Terminology (formal).
  - index 1 (term_p)     "primary"
  - index 2a (term_na)     "secondary a"
  - index 2b (term_nb)     "secondary b"
Transformer ratio.
The winding ratio is determined indirectly by the choice of nominal voltages.
It may be >  or <  1.
Tap changers.
For constant transformer ratio no tap changer input needed.
For variable transformer ratio tap changer input needed.
The sequence of the parameters
v_tc tc voltage levels v_tc[1], v_tc[2], v_tc[3], ...
must be defined in accordance with the input-signals of
tap index of tap voltage levels, v_tc[tap]
Set dynTC = true if tap-index changes during simulation.