Ultra high current
Regularly, currents suddenly decide to leave the paved roads that power system designers have laid-out for them. This path of least resistance is so enjoyable, that current magnitude increases tremendously: short circuit. Much is done to avoid this situation, but still, in transmission systems roughly one short circuit has to be reckoned with per 100 km of overhead line per year. The lower the system voltage the more frequently “circuits short”.
There is one location where short-circuit current can be particularly high: this is directly at the output of a power plant. There, before transformation of the voltage to transmission levels, at the output of the generators, even normal, continuous current can go up to tremendous values. Generator circuit breakers need to master faults in that location.
Already in an early stage of electricity generation, it was understood that these devices are “special” and when the first designs of such specialized breakers were conceived, KEMA Laboratories were actively cooperating with the manufacturers of such apparatus.
The reason was and is sheer power. With four generators at hand in lab 2, the high-power laboratory can simulate a faulted power plant rather accurately. The designers of lab 4 had the vision to incorporate the generation of extreme current, not only for circuit breakers, but also for through fault testing of generator busducts, the connection from power plant to the outside world. In the initial design, a high-current busbar was constructed directly connecting the lab generators, all in parallel, with the test bay. In this way current op to 400 kA in the test bay became possible. With the increase of current, the main challenge became the management of the huge Lorentz forces involved. H.A. Lorentz was born in Arnhem in 1853, and apart from his major contribution to Einstein’s theory of relativity, he is best known because of his description of the forces between current carrying conductors, more precisely, the forces on a current carrying conductor in a magnetic field, summarized in the well-known “left hand rule” from secondary school physics.
Lorentz would be very pleased to see that “his” forces, taken to the extreme, were controlled in the laboratory based in his town of birth. Over the years, mechanically complicated but extremely robust constructions were developed to deal with the forces associated with current exceeding 1 million of amperes as a peak value. Again, KEMA Laboratories kept pace with the market requirements. Even when generator breakers needed to be tested for the largest nuclear power stations, the challenge was met.
Not only fault situations had to be implemented in testing, also the verification of thermal withstand of long duration continuous current had to be taken to the extreme. In the nineties, temperature rise tests in the high-voltage laboratory reached levels up to several ten-thousands of amperes.
Generator circuit breakers passed a development, from airblast, to SF6 and even vacuum technology. KEMA Laboratories also internally benefitted from the ongoing developments, since its own generators must be protected. From the seventies, airblast generator circuit breakers, better known as “master breakers” from AEG were employed in the Arnhem laboratories. These shall not only protect our own installation, but they must also protect the apparatus under test. Here, speed is the keyword, because if the huge test current is not switched off very quickly after a fault in the test object develops, there is no way to detect the root cause of failure. Airblast technology, now abandoned in commercial applications, appeared to be very reliable for master breakers, and is now continues to safeguard the new generators.