Mains frequency monitoring


Playing around with measuring the UK mains frequency and got the following results


where blue is my measurement and yellow is a reference measurement i obtained from a real time tracker on the internet. Pretty good agreement with in 0.01 of a Hz (mostly)

What i’m really interested in are the variations between blue and yellow, they are not constant and at times look cyclical such as


What am i seeing there? is that the real time frequency regulation of the mains kicking in and out on 10-20 second duty cycles? as generating capacity is added and removed to regulate the frequency? Or am i detecting local distortions that are offsetting my zero crossing detection and causing error in my frequency graphs?

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