On the accuracy of power flow and load margin calculation caused by incorrect logical PV/PQ switching: Analytics and improved methods

•It shows that the widely used logical PV/PQ switching method may converge to a non-physical power flow solution for both small and large size power flow models.•It shows that a non-physical power flow solution can lead to an incorrect load margin calculation, and the reason behind this incorrect load margin calculation is also explained.•It proposes two smooth piecewise functions, termed Type-I and Type-II, to compute a feasible/physical power flow solution. The Type-I function is suitable for power flow calculation with a faster and better convergence than the logical PV/PQ switching method. The proposed Type-II function is more suitable for the continuation power flow calculation.•A smooth-model-based and active-set-assisted power flow method is presented for quick computation of a feasible power flow solution. In addition, accurate power flow solution trajectories and accurate load margins are obtained.