Sulphur Hexafluoride: Electrical applications: Technical & Safety Aspects
Technical
The gas SF6 provides a combination of favourable properties such as high dielectric strength, excellent arc quenching ability, chemical inertness, high thermal stability, non-flammability and very low toxicity. It is colourless and odourless. The excellent quenching and insulating properties of SF6 have permitted the construction of completely new types of high-voltage circuit breakers and switching stations with outstanding features:
- compact and space-saving design
- low noise levels
- protection against accidental contact of live parts
- protection against intrusion of impurities
- elimination of fire hazard
Substations using SF6 are particularly in demand where, on account of limited space, a compact design is required. These substations occupy only 10-15 % of the space required by conventional units. New SF6 filled equipment can thus be installed in densely-populated areas where site costs would prohibit the use of traditional methods. Thanks to their insensitivity to polluted air, enclosed outdoor versions of SF6-insulated substations are installed in the chemical industry, in desert regions and in coastal areas.
Safety aspects
SF6 is non-flammable and has very low toxicity. It is transported as a pressurized liquified gas.
Pure (new) SF6 according to IEC 376:
- Potential hazard: Asphyxiation
- Protective measures: Natural and forced ventilation
Contaminated SF6 (e.g. caused by electrical discharges):
- Potential hazard: Formation of small quantities of toxic gaseous decomposition products could have occured.
Technical and safety aspects If health hazards associated with the handling of contaminated SF6 cannot be totally excluded with the aid of technical safety measures, then personal protective clothing must be worn.
Recovery and recycling
SF6 is securely contained in switchgears, which are hermetically sealed or closed systems. Leakage is generally less than 0.5 % per annum due to the technology used. Additional emissions occur during handling. Total emission from equipment in service in the EU in 1995 was about 3 % of the about 4000 tonnes installed according to the result of a comprehensive inquiry by switchgear manufacturers and utilities. These around 2.5 % handling losses (plus 0,5% leakage losses) can be considered as the highest limit, since recovery processes and handling procedures are constantly being improved.
Additionally "ReUse" concepts of SF6 producers ensure that a closed product cycle is established including reclamation of contaminated gas to original quality in cases when on-site recycling is impossible due to the kind or concentration of impurities.