Switching in Electrical Transmission and Distribution Systems

9 Vacuum Circuit-Breakers

9.1 General Features of Vacuum Interrupters

In comparison with other types of circuit-breakers, vacuum circuit-breakers are mechanically the most simple. They basically consist of a fixed and a movable contact mounted in a vacuum bottle. When the contacts separate, the arc is supported by ionized metal vapour released from the cathode (the negative contact) that thus provides the arcing medium. This is unlike gas or oil-filled interrupters, where ionized gas is provided by the extinguishing medium between the contacts. When the current approaches zero, the collapse of ionization and the condensation of vapour are very fast, ensuring efficient current interruption, virtually independent of the rate-of-rise of the transient recovery voltage, see Section 8.2.

The first serious demonstration of switching in vacuum took place in 1926 (see Section 8.1), and for many years, switchgear designers were fascinated by the great advantages of vacuum interrupters.

The development of vacuum circuit-breakers [1–3], in spite of the rather simple concept, required far more time and research effort than other interruption devices, due to the lack of supporting technology.

First, there were problems associated with the production of high-degree degassed contact materials, called gas-free contacts. Degassing is required to prevent deterioration of the initial vacuum due to the release of gases that are normally trapped inside the metals. These gases accumulate and affect ...