Ideal voltage source with constant amplitude and phase when 'vPhasor_in' unconnected,
with variable amplitude and phase when 'vPhasor_in' connected to a signal-input.
Optional input:
  vPhasor_in         {norm(v), phase(v)}
   vPhasor_in[1]     in SI or pu, depending on choice of 'units'
   vPhasor_in[2]     in rad
Frequency: the source has always system-frequency.