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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