The class implements a model of a simple proportional governor with saturation.
If fixInitialControlledVariable = true, an initial equation is added to ensure that the mechanical power output PMechPu is equal to its start value PMechPuStart. In order to fulfill this condition, the initial value of the reference signal should be free, so that the Modelica tool can back-compute it automatically. For this purpose, the PowerGrids.Controls.FreeOffset block should be used to generate the reference signal to be connected to the VrefPu input.
In case the condition PMechPu = PMechPuStart cannot be fulfilled without violating the controller saturations, then an alternative initial condition is enforced, which prescribes the saturation block input so as to obtain a certain degree of oversaturation oversaturationPu. This allows to back-compute a unique initial value of the reference signal, which is of course dependent on oversaturationPu and hence somewhat arbitrary.