sharktooth meets resonance
Self-commutated rectifiers and inverters often create switching ripple effects in the grids. As in the picture "Zoom UL1", the familiar sharktooth can be seen in the voltage. Apart from the fact that these effects can be quite diverse, depending on the technology used, it must be considered, that this alone can lead to additional losses in electrical equipment operated in parallel or in series, up to thermal overload of components and direct interference with other consumers. In the picture, multiple zero crossings of the voltage occur at the very beginning, which often lead to disturbances of UPS systems, parallel working frequency converters and other automated processes. Experience shows that this condition alone is unacceptable and requires filter measures.
But it could be worse.
In the case shown above, a frequency converter with a switched input circuit - often also called AFE or IGBT converter - "finds" the parallel resonance of the low-voltage grid and begins to excite it. The oscillation process can be seen in the increasing and extreme voltage distortions. At a certain point, the causing inverter switches off and the resonant effects disappear abruptly. However, this switch-off does not occur due to a control loop that detects the extreme condition. No, the inverter switches off with an unclear error message with reference to the voltage.
Only a power quality and fault analysis by PQ Professionals figured out the real cause.
In summary, the IGBT converters in the present case not only create switching ripple distortions but also repeatedly excite the parallel resonance with extreme effects on all parallel loads. Premature aging and unexpected disturbances of the plant operation must be expected.