A circuit breaker is an electrical switch designed to protect an electrical circuit. The main feature of a vacuum circuit breaker is its ability to switch off short-circuit currents.
The requirements for medium and high voltage switches are formulated in the international standards IEC 60694 and IEC 62271-100.
Medium-voltage circuit breakers, like other electromechanical switching devices, differ depending on the medium in which their contacts open and close. There are, in particular, vacuum, oil, gas and air circuit breakers, the first three types being used in AC networks, while air circuit breakers, which were previously used in AC and DC networks, are currently only used in DC networks.
The application of one or another arc suppression medium is determined by its electrophysical properties, as well as by technological and operational factors. The curves presented in Fig. 1a allow comparing the electrical strength of different media, of which air is the worst. Elegas (sulfur hexafluoride) at a pressure close to atmospheric pressure (1 bar ≈ 1 kGs/cm2) has about 2.5 times higher electrical strength. An even better dielectric is transformer oil, which is also used in circuit breakers.
However, transformer oil and gas are considerably inferior to vacuum in terms of electrical strength, with which high-pressure gas (5 bar and more) competes successfully.
Vacuum is the most efficient medium in terms of minimizing the energy released in the arc during its extinguishing (Fig. 1b). Thus, vacuum switching modules or vacuum interrupter chambers have the smallest dimensions. Vacuum circuit breakers are the least demanding in operation.
Vacuum circuit breakers as a type of switching device appeared on the electrical market in the late 1970s. By overcoming the disadvantages of vacuum switching that were discovered during the initial research phase, such as current clipping and associated overvoltages, re-ignition of the arc after the current has passed through zero, welding of contacts during short-circuit currents, the development of suitable contact materials and technical solutions for the design of vacuum interrupters, the production of vacuum circuit breakers began to grow rapidly. Today, in the medium-voltage circuit breaker segment, vacuum circuit breakers occupy approximately 80% of the market, and this figure is on an upward trend.
The vacuum circuit breaker is based on a vacuum chamber, which successfully cuts short-circuit currents of up to 50 kA and ensures arc extinguishing from the first passage of current through zero ( Image 2).
Vacuum switches are available with both spring-motor and electromagnetic actuators.
Vacuum circuit breakers cover a wide range of applications, from industrial plants to urban infrastructure, where they are used to protect and control electrical networks. They provide high operational reliability and safety, minimizing the risk of accidents and facilitating network maintenance.
Vacuum circuit breakers from AVM Ampere utilize a spring-motor drive design with two-stage contact separation.
1 – fixed contact, 2 – vacuum chamber, 3 – insulating pole of the switch, 4 – trigger mechanism of the drive, 5 – movable contact, 6 – compression spring, 7 – main drive of the switch, 8 – traction insulator, 9 – drive lever.
The main advantage of the VB4 actuator is high mechanical resistance and a special diagram of power contacts separation. When the circuit breaker is tripped, the contacts are not separated instantaneously, which avoids high level of overvoltages.
The actuator also provides a mechanism for breaking the welded contacts of the vacuum chamber.
