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.