Nothing at all
Current interruption technology started with just drawing electric arcs in oil in wooden barrels. During many decades the development was aimed at reducing the amount of oil while at the same time increasing the interrupting power. All too often, big explosions followed operation of those vintage breakers. The idea was to exploit the hydrogen gas, developed by the heat of the arc, for removal of the arc’s heat under very high pressure. Design of breakers at that time was aimed at keeping the high pressure inside its housing.
As early as the mid-twenties, it was discovered in the US rather unexpectedly that an arc burning in vacuum had very good arc quenching properties. Pilot projects with “vacuum switches” in a 16 kV system were conducted in de era of KEMA’s foundation. However, it would take decades before this technology of using no medium at all, could be exploited commercially. The reason was that manufacturing technology of vacuum sealed devices was not mature enough to keep the required degree of vacuum inside a “bottle” during a breaker’s lifetime. Only in the seventies, vacuum circuit breakers reached the medium voltage level.
Around that time the first vacuum circuit breakers also reached KEMA Laboratories, at that time from many manufacturers (foto beschikbaar). Since these breakers are basically medium voltage devices, testing was and has always been in a direct three-phase circuit, first in laboratory 2, then 4 and starting the new millennium in laboratory 6. In the first years, vacuum suffered from a reputation of “chopping” the current slightly before current zero, thereby causing overvoltages in certain switching duties, challenging KEMA’s engineers to manage these properly. In the eighties, this issue had disappeared, after intense corporate research led to a proper choice of contact material. KEMA started to understand the benefits of vacuum switches by exploiting these in its “re-ignition installation”, where highly specialized, military grade triggered vacuum gaps were applied to realistically perform synthetic high-voltage breaker tests.
In the early nineties, in Japan, vacuum circuit breakers were developed up to 145 kV, reaching far beyond their traditional medium voltage applications. The driver was reduction of maintenance. Twenty years later, a new driver emerged, this time environmental: the wish to reduce SF6, a very strong greenhouse gas, the workhorse of high-voltage equipment. Now, development was initiated in Europe, China and again in Japan. This time, KEMA Laboratories had a key selling point: having available power to test the sub-transmission vacuum breakers (below 100 kV of rated voltage) in direct three-phase circuits. Vacuum breakers at 145 kV need to be tested in a synthetic way, which causes a major challenge in arc extension, because even KEMA’s advanced re-ignition installation is too slow to deal with the very responsive vacuum interruption technology. Specially designed, re-ignition devices had to be ordered to deal with the new situation.
In the meantime, also the limits regarding current shifted considerably. Vacuum generator breakers became attractive for use in renewable power plants. Various designs were tested in synthetic circuits. KEMA’s innovation here was the preparation of a hybrid circuit, combining the benefits of a direct with a synthetic circuit. The result, stressing the test-object with a constant power frequency voltage, avoids any discussion on dielectric withstand capability after the interruption.