  To ensure reliable operation of power grids, their frequency shall stay
within strict bounds. Multiple sources of disturbances cause fluctuations of
the grid frequency, ranging from changing demand over volatile feed-in to
energy trading. Here, we analyze frequency time series from the continental
European grid in 2011 and 2017 as a case study to isolate the impact of
trading. We find that trading at typical trading intervals such as full hours
modifies the frequency fluctuation statistics. While particularly large
frequency deviations in 2017 are not as frequent as in 2011, large deviations
are more likely to occur shortly after the trading instances. A comparison
between the two years indicates that trading at shorter intervals might be
beneficial for frequency quality and grid stability, because particularly large
fluctuations are substantially diminished. Furthermore, we observe that the
statistics of the frequency fluctuations do not follow Gaussian distributions
but are better described using heavy-tailed and asymmetric distributions, for
example L\'evy-stable distributions. Comparing intervals without trading to
those with trading instances indicates that frequency deviations near the
trading times are distributed more widely and thus extreme deviations are
orders of magnitude more likely. Finally, we briefly review a stochastic
analysis that allows a quantitative description of power grid frequency
fluctuations.
