The logic behind calling 1000 V to 72.5 kV a medium-voltage range is not obvious unless it is compared with the maximum North American grid voltages of 800 kV and more common transmission system voltages of 60 to 500 kV.

Nevertheless, medium-voltage circuit breakers can protect AC generators, some transmission and subtransmission lines, and distribution substations.

The industry classifies circuit breakers in the following way:
# Medium-voltage power (1000 V to 72.5 kV)
# Low-voltage power (1000 V and below)
# Industrial molded case (600 V and below)
# Miniature or branch circuit (240 V and below)

All circuit breakers are electromechanical devices that make and break currents under normal conditions and make, carry for a specified time, and break currents under abnormal conditions such as short circuits. Circuit breakers, like transformers and batteries, are made in a wide range of ratings.

Because the higher-voltage circuit breaker contacts can be damaged or destroyed by the burning action of electric arcs when the contacts of a high-voltage circuit are opened, various methods have been developed to provide an appropriate quenching medium around the contacts that will assist in extinguishing any arcs formed as rapidly as possible.

The names of mediums used for extinguishing the arc are included in the descriptions of the circuit breaker. For example, there are oil circuit breakers, air-blast circuit breakers, and magnetic-air circuit breakers.

The selection of the appropriate method for quenching the arcs depends on the cost-effectiveness and availability of sources of and means for providing air blasts, insulating gas, insulating oil, magnetic fields, or vacuums.

The two basic designs for high-voltage circuit breakers are oil and oil-less. The oil type circuit breaker had been the most popular for outdoor service up to 362 kV, but the air-blast and gas-type versions have been gaining in popularity.

At 550 and 800 Kv, oil-less breakers predominate. For new indoor applications magnetic-air and vacuum circuit breakers predominate, along with some gas-type. Indoor magnetic-air, air-blast, and vacuum breakers have been adapted for outdoor use in the 2.5- to 34.5-kV range by protecting them with metal covers.

It is essential that the correct circuit breakers, fuses, and switches be selected for each power control application because of their importance in the design and function of the overall electrical system.

Immediately upon sensing a short circuit or break in the supply line, fuses and circuit breakers must isolate the sections of the electrical network where the fault occurred, to prevent further damage while permitting the remainder of the network to remain operational.

No comments:

Post a Comment