frequency - Serbia, Kosovo power grid row delays European clocks. Why?

According to this article (and a lot more published today on the same topic), Kosovo electricity net production balance has decreased during the last weeks.
This has led to a small deviation of the European network’s frequency (from 50Hz to 49.996Hz). In turn, this frequency deviation led to some electric clocks (like ones in ovens) being out of sync (up to 6 min since January).

• How can a decrease in electricity production lead to a decrease of the frequency on the grid on the long term? Isn't the frequency a parameter controlled by the power plant at the end of the day?


• If the loss of power from some countries causes a frequency deviation, shouldn't we also observe other impacts, like a drop of the output voltage? Does this mean that we've also been experiencing a drop of voltage for weeks here in Europe?



• Why do some electric devices directly use the network frequency to sync their clocks, instead of a quartz crystal technology? This means the same oven needs two different firmwares for countries with different electric network frequencies, while, with a crystal (that should be needed anyway to run all the embedded circuits), the same device would run unmodified everywhere.

Answer

How a decrease in electricity production can lead to a decrease of the frequency on the grid on the long term? Isn't the frequency a parameter controlled by the power plant at the end of the day?

Answered before: Why do generators have to rotate at a slower frequency if demand outpaces the supply?

If the loss of power from some countries causes a frequency deviation, shouldn't we also observe other impacts, like a drop of the output voltage? This means we've also been experiencing a drop of voltage for weeks here in Europe?

frequency reduction is primary the means to regulate, not voltage. But, yes, you'll see some amplitude (envelope) variation.

Why some electric devices directly use the network frequency to sync their clocks, instead of a quartz crystal technology? This means the same oven needs 2 different firmwares for countries with different electric network frequencies, while, with a crystal (that should be needed anyway to run all the embedded circuits), the same device would run unmodified everywhere.

Now coming to the relevant question:

Nope, these won't have firmwares the way you think. Also, you can't sell 100% the same oven in countries with different grid frequencies, anyway, because 60 Hz countries tend to use different grid voltages.

These clockkeeping circuits will more look like zero-crossing detectors (that is, a very minimal comparator with a cap on one input, fed directly from the power line, most likely through a resistive voltage divider) and counters, feeding a very minimalistic dedicated clock IC. We're talking low-tech here. Things produced in the millions where using a decades-old ASIC is cheaper than using a quartz + microcontroller.

Also, notice that with a minimal amount of additional complexity (an RC filter) it would be relatively easy to figure out whether you're running on nominally 50 Hz or 60 Hz, so not even your "needs to be different for different grids" argument works!