Vacuum Circuit Breaker Selection Guide: Parameters & Applications
I. Selection of Vacuum Circuit Breakers
Vacuum circuit breakers should be selected based on rated current and rated short-circuit current, using the actual capacity of the power grid as the reference. The tendency to adopt excessively high safety factors should be avoided. Overly conservative selection not only leads to uneconomical "over-sizing" (large breaker for small load), but also affects the breaker’s performance in interrupting small inductive or capacitive currents, potentially causing current chopping overvoltages.
According to relevant literature, approximately 93.1% of 10kV feeder circuits in China’s operational power grids have a rated current of 2000A or less. Therefore, the selection of rated operating current should primarily focus on values of 2000A and below. The selection of maximum short-circuit current should follow the requirements of the "Guidelines for Urban Network Planning and Reconstruction," avoiding blind pursuit of excessive safety margins.
Currently in the Chinese market, commonly used imported brand circuit breakers include Schneider's HVX, ABB's VD4, and Siemens' 3AE series. Domestic brands include Changshu Switchgear's CV1, Shanglian's RMVS1, and Baoguang's ZN172 series. The quality difference between domestic and imported brands is now negligible.
II. Vacuum Circuit Breakers and Their Characteristics
A circuit breaker is a switching device equipped with a special arc-extinguishing chamber. It can close, carry, and interrupt currents under normal circuit conditions, and can close, carry, and interrupt abnormal circuit conditions (e.g., short circuits) within specified time periods. It is suitable for power grids with a frequency of 50Hz and voltage levels of 3.6kV and above, used to switch on and off load currents (typically not exceeding 4000A), overload currents, and rated short-circuit currents (typically not exceeding 63kA).
It can also be used in special applications to switch unloaded long transmission lines, unloaded transformers, capacitor banks, etc., and to carry short-circuit currents (typically not exceeding 63kA) within specified durations (1s, 3s, 4s), as well as close onto short-circuit currents (typically not exceeding 160kA). The mechanical life of circuit breakers is generally 10,000 operations, with special models reaching 30,000 or 60,000 operations. When equipped with a permanent magnet actuator, it can reach up to 100,000 operations. According to CB1984-2014, the electrical life of a circuit breaker is 274 operations.
Circuit breakers generally have auto-reclosing capability, allowing rapid restoration of power supply after fault clearance, and are typically used in critical applications. However, circuit breakers are relatively expensive (requiring corresponding relay or microprocessor-based protection), and their fault interruption time is within 80ms (dependent on protection relay response time, breaker tripping time, and arcing time). Their fault current interruption speed is slower than that of switchgear assemblies, thus requiring the protected equipment to have sufficient short-time withstand current capability.
III. Main Applications of Circuit Breakers
Circuit breakers are mainly used in industrial and mining enterprises, power plants, and substations for receiving, controlling, and protecting power systems. A typical configuration (using 12kV as an example) consists of two incoming circuit breakers and one or more outgoing circuit breakers (see diagram). The incoming circuit breaker current generally does not exceed 4000A, with a short-circuit breaking current generally not exceeding 50kA. The rated current of outgoing circuit breakers generally does not exceed 1600A, with a short-circuit breaking current generally not exceeding 40kA.
IV. Selection Criteria for Circuit Breakers
Use a circuit breaker when controlling load currents exceeding 630A.
Use a circuit breaker when protecting transformers with a capacity greater than 1600kVA at the power supply end.
Use a circuit breaker when protecting motors with a capacity greater than 1200kW.
Use a circuit breaker when switching capacitor banks.
Use a dedicated generator circuit breaker when protecting generators.
Use a circuit breaker when protecting power lines or critical equipment.
Application examples of circuit breakers
V. Precautions During Vacuum Circuit Breaker Operation
During operation, maintenance of vacuum circuit breakers should be determined based on usage conditions and operational frequency. For breakers with infrequent operation (annual operations not exceeding 1/5 of mechanical life), a routine inspection once a year is sufficient within the mechanical life period. For frequently operated breakers, the number of operations between inspections should not exceed 1/5 of the mechanical life.
When operation frequency is extremely high or mechanical/electrical life is nearing its end, inspection intervals should be shortened. Inspection and adjustment items include vacuum level, travel, contact travel, synchronization, opening/closing speed, as well as checks on major components of the operating mechanism, external electrical connections, insulation, and control power supply auxiliary contacts.
The following issues should be noted during vacuum circuit breaker operation:
(1) Overvoltage Issues
Vacuum circuit breakers often produce high overvoltages when interrupting small currents, especially small inductive currents such as transformer magnetizing currents, due to significant current chopping. Additionally, when interrupting capacitive currents of capacitor banks, arc re-ignition is difficult to avoid; once re-ignition occurs, it can generate re-ignition overvoltages. Therefore, high-performance metal-oxide surge arresters or RC (resistor-capacitor) protection devices should be installed for protection.
(2) Monitoring of Vacuum Integrity in the Interrupter Chamber
The vacuum level inside the vacuum interrupter is typically maintained between 10⁻⁴ and 10⁻⁶ Pa. As the interrupter ages and accumulates more switching operations, or due to external influences, the vacuum level gradually deteriorates. Once it drops below a critical threshold, the interrupting capability and dielectric strength will be compromised. Therefore, the vacuum level inside the interrupter must be regularly tested during operation.
(3) Monitoring of Contact Wear
The contact surfaces of the vacuum interrupter wear gradually after multiple current interruptions. As contact wear increases, contact travel increases, which in turn increases the working stroke of the bellows, significantly reducing its service life. Typically, the maximum allowable electrical wear is around 3mm. When the cumulative wear reaches or exceeds this value, both the interrupting performance and conductivity of the vacuum interrupter degrade, indicating the end of its service life.
VI. Conclusion
In the selection of vacuum circuit breakers, full consideration should be given to the actual power supply conditions and the real load characteristics on the load side. Correct and rational selection of circuit breakers plays a significant role in enhancing the safe and reliable operation of the system.
