Selection Principle and Empirical Data of Mechanical Parameters of Vacuum Circuit Breaker (1)

1.Rated distance

When the vacuum circuit breaker is in the open state, the distance between the moving and static contacts of the vacuum interrupter be selected should consider factors including the voltage of the vacuum circuit breaker, operating conditions, the nature of the breaking current,contacts materials and strength withstand of voltage of the vacuum gap.Mainly depending on the rated voltage and the material of the contacts.

Since the rated spacing of the vacuum circuit breaker has a great influence on the insulation performance, when the rated spacing increases from zero spacing, the insulation level will also increase, but when the spacing increases to a certain value, the opening rejection will not affect the insulation. The influence of performance is not big. If the distance is further increased, it will seriously affect the mechanical life of the vacuum circuit breaker. Through the installation, operation and maintenance of the vacuum circuit breaker, it is concluded that the rated distance of the vacuum circuit breaker is generally selected as: 6kV and below are generally 4~8mm, 10kV and below are generally 8~12mm, and 35kV is generally 20~40mm.

2. Contact stroke of contacts

The selection of the contact stroke of the contact must ensure that the contact can still maintain a certain pressure after wear; the moving contact can obtain a certain initial kinetic energy during opening, improve the initial opening speed of the switch, break the fuse point, reduce Burning time, improve medium recovery speed: When closing, the contact spring can be used to obtain smooth buffering and reduce bounce.

The contact stroke of the vacuum circuit breaker is too small to ensure the contact pressure after the contact is burned out. The initial speed of the switch is too small, which affects the breaking and dynamic thermal stability of the vacuum circuit breaker, and produces serious closing bombs. vibrate. The contact stroke of the switch is too large, which will increase the closing work of the operating mechanism, or make the closing extremely unreliable. Usually, the contact stroke of the vacuum circuit breaker is generally 20%~40% of the rated opening distance, and the 10kV vacuum circuit breaker The contact stroke is generally 3~4mm.

3.Contact working pressure

The working pressure of the contacts of the vacuum circuit breaker has a great influence on the performance of the vacuum circuit breaker, and its pressure is equal to the sum of the self-closing force of the vacuum interrupter and the spring force of the contacts. The working pressure selection of the circuit breaker contact should meet the requirements of 4 aspects:

A. Keep the contact resistance of the vacuum interrupter within the specified range.

B. Meet the requirements of dynamic stability test.

C. Inhibit combined bounce.

D. Reduce the spring vibration of opening.

When the vacuum circuit breaker is closing the short-circuit current, the contact will generate arc and electric repulsion after the breakdown, the contact will bounce, and the closing speed of the mechanism is also the slowest. Therefore, closing the short-circuit current is to check whether the working pressure of the contact is Meet the most demanding conditions required.

If the working pressure of the contact is too small, the bounce time when the contact is closed will increase, and at the same time, the resistance of the primary circuit will increase, which will directly affect the long-term working temperature rise of the new vacuum circuit breaker. If the working pressure of the contact is too high, since the self-closing force of the vacuum interrupter is a constant value, the working pressure will increase, thereby increasing the spring force of the contact, resulting in an increase in the combined work of the operating mechanism, and increasing the impact and vibration on the vacuum interrupter.

In actual work, not only the electromotive force between the contacts is related to the short-circuit current peak value, but also the contact structure and size of the vacuum circuit breaker must be considered. At the same time, the hardness of the contacts, the opening speed and other factors must also be considered. in short, a comprehensive consideration must be made in the solid line.

The contact pressure of the contacts of the vacuum circuit breaker is obtained from the empirical data according to the breaking current. When the breaking current is 12.5kA, the selected pressure is 50kg. When the breaking current is 16kA, the selection pressure is 70kg, and when the breaking current is 20kA, the selection pressure is 90~120kg. When the breaking current is 31.5kA, the selection pressure is 140~180kg, and the breaking current is 40kA. The selection pressure is 230~250kg.

4. Opening speed

Since the opening speed directly affects the recovery speed of the dielectric strength between the contacts after the current crosses zero, if the recovery speed of the dielectric strength between the contacts is less than the recovery voltage after the arc is extinguished, it will cause the arc to reignite. The arc time must meet the vacuum circuit breaker opening speed. vacuum circuit breaker opening speed mainly depends on the rated voltage. When the rated voltage and the contact opening distance are fixed, the fluctuation range of the opening speed depends on the size of the breaking current, the nature of the load, the recovery voltage and other factors. When the breaking current is large , the opening speed should also be larger. When breaking the capacitor current, due to the higher recovery voltage, in order to reduce the probability of re-ignition, the opening speed should also be larger. The opening speed of 10kV vacuum switch is usually 0.8-1.2m/s, and can be higher than 1.5m/s if necessary.

In fact, the initial opening speed, not the average opening speed, has the greatest impact on the breaking capacity. Therefore, some high-performance vacuum circuit breakers and 35kv vacuum circuit breakers usually evaluate the opening speed within a few millimeters when they are just broken. It seems that the higher the opening speed, the better, but it is not the case. The more serious the compression, the easier it is to cause the bellows to be damaged early and leak air. At the same time, the greater the vibration to the whole machine, it is easy to cause damage to the parts.