SF6 properties and use in MV and HV switchgear

Handling of SF6 gas

The general properties of SF₆ gas and its SF₆ by-products are presented. A brief history of the use of SF₆ in switchgear is given. The effects of SF₆ on the environment are discussed. Guidance is given for working with SF₆ gas and SF₆ filled equipment under normal and abnormal conditions of service. The content of the present document is based on the technical report IEC 1634, entitled « the use and handling of SF₆ HV Switchgear and controlgear ».

A brief history of use of SF₆

Sulphur hexafluoride was first synthesised in the laboratories of the Faculté de Pharmacie de Paris in 1900 by Moissan and Lebeau. Fluorine, obtained by electrolysis, was allowed to react with sulphur and a strongly exothermic reaction, giving rise to a remarkably stable gas. Gradually thereafter the physical and chemical properties of the gas were established, with publications by Pridaux (1906), Schlumb and Gamble (1930), Klemm and Henkel (1932-35) and Yest and Clausson (1933) concerning particularly the chemical and dielectric properties.

The first research into industrial applications was by the General Electric Company in 1937 who realised that the gas could be used for insulation in electrical plant. In 1939 Thomson-Houston patented the principle of using SF₆ for insulating cables and capacitors. Immediately after the 2nd world war, publications and applications began to appear in rapid succession:

• towards 1947, work on transformer insulation,
• development of industrial manufacture of SF₆ in the USA in 1948 by Allied Chemical Corporation and Pennsalt,
• large scale commercialisation of SF₆ manufacture for use in electrical plant construction towards 1960 in the USA and in Europe, coinciding with the appearance of the first SF₆ circuit-breakers and switches at High Voltage – HV – and Extra High Voltage – EHV.

At Merlin Gerin, the research work, concerning theuse of SF₆ gas for insulation and circuit-breaking was initiated around 1955. This coïncides with the appearence of the first industrial products in the U.S. The first industrial applications from Merlin Gerin were at EHV, then in Medium Voltage – MV – applications following:

• 1964: the first SF₆ – insulated substation, ordered by EDF and put into service in the Paris region in 1966.
• 1967: the FA circuit-breaker was launched and progressively replaced the compressed air equipment which had established its position in France and elsewhere during the previous 25 years.
• 1971: changes in the needs of the industry led Merlin Gerin to launch the Fluarc SF₆ MV circuitbreaker.
• More recently, SF₆ has been adopted for use in MV switches, ring main units, contactors and circuit-breakers, GIS, covering all the needs of the elecrical distribution industry.

Specific heat

The volumetric specific heat of SF₆ is 3.7 times that of air. This has important consequences for reducing the effects of heating within electrical equipment.

Thermal conductivity

The thermal conductivity of SF₆ is below that of air but its overall heat transfer capability, in particular when convection is taken into account, is excellent, being similar to that of gases such as hydrogen and helium and higher than that of air. At high temperatures, the thermal conductivity curve of SF₆ (see fig. 4 ) reveals one of the exceptional qualities of the gas, which allows it to be used for extinguishing arcs by thermal transport.

The peak of the thermal conductivity corresponds to the dissociation temperature of the SF₆ molecule at 2100 to 2500 K. The dissociation process absorbs a considerable amount of heat which is released when the molecules reform at the periphery of the arc, facilitating a rapid exchange of heat between the hot and cooler regions.

Electrical Properties

The excellent dielectric properties of SF₆ are due to the electronegative character of its molecule. It has a pronounced tendency to capture free electrons forming heavy ions with low mobility making the development of electron avalanches very difficult. The dielectric strength of SF₆ is about 2.5 times higher than that of air under the same conditions.

The advantage of SF₆ over nitrogen as a dielectric is clearly illustrated by the curve (see fig. 5 ). Because of its low dissociation temperature and high dissociation energy, SF₆ is an excellent arc quenching gas. When an electric arc cools in SF₆, it remains conductive to a relatively low temperature, thus minimising current chopping before current zero, and thereby avoiding high overvoltages.

AUTHOR: Schneider Electric expert | D. KOCH