Resonance or switching ripple? Neither nor!

Even we thought about resonance effects when first observed these extreme voltage distortions in a 400 V grid. The cause are large active frontend inverters with IGBT technology. After analysis, the switching frequency of the inverters could be discarded as possible source.

Real cause is a malfunction in the control of the inverters, which occurs under certain circumstances.

Filter applications would be suitable for damping those effects and are already used on an experimental basis. But the problem should be solved on the real source, in the inverter control.