SF6 circuit breaker

An SF6 circuit breaker is a breaker in which the contacts separate in sulphur hexafluoride (SF6) gas under pressure. SF6 is electronegative: it captures free electrons to form heavy negative ions, which rapidly de-ionises the arc path and restores dielectric strength after current zero.

Construction

        Fixed contact (hollow)
     +---------[  ]---------+
     |   SF6 gas at about    |
     |   2.8 to 5 bar        |
     |   arc  ->  nozzle     |
     |       [moving]        |
     |   insulating nozzle   |
     +----------+------------+
                |
  HP SF6 tank --valve-- LP tank --compressor
                |
       operating mechanism

Main parts:

1. Interrupter unit: fixed (hollow) contact and moving contact with arcing probes/contacts and current-carrying main contacts; an insulating (PTFE) nozzle directs gas flow.
2. Gas system: in the two-pressure design, a high-pressure reservoir (about 14 bar) and low-pressure chamber (about 2.8 bar), with compressor, filters, valves and gas monitoring (density switches). Modern puffer breakers use a single pressure (about 5–6 bar).
3. Arc-quenching chamber of porcelain or metal enclosure (live tank, dead tank or GIS).
4. Operating mechanism: spring, hydraulic or pneumatic.
5. Alarms and lockouts for low gas pressure, and absorbers (activated alumina) to take up decomposition products.

Principle of operation

1. In the closed position the contacts are surrounded by SF6 at low pressure.
2. On a trip signal the moving contact is pulled away; an arc forms between the arcing contacts.
3. At the same time a valve opens (two-pressure type) or the puffer piston compresses gas, and SF6 is blown axially along the arc through the nozzle.
4. SF6 molecules capture the free electrons and form heavy, slow negative ions (SF6⁻, SF5⁻), so the conductivity of the arc column falls sharply.
5. The gas also cools the arc (high thermal conductivity near arc temperatures).
6. At current zero the arc path de-ionises quickly and the dielectric strength rises faster than the restriking voltage, so the arc is extinguished.
7. The gas returns to the low-pressure side and is recompressed and reused; it recombines after the arc, so little is lost.

Applications

• 33 kV to 765 kV outdoor switchyards and gas-insulated substations (GIS).
• 132 kV and 220 kV substations in Nepal's grid widely use SF6 breakers.

Advantages

• Excellent arc quenching and dielectric strength (about 2.5–3 times air); silent operation.
• Compact, closed system, no moisture or fire risk; low maintenance.
• No current chopping problem; suitable for high RRRV.
• Disadvantage: SF6 is a strong greenhouse gas and needs careful handling; costly, needs leak monitoring.