SF6 CB: properties and limitations

An SF6 circuit breaker is a breaker in which the contacts separate inside a sealed chamber filled with sulphur hexafluoride (SF6) gas under pressure, and the arc is quenched by blowing SF6 over it. It is the standard breaker for 33 kV to 765 kV.

Construction

• Interrupter unit (arc chamber): a sealed metal/porcelain chamber filled with SF6 at about 3–5 bar (gauge); it holds a fixed contact and a hollow moving contact with a nozzle.
• Contacts: main current-carrying contacts and arcing contacts (copper-tungsten tips) so that the arc does not burn the main contacts.
• Gas system: high-pressure reservoir (about 14–16 bar in the older double-pressure type), low-pressure chamber, compressor, filters (activated alumina to absorb decomposed products), pressure and density monitors.
• Operating mechanism: spring, pneumatic or hydraulic, linked to the moving contact through an insulated rod.

  Fixed contact      Nozzle      Moving contact
   ┌──────┐        ┌──\/──┐      ┌──────────┐
   │      │  arc   │      │◄═════│ (hollow) │◄── drive rod
   └──────┘ ~~~~~~ └──────┘      └──────────┘
        ▲ SF6 blast from piston/reservoir ▲
  ─────── sealed chamber filled with SF6 ───────

Working

1. In the closed position the contacts touch and the chamber is filled with SF6 at rated pressure.
2. On a trip signal the moving contact is pulled away; an arc forms between the arcing contacts.
3. In the puffer type the same motion compresses gas in a cylinder; in the double-pressure type a valve opens from the high-pressure reservoir. Either way SF6 flows axially through the nozzle along the arc.
4. SF6 is electronegative: its molecules capture free electrons to form heavy negative ions (SF6⁻, F⁻) that move slowly. The arc column loses conductivity very fast.
5. At the next current zero the arc is extinguished and the gap regains dielectric strength quickly, so restriking does not occur.
6. The gas recombines almost fully into SF6 after cooling; small by-products are absorbed by filters.

Dielectric properties of SF6

• Dielectric strength about 2.5–3 times air at the same pressure; at 3 bar it is close to transformer oil.
• High electronegativity, so fast recovery of dielectric strength after current zero.
• Colourless, odourless, non-toxic, non-inflammable and chemically inert at normal temperature.
• High thermal conductivity and heat transfer, good cooling of the arc.

Arc quenching characteristics

• Arc time constant very small (a few μs), so even high rates of rise of restriking voltage are handled.
• Low arc voltage and low arc energy, hence little contact erosion.
• No current chopping, so low switching overvoltages.
• Silent operation; no exhaust to atmosphere.

Limitations

• SF6 is a very strong greenhouse gas (GWP about 23,500 times CO₂); leaks must be prevented and gas recovered.
• Needs leak-proof sealing and gas handling plant; costly.
• Arc by-products (SF4, S2F2, metal fluorides) are toxic and corrosive; moisture must be kept out.
• Gas liquefies at low temperature and high pressure (about 10 °C at 15 bar), so heaters may be needed.
• Dielectric strength falls sharply if pressure drops, so continuous pressure monitoring and lock-out are required.