Electrical Distribution Systems
CIRCUIT BREAKERS
High-Voltage Circuit Breakers
High-voltage circuit breakers (including breakers rated at intermediate voltage) are used for
service on circuits with voltage ratings higher than 600 volts. Standard voltage ratings for these
circuit breakers are from 4,160 to 765,000 volts and three-phase interrupting ratings of 50,000
to 50,000,000 kVA.
In the early stages of electrical system development, the major portion of high-voltage circuit
breakers were oil circuit breakers. However, magnetic and compressed-air type air circuit
breakers have been developed and are in use today.
The magnetic air circuit breaker is rated up to 750,000 kVA at 13,800 volts. This type of circuit
breaker interrupts in air between two separable contacts with the aid of magnetic blowout coils.
As the current-carrying contacts separate during a fault condition, the arc is drawn out
horizontally and transferred to a set of arcing contacts. Simultaneously, the blowout coil provides
a magnetic field to draw the arc upward into the arc chutes. The arc, aided by the blowout coil
magnetic field and thermal effects, accelerates upward into the arc chute, where it is elongated
and divided into many small segments.
The construction of this type of circuit breaker is similar to that of a large air circuit breaker used
for low-voltage applications, except that they are all electrically operated.
Compressed-air circuit breakers, or air-blast circuit breakers, depend on a stream of compressed
air directed toward the separable contacts of the breaker to interrupt the arc formed when the
breaker is opened. Air-blast circuit breakers have recently been developed for use in extra
high-voltage applications with standard ratings up to 765,000 volts.
Oil circuit breakers (OCBs) are circuit breakers that have their contacts immersed in oil. Current
interruption takes place in oil which cools the arc developed and thereby quenches the arc. The
poles of small oil circuit breakers can be placed in one oil tank; however, the large high-voltage
circuit breakers have each pole in a separate oil tank. The oil tanks in oil circuit breakers are
normally sealed. The electrical connections between the contacts and external circuits are made
through porcelain bushings.
Circuit Breaker Control
As we have discussed, circuit breakers may be remotely operated. In order to operate the
breakers from a remote location, there must be an electrical control circuit incorporated. Figure
7 shows a simple control circuit for a remotely-operated breaker.
Control power is supplied by an AC source and then rectified to DC. The major components of
a simple control circuit are: the rectifier unit, the closing relay, the closing coil, the tripping coil,
the auxiliary contacts, and the circuit breaker control switch.
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