Oil Circuit Breaker
a high-voltage AC electrical switch whose main contacts are located in a space filled with mineral (transformer) oil. Upon interruption of the electric circuit, an electric arc forms between the contacts of the circuit breaker. Because of the high temperature of the arc the oil is evaporated rapidly and oil vapors are partially decomposed, liberating ethylene, methane, and other gases. A gas bubble is formed in the arcing zone; the pressure in the bubble may be as high as several dozen meganewtons per sq m. The arc is then extinguished, both because of its elongation upon parting of contacts and because of intensive cooling by the gases and oil vapor.
In an oil circuit breaker with simple interruption under oil, the duration of arcing is 0.02-0.05 sec. To extinguish the arc more efficiently, arc-quenching chambers are used. In a longitudinal blast chamber the vapors and gases evolved travel upward along the arc, thus cooling it. In addition, the arc is in contact with the cold oil that fills the annular slots of the chamber, which also accelerates cooling of the arc. In a transverse blast chamber a drastic pressure increase within the gas bubble causes a stream of oil and gases to flow across the arc, thus accelerating the cooling process.
In terms of design, a distinction is made between tank-type oil circuit breakers and oil-minimum, or low-oil-capacity, circuit breakers. In the first type, the main contacts and the arc-quenching devices are located in a grounded metal tank; in the second type they are in an insulating or ungrounded metal enclosure filled with oil. Tank-type oil circuit breakers are inferior to other types of high-voltage breakers in many regards. However, their low cost and high reliability have led to their continued use in the USSR, the USA, and Canada.
In the USSR, tank-type oil circuit breakers are manufactured for voltages from 6 to 220 kilovolts (kV); maximum rated current, 3.2 kiloamperes (kA); breaking current, 50 kA. For voltages of 10 kV or less and breaking currents of 15 kA or less, all three poles of the oil circuit breaker are located in the same tank. For higher voltages and breaking currents, each pole is located in a separate tank. Oil-minimum circuit breakers are used in the USSR, the Federal Republic of Germany, and France. They are manufactured for 3 to 420 kV; since the late 1960’s they have also been manufactured for higher voltages.
Kate Koltunova
Mike Gurfing