Strange frequency run in Turkey:
Maybe it is related to a power outage in Turkey: https://www.livik.net/2021/08/02/there-are-power-outages-across-turkey/
At 07:34:48 UTC power at the measurement device dropped off, at 07:38:10 UTC power returned.
From 07:38:10 UTC power behaves “differently”. When enabling a second graph (here: measurement from France), it can be seen, that the frequency in Istanbul stays closer to 50 Hz but is overlayed with much more glitches.
My interpretation is: an emergency power system was activated, which provides power only to a quite small number of rather small consumers (frequency closer to 50 Hz, no large deviations) but which in turn is less robust against disturbances and reacts with significant glitches. Or it has more harmonic components, which are not filtered by the measurement device and these glitches are measurement artifacts.
Finally, at 08:28:25 UTC the really wild course of the Istanbul graph ends and it is synchronous with the France graph again.
My conclusion: at 07:35 power dropped off, between 07:38 and 08:28 power returned, but it wasn't power from the common European grid and finally at 08:28 power from the common European grid was back again.