  This letter analyses the sources of asymmetry of frequency probability
distributions (PDs) and their impact on the dynamic behaviour of power systems.
The letter also discusses on how secondary control can reduce this asymmetry.
We also propose an asymmetry index based on the difference between the left and
right-hand side standard deviations of the frequency PDs. The IEEE 9-bus system
and real-world data obtained from the Irish transmission system serve to show
that losses, saturation's and wind generation lead to asymmetric PDs. A
relevant result is that the droop-based frequency support provided by wind
generation using a tight deadband of 15 mHz leads to significantly increase the
asymmetry of the frequency PDs.
