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