The tap changer controls a monitored voltage to keep it within a voltage range defined by [UMin ; UMax]. When the voltage goes above UMax or below UMin, the tap-changer is ready to begin increasing its tap until the voltage value comes back to an acceptable value.
The time interval before the first time change is specified with a first timer and a second timer indicates the time interval between further changes. The automaton can be locked by an external controller: in this case, it stops acting.
The detailed tap-changer behavior is explained in the following state diagram:
.Dynawo.Electrical.Controls.Transformers.TapChanger