What Is An SF6 Power Circuit
Breaker?
SF6 gas has proven to be an excellent
arc quenching and insulating medium for circuit breakers. SF6 is a
very stable compound, inert up to about 500 degrees C, non-flammable,
non-toxic, odorless, and colorless. At a temperature of about 2000K
SF6 has a very high specific heat, and high thermal conductivity,
which promotes cooling of the arc plasma just before and at current
zero, and thus facilitates quenching of the arc.
The electronegativity behavior of the
SF6, that is, the property of capturing free electrons and forming
negative ions, results in high dielectric strength and also promotes
rapid dielectric recovery of the arc channel after arc quenching. SF6
breakers are available for all voltages up to 1100 kV, continuous
currents up to 5000 A for conventional breakers (higher for generator
breakers), and shortcircuit interruption up to 80 kA.
SF6 breakers of the indoor type have
been incorporated into metal-clad switchgear. Outdoor designs include
both dead tank and live tank circuit breakers.
Over the years, SF6 circuit breakers
have reached a high degree of reliability; thus they can cope with
all known switching phenomena. Their closed-gas system eliminates
external exhaust during switching operations and thus perfectly
adapts to environmental requirements. Their compact design
considerably reduces space requirements and building and installation
costs.
In addition, SF6 circuit breakers
require very little maintenance. All ratings are economically
satisfied by the modular design. Each pole is equipped with one or
more interrupters; stored energy, spring, hydraulic, or pneumatic
driving mechanisms are provided for each pole or 3-pole unit.
Gas-density monitors are standard. In
the closed position, the current flows over the continuous current
contacts and the complete volume of the breaker pole is under the
same pressure of SF6 gas.
The precompression of the SF6 gas
commences with the opening operation. The continuous current contacts
separate and the current is transferred to the arcing contacts. At
the instant of separation of the arcing contacts, the pressure
required to extinguish the arc is reached.
The arc produced is drawn and at the
same time exposed to the gas, which escapes through the ring shaped
space between the extinction nozzle and the moving arcing contact.
The escaping gas has the effect of a double blast in both axial
directions.
Until the open position is reached, SF6
gas flows out of the puffer cylinder. The existing overpressure
maintains stability of the dielectric strength until the full value
of the open contacts at the rated service pressure is reached.
In the case of high-current
interruption, arc energy heats the gas, resulting in a pressure rise
in the static volume (heating volume) V1. This pressure then quenches
the arc at an ensuing current zero. In the low-current case an
auxiliary puffer (volume, V2) generates sufficient pressure for
interruption.
Necessary force requirements for the
mechanical system are therefore drastically reduced. All ancillary
equipments, including the oil pump and accumulator associated with
the drive, form a modular assembly that is mounted directly on the
circuit breaker, thus eliminating installation of piping on the site.
The metal-enclosed GIS breaker is provided with the necessary items
to fit into the substation arrangement.
The main equipment flanges of the
breaker are fitted with contact assemblies to accept the isolator
moving contacts. Other equipment modules can be coupled to the same
flanges. On the fixed-contact end of the circuit breaker, provision
is made for coupling two modules, facilitating the mounting of an
extension module to connect the second busbar isolator.
Dead tank SF6 breakers typically employ
gas-filled bushings. Such bushings are usually integral to the
circuit breaker itself and are not interchangeable with other
apparatus bushings.
Electrical grading is provided by a
lower throat shield. Ring-type bushing current transformers are
located at the base of the bushing. Potential taps are not generally
available in SF6 bushings because of the lack of a capacitive grading
structure.
Porcelain alternatives, such as
composites, have been used to provide greater safety (explosion
resistance), easier handling (lighter and nonbrittle), seismic
performance (lighter and stronger), and pollution performance.
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