Modern medium voltage circuit breakers

modern-circuit-breakerFor distribution units medium voltage oil circuit breakers are in the past. There are disadvantages of oil as means for cooling and extinguishing the arc, such as easy flammability and high operating costs. Manufacturers have gone through a number of alternatives – Air flow, electromagnetic switches and etc., but they failed to stay on the market because of various shortcomings. New research led to the simultaneous development of two types of switches – gas-electric elements and vacuumed. These switches have successfully replaced the older types and have already become a major element of the medium voltage substations. The journal “ENERGY” considers these two types of switches, each with its advantages and disadvantages. We present both technology and show what their advantages make them preferable in different situations.

Vacuum breakers are used for switching electrical circuits for all possible cases of exploitation – nominal operation, overload, short circuit, etc. When the contacts separate vacuum breaker, they began burning the arc in the middle of the metal, from vapors and gas of the molten metal of the contact bodies, until the first passage of current through zero value. Then the arc extinguishes and the conductive metal vapor condenses on the metal surfaces within a few micro seconds. In consequence of which the dielectric strength of the breaker is restored very quickly. The properties of vacuum circuit breakers are largely dependent on the material and shape of their contacts. For the period of their progress, they have been used with different materials. We assume that oxygen-free copper alloy and chrome is the best material for high voltage circuit breakers. In this alloy, chromium is distributed in honey in the form of fine grains. This material combines good arc extinguishing feature with reduced tendency to weld the contacts and the current low shear when switching inductive loads. The use of this special material limits the flow point to 4-5 Amps.

There is also a new design of the vacuum breaker, with which and arc is formed of diffusion into contracted form and divided into several branches, by subjecting the arc of the axial magnetic field. Such field can be obtained by directing the current of the arc of the coil located appropriately outside the vacuum chamber. Alternatively, the field can be created through the development of the contact system in an appropriate way to obtain the necessary contact time. These systems are called contact systems with axial magnetic field. This principle has its advantages when short-circuit current is above 31.5 kAamps.

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