Discussion on using the hottest vacuum circuit bre

Discussion on vacuum circuit breaker used as generator outlet circuit breaker

Abstract: early vacuum circuit breakers were less used as generator outlet circuit breakers because of their fatal shortcomings of high cut-off value, high overvoltage and easy breakdown of insulating media. The first generator outlet circuit breaker in the world is the air circuit breaker produced by ABB, followed by SF6 circuit breaker

keyword: air circuit breaker; Vacuum circuit breaker; Arc extinguishing principle

in China, the introduction of SF6 generator outlet circuit breaker is also relatively early, and it is widely used in hydropower plants and small and medium-sized thermal power plants. However, since the 1970s, with the research of vacuum arc and the development of contact material technology, Cu Cr alloy has been used as the manufacturing material of contact. The shape of contact has also been improved from the early simple cylindrical shape to the spiral shape with transverse teeth. These improvements have reduced the cut-off value of true air circuit breaker from the original 8 ~ 12 A to the average 3 A, and the maximum is 5 a. The improvement of technology and materials has gradually eliminated the main obstacles for the vacuum circuit breaker to be used as the generator outlet circuit breaker. At the same time, the advantages of the base, such as simple structure, high reliability, small maintenance and long service life, have become increasingly apparent. Vacuum circuit breakers are more and more widely used as generator outlet circuit breakers. Taking 3ah vacuum circuit breaker produced by Sie Mens company as an example, the main characteristics of vacuum circuit breaker are briefly analyzed

1 technical characteristics of vacuum circuit breaker

1.1 arc extinguishing principle and basic structure have wide speed regulation limitation structure

the arc extinguishing principle of vacuum circuit breaker is to generate an arc when the contact in the vacuum bubble is opened, and the arc carries evaporated metal gas, which is generated from the surface of the contact, and finally will be cooled on the surface of the contact and the inner surface of the closed cover. When the current crosses zero, the original vacuum state recovers rapidly, the dielectric strength at the gap increases rapidly, and the circuit is cut off

since vacuum is used as the arc extinguishing medium, the structure of the circuit breaker is relatively simple and the overall components are few. Siemens' 3 ah (6300 a, 50 KA) vacuum circuit breaker is assembled in a frame similar to the high-voltage switchgear, and the overall weight is light. The operating structure is a spring operating mechanism, and the dynamic stress of operation is small; When installed on the door, the requirements for civil foundation are simple. Only a few flat irons need to be embedded for welding the frame during installation, and the installation time is short

1.2 reliability and service life

vacuum interrupter the annual consumption of its flexible packaging products accounts for more than 1.5% of the total. Under normal circumstances, the vacuum bubble of the circuit breaker is sealed during the whole service life, so it can ensure a high vacuum degree. After the operation of the circuit breaker, the vacuum degree will not be reduced, which ensures that the rated current and rated breaking current will not be reduced, effectively improving the reliability of the circuit breaker. When an arc is generated, the contact of the circuit breaker will be worn due to the generation of metal gas, but the degree of wear is very small, completely within the controllable range, and it is completely maintenance free within the required 10000 times of mechanical operation life. Therefore, it can be said that the normal service life of vacuum circuit breaker can reach hundreds of years, exceeding the service life required by ANSI and IEC standards

1.3 impact on the environment

the vacuum circuit breaker produces a small amount of pure metal gas, Cu, Cr gas, and is basically recaptured by the contact. Even in the worst case, that is, the vacuum bubble breaks, the impact of the released gas on the environment is negligible

2 analysis of being used as generator outlet circuit breaker

when the generator outlet is short circuited, the short-circuit current contains a high DC component, which depends on the time constant of the stator winding τ A = XD ″/2 π FRA attenuated (XD ″ is the generator subtransient reactance, f is the generator frequency, RA is the resistance of the stator winding), the outlet growth is very difficult, and the short-circuit current may have a current zero of 100 ms or more. After the fault, the arc resistance generated by the contact separation further reduces the time constant of the DC component, making the DC component decay faster, thus generating the current zero crossing. According to the example provided by Siemens, its vacuum circuit breaker can meet the requirements of cutting off current

good arc extinguishing medium and contact material make the vacuum circuit breaker meet the transient recovery voltage rate required by IEEE. The transient recovery voltage parameter of generator source fault specified in IEEE standard is 1.8kv/μ S (the rated capacity of the generator is 101 ~ 400 MVA). The transient recovery voltage of Siemens vacuum circuit breaker is 4.5kv/μ s. When the breaking capacity is 50 Ka, 80% of the DC component can be broken

3 conclusion

based on the above characteristics, the vacuum circuit breaker has the advantages of high reliability, maintenance free, long service life and so on, which can technically meet the requirements of being used as the generator outlet circuit breaker. In addition, due to the simple overall structure, the price of vacuum circuit breaker is about 10% cheaper than SF6 circuit breaker under the same technical parameters. Therefore, vacuum circuit breakers are worth popularizing in hydropower plants and small and medium-sized thermal power plants that need to install generator outlet circuit breakers. (end)