Model that can be used to represent the electrical grid supply.
The model has an output connector named P[n] with
n = 3 that contains information about the power
supplied by the grid to the network. The convention is that
P[i].real is positive if real power is consumed from
the grid, and negative if it is fed into the grid. The connector
has size equal to three because each element of the vector refers
to a single phase.
The parameter V is the root means square of the
voltage. In US, a typical value 480 Volts.
P.