This model implements a transformer with a tap changer which takes
a phase-to-phase voltage magnitude target and a dead band, and
tries to maintain the magnitude of phase-to-phase voltage at portB
in the acceptable dead-band next to the target value.
It extends the class
TransformerWithTapChangerInterval, which implements a
transformer with tap-changer and Interval logic, redefining said
interval in terms of target value and dead-band.