Ideal source with constant power and voltage when 'pv' unconnected,
with variable power and voltage when 'pv' connected to a signal-input.
Optional input:
pv {p_active, norm(v)} pv[1], pv[2] in SI or pu, depending on choice of 'units'
Frequency: the source has always system-frequency.
Use only if prescibed values for p and v are compatible with the properties of the connected system.