early 1960s. Operation of 800-kV equipment has proved successful since the end of 1979. Prototype testing of 1100 through 1600-kV substation equipment proved the feasibility of this equipment at the next generation of voltage levels.

The basic principle of gas-insulated equipment is that the high-voltage current-carrying parts are within a metal enclosure and are held in a concentric configuration by cast epoxy spacer insulators. The space between the conductor and the enclosure is filled with sulfur hexafluoride gas under moderate pressure.

Medium-voltage to 170-kV equipment is available in three phases in one enclosure; for higher voltages, it is generally in a single-phase enclosure arrangement. The equipment can be installed indoors or outdoors, and it can be designed for any bus scheme.

Depending on the voltage level, bus scheme, and whether connecting lines are installed underground or overhead, the land area required for gas-insulated equipment is 10% for 800 kV to 20% for 145 kV of the space required for comparable air-insulated equipment.

Because of its smaller size and enclosed current-carrying parts, this equipment is excellently suited for installation where real estate is at a premium, where the environmental constraints dictate a minimum of visual exposure, and where the continuity of service may be threatened by airborne contamination.

The dielectric medium is the sulfur hexafluoride (SF6) gas, which became commercially available in 1947. SF6 has been used as an insulating medium in electronic devices, power apparatus, and HVDC converter stations. Its excellent properties make it ideally suited both as an insulating and as an arc-quenching agent. SF6 gas is colorless, odorless, chemically inert, nontoxic, nonflammable, and noncorrosive.

Its dielectric strength is greatly superior to that of air, and it is close to 100 times as effective as air in quenching an electric arc. Pure SF6 is heavier than air, which causes it to settle in low areas, thus diluting oxygen in air. It is therefore necessary to learn proper safety rules before entering any area where pockets of SF6 could accumulate.

Although the gas is self-restoring, during its exposure to an electric arc it will yield decomposition by products. In the presence of moisture, which is especially the case in failed and ruptured equipment, these by-products will hydrolyze, and all resulting reaction products must be considered hazardous.

The level of gas pressure at which the equipment will operate to meet specified ratings is a function of the relationship between diameters of the conductor and the enclosure (the size of the gap), and the temperature at which the equipment will operate. At the higher pressures, the gas would liquefy at higher temperatures.

At lower pressures, dielectric strength and arc-quenching qualities of the gas would be reduced. Therefore, the gas-insulated equipment operating pressure is usually between 0.35 and 0.52 MPa (50 and 75 lb/in2, gage).

Environmental effects of SF6 that might be released to the atmosphere from GIS have been thoroughly studied. SF6 does not affect the earth’s ozone layer, but it is a strong greenhouse gas. Relative to CO2, it has a global warming potential of 23,400 due to its infrared absorption and emission characteristics and very long life in the atmosphere (half-life is projected to be 3200 years).

Fortunately, the concentration of SF6 in the atmosphere is very low, and with proper handling, leak checking, and recycling, the contribution of SF6 to anthropogenic global warming due to its use in electrical equipment can be kept below 0.1%.

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