SF6 gas (Part 1)

• Since 1960, SF6 has been used as arc quenching and insulating medium for high and medium voltage switchgear systems. The favourable electrotechnical, chemical and physical characteristics of the gas have considerably influenced the development of the switchgear technology. SF6 is an alternative to other conventional insulating and quenching media such as e.g. oil, and air. The use of SF6 gas considerably increases, in some applications, the efficient utilisation of resources in energy transmission and distribution with respect to technology, finances and personnel [1]. At the same time SF6 in comparison to oil reduces the risk of hazard (e.g. fire, explosion) to personnel and environment. An overall evaluation considering all ecological, economic, safety and technological aspects has proven that SF6 is still an excellent choice as insulating medium [2]. The existing SF6 technology in the field of energy transmission and distribution is the result of decades of optimisation and contributes essentially to the further development of the economically efficient power distribution.

Modern, state of the art, High Voltage and Extra High Voltage switching devices (e.g. circuit breakers) use SF6 as an arc-quenching medium, almost exclusively. More integrated solutions, like the Gas Insulated Switchgear (GIS) use the gas not only as arc-quenching medium but as insulation as well.

• In the Medium Voltage range circuit breakers, switches, etc., using different quenching media including SF6, are normally installed inside a metal enclosure (traditional metal-enclosed switchgear).Another design is the SF6 insulated metal-enclosed switchgear, where the SF6 gas provide the main insulation of the equipment. Vacuum or SF6 interrupters can be used. A particular case, broadly used in the public secondary network, is the SF6 insulated Ring Main Unit (RMU). In this type of equipment switching devices are contained in a single enclosure, where the SF6 gas ensures simultaneously two tasks: general insulation and arc-quenching. It gives to the equipment a great simplicity, compactness, reliability and safety.

In all these types of equipment there is a common design requirement. Gas-tight enclosure is a functional and essential requirement, for obvious reasons. Although the International Standard IEC 60694 still describes three different types of pressure systems when qualifying the tightness of the enclosure, nowadays switchgear design is based on only two of these pressure systems:

 • Closed pressure systems, which can be re-filled periodically. IEC 60694 allows two standardised relative leakage rates of 1% and 3% per year. The present generation of switchgear is according to the 1% criteria.

 • Sealed pressure systems are designed and manufactured to have no emission. Therefore they do not need to be refilled during the expected operating life (generally 20 to 40 years). In High Voltage Switchgear, both economical and functional/operational reasons make the SF6 usage as arc-quenching medium the only practical choice. At Medium Voltage level most frequently used quenching systems are SF6 and vacuum. Air or oil switching devices are still also available, although they can present some disadvantages or limitations: economical in some cases, functional in other cases (performance, expected service life, maintenance requirements, risk of fire, etc.). In the case of SF6 insulated switchgear the combination of the outstanding characteristics of the gas with the gas-tight metal enclosure, following features are available.

 • Low operating energy for the switching devices. 

• No risk of fire. • Not toxic hazard. (See IEC 61634, Annex C) [3]

 • Independence of the installation site altitude, allowing the use of standard products at altitudes higher than 1000 m above sea level. 

• High protection against ambient conditions resulting in two very important properties: - Contaminant conductive deposits can not build up to degrade solid insulators, so preventing one of the most frequent causes of serious failures. Electrical contacts are protected against chemical corrosion that can reduce their performance, and potentially lead also to a final failure of the equipment. -

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