Abstract: Early vacuum circuit breakers were rarely used as generator outlet circuit breakers due to their fatal drawbacks of high current-cutting capacity, resulting in high overvoltage and easy breakdown of the insulating medium. The world's first generator outlet circuit breaker was an air circuit breaker manufactured by ABB, followed by SF6 circuit breakers. Keywords: Air circuit breaker, Vacuum circuit breaker, Arc extinguishing principle. In China, SF6 generator outlet circuit breakers were introduced relatively early and are widely used in hydropower plants and small and medium-sized thermal power plants. However, since the 1970s, with the development of research on vacuum arcs and contact material technology, Cu-Cr alloys began to be used in the manufacture of contacts, and the shape of the contacts has been improved from the early simple cylindrical shape to a spiral shape with transverse teeth. These improvements have reduced the current-cutting capacity of vacuum circuit breakers from the original 8-12 A to an average of 3 A and a maximum of 5 A. Technological and material improvements have gradually eliminated the main obstacles to using vacuum circuit breakers as generator outlet circuit breakers, while at the same time making its advantages such as simple structure, high reliability, low maintenance, and long service life increasingly apparent. Vacuum circuit breakers are increasingly widely used as generator outlet circuit breakers. Taking the Siemens 3AH vacuum circuit breaker as an example, this paper briefly analyzes the main characteristics of vacuum circuit breakers. 1. Technical Characteristics of Vacuum Circuit Breakers 1.1 Arc Extinguishing Principle and Basic Structure The arc extinguishing principle of a vacuum circuit breaker is that when the contacts inside the vacuum bulb are pulled apart, an electric arc is generated. The arc carries evaporated metallic gas, which is generated from the contact surface and eventually cools onto the contact surface and the inner surface of the enclosure. When the current crosses zero, the original vacuum state is rapidly restored, the dielectric strength at the gap increases rapidly, and the circuit is interrupted. Because vacuum is used as the arc extinguishing medium, the circuit breaker structure is relatively simple, with fewer overall components. The Siemens 3AH (6300 A, 50 kA) vacuum circuit breaker is assembled in a frame similar to a high-voltage switchgear, making it lightweight. The operating structure is a spring-operated mechanism, resulting in low dynamic stress during operation; it is installed on a door, requiring simple civil engineering foundations, only needing to pre-embed a few flat irons for welding the frame during installation, thus shortening the installation time. 1.2 Reliability and Service Life Under normal conditions, the vacuum bulb of a vacuum circuit breaker remains sealed throughout its entire service life, ensuring a high degree of vacuum. This vacuum level does not decrease after circuit breaker operation, guaranteeing that the rated current and rated breaking current values ​​will not decrease, effectively improving the reliability of the circuit breaker. During arcing, metallic gases are generated, causing some wear on the circuit breaker contacts. However, this wear is minimal and completely within a controllable range, requiring no maintenance within the required 10,000 mechanical operation cycles. Therefore, it can be said that the normal service life of a vacuum circuit breaker can reach several hundred years, exceeding the service life required by ANSI and IEC standards. 1.3 Environmental Impact Vacuum circuit breakers generate only a small amount of pure metallic gases, Cu and Cr, which are largely recaptured by the contacts. Even under the worst-case scenario, i.e., when the vacuum bulb ruptures, the released gases have a negligible impact on the environment. 2. Analysis of the Vacuum Circuit Breaker Used as a Generator Outlet Circuit Breaker When a generator outlet short circuit occurs, the short-circuit current contains a high DC component. This DC component decays according to the stator winding time constant τa = Xd″/2πfRa (where Xd″ is the generator subtransient reactance, f is the generator frequency, and Ra is the stator winding resistance). The short-circuit current may have a current zero point of 100 ms or longer. After the fault, the arc resistance generated by the contact separation further reduces the time constant of the DC component, causing it to decay even faster, thus generating a current zero-crossing point. Calculations based on examples provided by Siemens show that their vacuum circuit breakers can meet the interruption current requirements. The excellent arc-extinguishing medium and contact materials enable the vacuum circuit breaker to meet the transient recovery voltage rate required by IEEE. The IEEE standard specifies a transient recovery voltage parameter of 1.8 kV/μs for generator source faults (generator rated capacity of 101–400 MVA). Siemens' vacuum circuit breaker has a transient recovery voltage of 4.5 kV/μs and can interrupt 80% of the DC component with a breaking capacity of 50 kA. 3. Conclusion In summary, vacuum circuit breakers possess advantages such as high reliability, maintenance-free operation, and long service life, and technically meet the requirements for generator output circuit breakers. Furthermore, due to their simple overall structure, vacuum circuit breakers are approximately 10% cheaper than SF6 circuit breakers with the same technical parameters. Therefore, vacuum circuit breakers are worth promoting in hydropower plants and small to medium-sized thermal power plants that require generator output circuit breakers.