A setting of the outputstream of OpenPDC

I am not very sure what mean of the Nominal Frequency and Frames Per Second in the OpenPDC is. Is it the nominal frequency of the grid?

Yes, nominal frequency is for the grid / power system - this metadata is included in the configuration frame for frequency offsets in the data frame.

Frames per second is simply the desired publication rate for the output data. Note that the output frame rate can be smaller than the frame rate of incoming data, in which case the system will downsample data. However, the setting the output frame rate to a value higher than the input frame rate will not inject artificial data, i.e., the system will only publish data at the highest frame rate of incoming data.

Hope that helps clarify.