Medium-voltage switchgear is typically used in three-phase AC 50Hz power systems with a rated voltage of 12kV. Taking the VS1 vacuum circuit breaker as an example, it is an indoor switchgear specifically designed for such systems. When selecting a switchgear, it is crucial to ensure that the circuit breaker's rated voltage and frequency are perfectly matched to the power system. A mismatch in rated voltage can lead to the circuit breaker malfunctioning or even causing safety accidents. For instance, using a circuit breaker with a lower rated voltage in a high-voltage system may damage it due to excessive voltage; conversely, using a circuit breaker with an excessively high rated voltage in a low-voltage system may result in wasted resources and, in some cases, may not achieve optimal protection and control.
The VS1 vacuum circuit breaker is suitable for applications requiring frequent operation at rated operating current or multiple short-circuit current interruptions, such as protection and control units for power grid equipment and industrial and mining power equipment. When selecting a model, suitability should be determined based on the actual usage scenario. For example, in industrial and mining enterprises requiring frequent equipment starts and stops, the VS1 vacuum circuit breaker can well meet the needs of frequent operation; while for applications with lower operating frequency requirements but extremely high short-circuit breaking capacity requirements, it is necessary to comprehensively consider whether its performance in repeatedly interrupting short-circuit currents meets the requirements.
Vacuum circuit breakers are named for their high-vacuum arc-extinguishing medium and the high-vacuum insulation medium in the contact gap after arc extinguishing. VS1 vacuum circuit breakers are characterized by a short arcing time, independent of the switching current, typically only half a cycle. The contact gap medium recovers quickly after arc extinguishing, resulting in good performance in breaking near-zone faults. When selecting a circuit breaker, good arc-extinguishing performance ensures that the arc is quickly extinguished when breaking the circuit, reducing damage to the contacts and extending the circuit breaker's service life. For example, in the event of a short-circuit fault, rapid and effective arc extinguishing can prevent further escalation of the fault and ensure the safe and stable operation of the power system.
VS1 vacuum circuit breakers feature a small contact gap; the contact gap of a 10kV vacuum circuit breaker is only about 10mm. This results in low operating power and short mechanical travel, leading to a long mechanical life. Simultaneously, due to minimal wear on the contacts during current interruption, the electrical life of the contacts is also long, reaching 30-50 full-capacity interruptions and over 5000 rated current interruptions. Contact life is a crucial consideration when selecting a circuit breaker. Choosing a circuit breaker with short contact life may require frequent contact replacements, increasing maintenance costs and power outage time. For locations with high power supply reliability requirements, such as hospitals and data centers, circuit breakers with long contact life better meet the needs.
When selecting a circuit breaker, the load current flowing through it needs to be calculated based on the actual load conditions. Generally, distribution circuit breakers are available with various rated currents such as 630A, 1000A, 1250A, and 1600A. For example, for a 2000KVA transformer, its rated current I = 2000 / (1.732 * 10) = 115.5A, a 630A circuit breaker can be initially selected, but further verification is required.
When determining the rated current, it's not sufficient to rely solely on the calculated load current; a certain margin must be allowed. This is because the load may fluctuate in actual operation, or there may be future load increases. If the margin is too small, the circuit breaker may trip due to overload when the load suddenly increases, affecting normal power supply; if the margin is too large, it will result in wasted resources. Generally, a margin of approximately 10% to 30% can be selected based on the actual situation.
Short-circuit current is one of the key parameters to consider when selecting a circuit breaker. The maximum short-circuit current value of the power system must be accurately determined based on professional specifications. Blindly pursuing excessively high safety factors will increase actual operating costs and waste resources. For example, in the power systems of some small factories, selecting circuit breakers with excessively large breaking capacities without considering the actual short-circuit current will not only increase equipment procurement costs, but also prevent the excessive breaking capacity from being effectively utilized during normal operation.
The short-circuit breaking capacity of the selected circuit breaker must be greater than or equal to the maximum short-circuit current of the power system. The minimum rated current for VS1-10kV high-voltage switches is 630A, with a breaking capacity of 20-25kA. Accurate calculations and comparisons must be performed during selection to ensure the circuit breaker can reliably disconnect the circuit in the event of a short-circuit fault, protecting equipment and personnel. If the circuit breaker's breaking capacity is insufficient, it may fail to disconnect the circuit in time during a short circuit, leading to the fault escalating and damaging more electrical equipment.
The VS1 vacuum circuit breaker uses a spring-operated mechanism and can be operated via AC or manually. When selecting a model, the appropriate operating method should be chosen based on actual needs. For locations requiring high operational flexibility, such as sites where frequent manual operation is necessary, the manual operation function is particularly important. Conversely, for highly automated locations such as substations, AC electric operation is more suitable to achieve remote control and automated operation.
The reliability of the operating mechanism directly affects the normal operation of the circuit breaker. Spring-cooled operating mechanisms have advantages such as simple structure and reliable operation. When selecting a model, attention should be paid to the quality and performance of the operating mechanism, choosing products with a good reliability record. For example, the reliability of operating mechanisms from some well-known brands has been widely recognized after extensive practical application verification. At the same time, the difficulty and cost of maintenance should also be considered, selecting operating mechanisms that are easy to maintain and have low maintenance costs.
To ensure equipment reliability, professional regulations require vacuum circuit breakers to be suitable for environments with an altitude below 1000m and an ambient temperature between -5℃ and 40℃, while outdoor products are required to be above -40℃. When selecting a circuit breaker, the appropriate temperature condition of the actual operating environment must be considered. If the ambient temperature is too high, the circuit breaker's heat dissipation may be affected, leading to a decrease in performance; if the ambient temperature is too low, some components of the circuit breaker may experience performance changes, even affecting the normal operation of the operating mechanism. For example, in cold northern regions, circuit breakers capable of withstanding low-temperature environments must be selected.
Humidity has a significant impact, particularly on outdoor products. When selecting circuit breakers, the applicable environment should have a daily average relative humidity below 95% and a monthly average below 90%, with a daily average saturated vapor pressure below 0.0022 MPa and a monthly average below 0.0018 MPa. High humidity can degrade the insulation performance of circuit breakers, potentially leading to safety issues such as leakage. Therefore, in areas with high humidity, circuit breakers with good moisture resistance should be selected. Furthermore, when operation is frequent or the mechanical or electrical lifespan is nearing its end, the equipment's operational status inspection cycle should be adjusted, and maintenance and monitoring should be strengthened.
In summary, selecting circuit breakers for medium-voltage switchgear (such as VS1 vacuum circuit breakers) is a complex process that requires comprehensive consideration of multiple factors. Only by fully understanding the various parameters and performance characteristics of the circuit breaker, and combining this with the actual operating environment and requirements, can the most suitable circuit breaker be selected to ensure the safe and stable operation of the medium-voltage switchgear and provide a guarantee for the reliable power supply of the power system.