ABOVE: SF6 gas-powered
In oil circuit breakers, the arc is drawn in oil. The intense heat of the arc
decomposes the oil, generating high pressure that produces a fluid
flow through the arc to carry energy away. At transmission voltages
below 345 kV, oil breakers used to be popular. They are increasingly
losing ground to gas-blast circuit breakers such as air-blast breakers
and SF6 circuit breakers.
In air-blast circuit breakers, air is compressed
to high pressures. When the contacts part, a blast valve is opened
to discharge the high-pressure air to ambient, thus creating a
very-high-velocity flow near the arc to dissipate the energy. In
SF6 circuit breakers, the same principle is employed, with SF6 as the medium instead of air. In the "puffer" SF6 breaker, the motion of the contacts compresses the gas and forces it to
flow through an orifice into the neighborhood of the arc. Both
types of SF6 breakers have been developed for ehv (extra high voltage) transmission systems.
Two other types of circuit breakers have been developed. The vacuum breaker,
another electromechanical device, uses the rapid dielectric recovery
and high dielectric strength of vacuum. A pair of contacts is hermetically
sealed in a vacuum envelope. Actuating motion is transmitted through
bellows to the movable contact. When the contacts are parted, an
arc is produced and supported by metallic vapor boiled from the electrodes.
Vapor particles expand into the vacuum and condense on solid surfaces.
At a natural current zero the vapor particles disappear, and the
arc is extinguished. Vacuum breakers of up to 242 kV have been built.
The other type of breaker uses a thyristor, a semiconductor
device which in the off state prevents current from flowing but which
can be turned on with a small electric current through a third electrode,
the gate. At the natural current zero, conduction ceases, as it does
in arc interrupters. This type of breaker does not require a mechanism.
Semiconductor breakers have been built to carry continuous currents
up to 10,000 A.