Puffer type SF6 circuit breaker

A puffer type SF6 circuit breaker is a single-pressure SF6 breaker in which the gas flow needed to quench the arc is produced by the motion of the moving contact itself: a cylinder attached to the moving contact compresses gas against a fixed piston and blows it through a nozzle onto the arc.

Construction

   Fixed contact (hollow, arcing tip)
           |    |
      =====|nozzle|=====   PTFE nozzle
           |  arc |
   +-------[moving]------+
   |  puffer cylinder    |  moves down with contact
   |  (gas compressed)   |
   +====[fixed piston]===+
           |
     operating rod -> spring/hydraulic mechanism
   Whole unit in SF6 at about 5-6 bar

1. Interrupter enclosure (porcelain or earthed metal tank) filled with SF6 at one pressure (about 5–6 bar).
2. Fixed contact with arcing tip, and main contacts that carry normal current.
3. Moving contact fixed to a puffer cylinder; a stationary piston sits inside the cylinder.
4. Insulating PTFE nozzle that guides the gas jet along the arc.
5. Operating mechanism, gas density monitor, and an absorbent (alumina) for decomposition products.

Working

1. On tripping, the moving contact and cylinder move away from the fixed contact.
2. The main contacts part first; the current transfers to the arcing contacts, which then part and draw an arc.
3. As the cylinder moves over the fixed piston, the gas inside is compressed (pressure rises 2–3 times).
4. When the arcing contact leaves the nozzle throat, the compressed SF6 is blown axially through the nozzle across the arc.
5. The gas cools the arc and its electronegative molecules capture electrons; at current zero the gap de-ionises rapidly and dielectric strength builds up faster than the restriking voltage.
6. The used gas recombines and remains in the enclosure; no compressor is needed.
7. Modern self-blast (auto-puffer) designs also use arc heat to raise pressure, reducing the operating energy.

Properties of SF6 that make it a good arc quencher

1. Electronegative: captures free electrons to form heavy, slow negative ions, so arc conductivity falls very quickly after current zero.
2. High dielectric strength: about 2.5–3 times that of air at the same pressure; at a few bar it is comparable to oil.
3. Fast dielectric recovery: strength builds up about 100 times faster than in air after current zero.
4. Good heat transfer: high thermal conductivity in the 2000–3000 K range cools the arc boundary.
5. Chemically stable and inert: non-flammable, non-toxic and non-corrosive in pure form; dissociated products recombine after the arc.
6. Low arc voltage and energy, so low contact erosion.
7. Heavy gas (about 5 times as dense as air) that liquefies easily under pressure, so it can be stored and transported as a liquid in cylinders.

These properties allow short arcing time, compact interrupters and use from 33 kV up to 765 kV and in GIS.