Singing circuit breakers in the metalworking industry

Problem

Risks for electrical equipment in installations are often only noticed by listening carefully.

This is also the case here, although it is not uncommon in the metalworking industry. The cause of the extreme voltage distortions shown above in a broad frequency spectrum up to slightly above 20 kHz are powerful converters in the presses. A common disturbance pattern is disturbingly loud whistling tones in open circuit breakers, through which the largest proportion of the causing clock frequency currents must flow. Rightly the responsible electricians are then concerned about the service life of the installation and a negative influence on the tripping behaviour of the circuit breakers. At the latest, however, this also applies to all other consumers in the grid if such repercussions have been proven on the basis of a qualified grid analysis. Then premature ageing of these systems and devices must be expected.

Causal research

As proof of cause and effect, it is not sufficient to carry out a network analysis according to EN 61000-4-30 with the limit values of EN 50160 or EN 61000-2-2 or EN 61000-2-4, especially in the case of high-frequency disturbances. Here, measuring instruments are required whose capabilities go far beyond the level required by the standards. And furthermore, a lot of experience and plant knowledge is required for the selection of the right measuring locations and for the device parameterisation.

And when it comes to interpreting the measurement results, unfortunately you can’t rely on the latest standard either. The feedback effects in the double-digit kilohertz range shown in the diagram would even be permissible according to the latest EN 61000-2-2 from 2019. And in EN 61000-2-4, which has been valid for industrial networks unchanged since 2003, there are not even any limit value recommendations for this.

Possible solution approach

Ideally, the causing inverters should not emit correspondingly high and high-frequency currents in the first place. This could also be achieved with a little additional effort in the inverters, but is apparently not in the interest of the manufacturers. For this reason, passive filters (suction circuits or broadband filters) are usually helpful for such problems.