The optimal use of the distribution grids needs a good knowledge base of the dependencies. The project „Swinging Grids“ aimed to investigate critical grids, where existing effects were measured and the limits of charging by regulated devices were shown by adequate models. On this base, recommendations for the assessment of connection requests in respect of control systems were elaborated.
The project „Swinging Grids“ monitored the whole frequency area between nearly 0 Hz and 150 Hz. Basically, low voltage grids with regulated transformers and converters with regulation functionalities are stable. It has been shown that the key factor of the stability of low voltage grids is the frequencydependent grid impedance. Instabilities are occurring especially by an unfavourable interaction of oscillations above the fundamental with the frequency-dependent grid impedance. Subharmonic oscillations are indeed measurable, but there is hardly any correlation with the instabilities. Single customer installations with an unfavourable combination of control parameters and frequency-dependent grid impedance can initiate harmonic distortions ending in an instable operation. This depends on the manufacturer, the device type and on the frequency-dependent grid impedance at the point of common coupling (PCC). Especially for customer installations with non-linear technologies, simple recommendations have been formulated, which help to analyse the grids ex post or to focus on the important parameters ex ante to identify the reasons for the disturbances.
Berner Fachhochschule Technik und Informatik