SF6 Circuit Breakers

115kV SF6 Gas Circuit Breaker
115kV SF6 Circuit Breaker. Photo: Wikimedia.

Sulphur hexafluoride (SF6) – an excellent insulating and arc quenching gas

Substantial properties of SF6 at a glance

  1. Colorless and odorless
  2. 5 times heavier than air
  3. Temperature-resistant up to 500 °C
  4. Chemically stable
  5. Non-toxic
  6. Excellent dielectric properties
  7. Non-inflammable
  8. Outstanding arc quenching properties

Advantages of SF6

SF6 has a number of advantages for use as an insulator. The dielectric strength of SF6 increases with pressure so smaller breakers can be used at higher voltages. The breakers have a smaller footprint, so they use less real estate which can be very expensive. They are very reliable and require less maintenance than air blast or oil breakers and can operate at higher voltages effectively.

SF6 in cold climates

Nitrogen and freon-14 are added to keep SF6 gasified at lower temperatures. SF6 has a tendency to liquefy and pool at colder temperatures. This reduces the internal pressure, which reduces the dielectric strength.

SF6 Circuit Breakers in Substation
SF6 Circuit Breakers in Substation. Photo: Wikimedia.

Dead Tanks vs. Live Tanks

Dead tank breakers have grounded tanks. Live tank breakers use the insulating column to house the mechanism and contact assemblies, so they are energized at system voltage (live).

Moisture in SF6 gas

ANSI/NETA MTS specifies a maximum of 200 ppm moisture of in-service SF6 gas. Reference ANSI/NETA MTS Table 100.13.

Hazardous Byproducts in SF6 gas

As SF6 decomposes during arcing metal fluorides are created, which are corrosive. When servicing SF6 breakers that have been faulted, full body protection and oxygenfed respirators may be needed to prevent the fluorides from contacting the skin or being inhaled. Any moisture on or in the body will create hydrofluoric acid.

4 Risk Categories of SF6 gas Contamination

  1. New gas – in cylinders from the gas manufacturer.
  2. Non-arced – has been used or handled, but contains no arcing byproducts
  3. Normally arced – corrosive byproducts less than 200 ppm
  4. Heavily arced – contains arcing byproducts greater than one percent by volume.
  5. SF6 Gas Tests Limits

    NETA ATS Table 100.13

    NETA MTS Table 100.13