POWER CIRCUIT BREAKERS ON POWERS SYSTEM TUTORIALS

A circuit breaker is defined as “a mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying and breaking for a specified time, and breaking currents under specified abnormal circuit conditions such as a short circuit” (IEEE Std. C37.100-1992).

Circuit breakers are generally classified according to the interrupting medium used to cool and elongate the electrical arc permitting interruption. The types are:
• Air magnetic
• Oil
• Air blast
• Vacuum
• SF6 gas

Air magnetic circuit breakers are limited to older switchgear and have generally been replaced by
vacuum or SF6 for switchgear applications. Vacuum is used for switchgear applications and some outdoor breakers, generally 38 kV class and below. Air blast breakers, used for high voltages ( ≥ 765 kV), are no longer manufactured and have been replaced by breakers using SF6 technology.

Oil circuit breakers have been widely used in the utility industry in the past but have been replaced by other breaker technologies for newer installations. Two designs exist — bulk oil (dead-tank designs) dominant in the U.S.; and oil minimum breaker technology (live-tank design).

Bulk oil circuit breakers were designed as single-tank or three-tank mechanisms; generally, at higher voltages, three-tank designs were dominant. Oil circuit breakers were large and required significant foundations to support the weight and impact loads occurring during operation.

Environmental concerns forcing the necessity of oil retention systems, maintenance costs, and the development of the SF6 gas circuit breaker have led to the gradual replacement of the oil circuit breaker for new installations.

Oil circuit breaker development has been relatively static for many years. The design of the interrupter employs the arc caused when the contacts are parted and the breaker starts to operate. The electrical arc generates hydrogen gas due to the decomposition of the insulating mineral oil.

The interrupter is designed to use the gas as a cooling mechanism to cool the arc and to use the pressure to elongate the arc through a grid (arc chutes), allowing extinguishing of the arc when the current passes through zero.

Vacuum circuit breakers use an interrupter that is a small cylinder enclosing the moving contacts under a high vacuum. When the contacts part, an arc is formed from contact erosion. The arc products are immediately forced to and deposited on a metallic shield surrounding the contacts. Without anything to sustain the arc, it is quickly extinguished.

Vacuum circuit breakers are widely employed for metal-clad switchgear up to 38 kV class. The small size of the breaker allows vertically stacked installations of breakers in a two-high configuration within one vertical section of switchgear, permitting significant savings in space and material compared to earlier designs employing air magnetic technology.
When used in outdoor circuit breaker designs, the vacuum cylinder is housed in a metal cabinet or oil-filled tank for dead tank construction popular in the U.S. Market.

Gas circuit breakers generally employ SF6 (sulfur hexaflouride) as an interrupting and sometimes as an insulating medium. In “single puffer” mechanisms, the interrupter is designed to compress the gas during the opening stroke and use the compressed gas as a transfer mechanism to cool the arc and to elongate the arc through a grid (arc chutes), allowing extinguishing of the arc when the current passes through zero.

In other designs, the arc heats the SF6 gas and the resulting pressure is used for elongating and interrupting the arc. Some older two-pressure SF6 breakers employed a pump to provide the highpressure SF6 gas for arc interruption.

Gas circuit breakers typically operate at pressures between six and seven atmospheres. The dielectric strength of SF6 gas reduces significantly at lower pressures, normally as a result of lower ambient temperatures. Monitoring of the density of the SF6 gas is critical and some designs will block operation of the circuit breaker in the event of low gas density.

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