When using indoor vacuum circuit breakers, the following issues should be noted: 1. Strictly control the contact travel. It is important not to mistakenly believe that a larger opening distance is beneficial for arc extinction and arbitrarily increase the contact travel of the vacuum circuit breaker. This is because the travel distance of a vacuum circuit breaker is relatively short. Generally, the contact travel of a vacuum circuit breaker with a rated voltage of 10~15kV is only 8~12mm, and the overtravel of the contact is only 2~3mm. If the contact travel is increased too much, it will cause excessive stress on the bellows after the circuit breaker is closed, resulting in damage to the bellows and disruption of the vacuum within the sealed enclosure of the circuit breaker.
2. Strictly control the closing and opening speeds of vacuum circuit breakers. When the closing speed of a vacuum circuit breaker is too low, it can increase the wear of the contacts due to a prolonged pre-breakdown time. Additionally, since the arc-extinguishing chamber of a vacuum circuit breaker generally employs brazing technology and undergoes degassing treatment at high temperatures, its mechanical strength is not high and its vibration resistance is poor. If the closing speed of the circuit breaker is too high, it can cause significant vibration and impact the bellows, reducing their lifespan. Typically, the closing speed of a vacuum circuit breaker ranges from 0.6 to 2 m/s. For vacuum circuit breakers with a certain structure, there is a specific closing speed. When the vacuum circuit breaker is opened, the arcing time is short, not exceeding 1.5 power frequency half-waves. It is required that the arc-extinguishing chamber has sufficient insulation strength when the current passes through zero. It is generally desired that the contact travel during opening reaches 50 to 80% of the full travel within the power frequency half-wave. Therefore, the opening speed of the circuit breaker needs to be strictly controlled.
In addition, it is required that the opening and closing buffers of the vacuum circuit breaker possess good characteristics to minimize the impact force during opening or closing, in order to protect the service life of the vacuum arc-extinguishing chamber.
3. Strictly control the load current. Vacuum circuit breakers have poor overload capacity. Due to the thermal insulation formed by the vacuum between the contacts and the housing of the vacuum circuit breaker, the heat on the contacts and conductive rod is mainly conducted away along the conductive rod. To ensure that the operating temperature of the vacuum circuit breaker does not exceed the allowable value, its working current must be strictly limited to make it lower than the rated current.
4. Prevention of overvoltage: Vacuum circuit breakers have excellent interruption performance. However, it should be noted that when interrupting inductive loads, high overvoltages may occur across the inductance due to rapid changes in circuit current.
When switching off a small-capacity motor, the starting current is relatively large, so measures should be taken to reduce the starting current.
For transformers, there are differences between those with different structures. Oil-immersed transformers not only have a higher impulse voltage withstand value but also have a large stray capacitance, so no special protection is needed. For dry-type transformers with a lower impulse voltage withstand value, it is advisable to install protection, such as installing zinc oxide arresters, or adopting measures such as utilizing the distributed capacitance of cables and installing capacitors.
For vacuum circuit breakers used for line protection, due to the long line, large stray capacitance, and multiple devices connected on the line, they will not generate high current-carrying overvoltages. Therefore, generally no special protective measures are required during use.
For capacitor banks, field tests have proven that the overvoltage generated by the closing of vacuum circuit breakers generally does not exceed twice the voltage. In China, shunt capacitors are generally used for voltages below 60 kV, and equipment at these voltage levels has a high insulation level. Therefore, the overvoltage generated by closing the capacitor bank generally does not pose a hazard to the equipment. However, vacuum circuit breakers with poor performance can cause high overvoltages due to prolonged contact vibration during switching, which has occurred in tests both inside and outside China and should be noted.
5. Vacuum Interrupter ChamberThe vacuum interrupter chamber is a key component of a vacuum circuit breaker. It is supported and sealed using glass or ceramic, and contains moving and stationary contacts as well as a shielding cover. The chamber maintains a negative pressure with a vacuum level of 1.33×10-5~1pa, ensuring its arc-extinguishing performance and insulation level during interruption. When the vacuum level decreases, its interruption performance significantly deteriorates. Therefore, the vacuum interrupter chamber must not be subjected to any external force impacts, and it is strictly prohibited to knock, hit with hands, or apply force during handling and maintenance. It is also forbidden to place any objects on the vacuum circuit breaker to prevent them from falling and damaging the vacuum interrupter chamber. Before leaving the factory, the vacuum circuit breaker undergoes rigorous parallelism inspection and assembly. During maintenance, all bolts of the interrupter chamber should be tightened to ensure uniform stress distribution.
6. Strict handover and acceptance: The vacuum circuit breaker has undergone rigorous acceptance inspection before leaving the factory. However, after being transported to the site and installed, relevant parameters must be tested and rechecked to prevent any changes during transportation and any incompatibility issues that may arise after mechanism adjustments, especially issues related to the connection between the operating mechanism and the vacuum circuit breaker. The main parameters that should be retested include: closing bounce, opening synchronization, opening distance, compression stroke, closing and opening speeds, closing and opening times, DC contact resistance, fracture insulation level, and transmission acceptance tests, all of which should meet the requirements of the vacuum circuit breaker.
7. Maintenance cycle of vacuum circuit breaker The maintenance cycle should be carried out in accordance with relevant regulations, and should be flexibly adjusted based on the actual operating conditions. It should not be mistakenly believed that vacuum circuit breakers do not require maintenance. Specific regulations are as follows: (1) The vacuum degree of the vacuum arc-extinguishing chamber fracture should be inspected using the power frequency voltage withstand method in conjunction with seasonal (annual) preventive tests.
(2) After the vacuum circuit breaker has been operated (with both on-load and off-load currents) 2000 times normally and has interrupted the rated short-circuit current 10 times, the screws of each part should be checked for looseness. The inspection method and requirements should follow the maintenance and inspection requirements for vacuum circuit breakers. If the technical parameters meet the specified standards, the vacuum circuit breaker can continue to be used.
(3) After a vacuum circuit breaker has been out of operation and stored for 20 years, its vacuum degree should be checked according to the internal inspection methods and requirements of the vacuum arc-extinguishing chamber. If it fails to meet the requirements, it should be replaced.
