Returns the generator active power set-point variable.

def _get_pgen_var(self, generators, base_mva):
        """ Returns the generator active power set-point variable.
        """
        Pg = array([g.p / base_mva for g in generators])

        Pmin = array([g.p_min / base_mva for g in generators])
        Pmax = array([g.p_max / base_mva for g in generators])

        return Variable("Pg", len(generators), Pg, Pmin, Pmax)