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.