Vacuum circuit breakers in electrical equipment
The arc extinguishing chamber (commonly known as vacuum bubble) of the vacuum circuit breaker is relatively vacuum, and the general vacuum degree is 13*10^-2~1.3x10 -4pa or more. With the generation of central cabinets and the rapid development of integrated automation devices, mobile (handcart) vacuum circuit breakers with spring operating mechanisms have been widely used, and because of their small size, they greatly reduce the floor space and space, and most of them are installed indoors.
1) Structural characteristics of vacuum circuit breaker.
Vacuum circuit breakers are particularly suitable for frequent operation. It can operate continuously for more than 10,000 times under rated conditions, and can operate continuously for dozens of times under the condition of rated blocking current, and the vacuum interrupter generally does not need to be overhauled, and there is no fire and danger.
The vacuum interrupter is a vacuum bubble made of glass or ceramic, the internal contact adopts the docking form, the dynamic and static contacts adopt a spiral structure, and the outside of the contact has a metal shielding cover to cool and absorb the metal vapor of the arc gap, which plays an important role in the arc extinguishing process. The function of the stainless steel bellows is to maintain the seal during the movement, and it expands and contracts with the movement of the moving contact rod to ensure the vacuum degree of the interrupter.
The contact opening distance of the vacuum circuit breaker is small, the action speed is fast, and the operating overvoltage is easy to generate, and the overvoltage protection can be realized by parallel arrester.
2) The arc extinguishing principle of the vacuum circuit breaker.
Any kind of high-voltage circuit breaker extinguishes the arc by relying on the arc extinguishing chamber, which is the heart of the high-voltage switch. When the dynamic and static contacts of the switch are separated, under the action of high electric field, the dielectric particles around the contact are ionized, thermally freed, and collided free, resulting in an electric arc. If the dynamic and static contacts are in an absolute vacuum, there will be no arc generation when the contacts are opened, and the circuit will be easily broken because there is no medium present. The vacuum degree of the interrupter of the vacuum circuit breaker is 13*10^-2~1.3x10 -4Pa or more, in this high vacuum, the trace ions and metal vapors generated by the arc will diffuse very quickly, so that they will be subjected to strong cold action, and once the current crosses zero and the arc is extinguished, the dielectric strength of the vacuum gap recovers very quickly, so that the arc will not be re-ignited. This is the basic principle of vacuum circuit breakers that use high vacuum to extinguish the arc.
3) Advantages and disadvantages of vacuum circuit breakers.
1) The structural characteristics of the vacuum circuit breaker determine that it has the following main advantages:
The vacuum circuit breaker is small in size and light in weight, especially suitable for central cabinets.
The contact opening distance is small, the action speed is fast, the arcing time is short, and the service life is long.
It has good arc extinguishing performance and can be operated frequently, especially for breaking capacitive currents.
The operation noise is small, and there is no adverse effect on the working environment during operation.
The operation and maintenance of the vacuum interrupter is simple, and there is no fire and danger.
2) The vacuum circuit breaker has the following disadvantages in operation:
If the speed is too fast during the breaking process, it is easy to generate operating overvoltage.
It is not convenient to measure its vacuum degree, and can only rely on the high-pressure test to indirectly detect the vacuum interrupter.
The interrupting voltage is not very high.