Vacuum interrupter
The use of a vacuum for switching electrical currents was motivated by the observation that a one-centimeter gap in an X-ray tube could withstand tens of thousands of volts.
Sorenson presented the results at an AIEE meeting that year, and predicted the switches' commercial use.
Five years later, Thomas H. Lee at General Electric produced the first vacuum circuit breakers[2][3] with a rated voltage of 15 kV at short-circuit breaking currents of 12.5 kA.
Special-purpose vacuum interrupters are also manufactured, such as those used in transformer tap changers or in electrical arc furnaces.
Research and investigation in the early 1990s allowed the employment of vacuum switching technology for generator applications.
Compared to circuit-breakers using other quenching media (such as SF6, air-blast or minimum oil), vacuum circuit-breakers have the advantages of: Vacuum GCBs are suitable for frequent switching duty and for interrupting low-frequency currents as found in pumped storage power plants.
The stainless-steel bellows isolates the vacuum inside the interrupter from the external atmosphere and moves the contact within a specified range, opening and closing the switch.
The shield also helps control the shape of the electric-field distribution inside the interrupter, contributing to a higher open-circuit voltage rating.
They are made of a variety of materials, depending on the vacuum interrupter's use and design for long contact life, rapid recovery of voltage withstand rating, and control of overvoltage due to current chopping.
An external operating mechanism drives the moving contact, which opens and closes the connected circuit.
The vacuum interrupter includes a guide sleeve to control the moving contact and protect the sealing bellows from twisting, which would drastically shorten its life.
The vacuum interrupter's contact materials has a great influence on its breaking capacity, electrical durability and level of current chopping.
The contact structure, or design, of a vacuum interrupter also affects the breaking capacity and the breakdown voltage curve during operation.
To ensure a high breakdown voltage, components are assembled in a cleanroom where dust is strictly controlled.
High-vacuum solder is applied at the joints of the components, the parts are aligned, and the interrupters are fixed.
Since the 1970s, interrupter subcomponents have been assembled in a high-vacuum brazing furnace by a combined brazing-and-evacuation process.
Tens (or hundreds) of bottles are processed in one batch, using a high-vacuum furnace that heats them at temperatures up to 900 °C and a pressure of 10−6 mbar.
Vacuum-interrupter manufacturers address these concerns by selecting contact materials and designs to minimize current chopping.
[12] Nowadays, with very low current chopping, vacuum circuit breakers will not induce an overvoltage that could reduce insulation from surrounding equipment.
