Returns the branch flow limit constraint.  The real power flows
        at the from end the lines are related to the bus voltage angles by
        Pf = Bf * Va + Pfinj.

def _branch_flow_dc(self, branches, Bf, Pfinj, base_mva):
        """ Returns the branch flow limit constraint.  The real power flows
        at the from end the lines are related to the bus voltage angles by
        Pf = Bf * Va + Pfinj.
        """
        # Indexes of constrained lines.
        il = array([i for i,l in enumerate(branches) if 0.0 < l.rate_a < 1e10])
        lpf = -Inf * ones(len(il))
        rate_a = array([l.rate_a / base_mva for l in branches])
        upf = rate_a[il] - Pfinj[il]
        upt = rate_a[il] + Pfinj[il]

        Pf = LinearConstraint("Pf",  Bf[il, :], lpf, upf, ["Va"])
        Pt = LinearConstraint("Pt", -Bf[il, :], lpf, upt, ["Va"])

        return Pf, Pt