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

Abstract

Power Flow (PF) and Continuation Power Flow (CPF) are key tools for power system static calculation and analysis. This paper shows that the widely used logical PV/PQ switching method can lead to calculating non-physical power flow solutions and incorrect load margin calculations. From a computational viewpoint, several small and large numerical examples are presented to show the non-physical power flow solution and inaccurate load margin calculation made by the outer loop PF method. The existence of a non-physical power flow solution caused by logical PV/PQ switching and its impact on incorrect load margin calculation will be demonstrated. To address these issues, two smooth functions to approximate the generator Q-V curve and an active-set scheme will be presented. The Type-I smooth model is suitable for calculating physically feasible power flow solution, in addition it will be shown to contribute to faster convergence than the logical PV/PQ switching methods. The type-II smooth model is suitable for continuation power flow calculation for accurate load margin and speed improvement. A smooth-model-based active-set-assisted power flow method is then developed and its effectiveness in converging to the feasible (physical) power flow solution and the correct calculation of load margins will be shown. Numerical studies on several test systems were conducted to demonstrate the effectiveness of the proposed method in computing physical power flow solutions and achieving faster speed. For example, the proposed method for 70,000-bus PF computation is 1.7 times faster than that of the inner loop-based method and for the 25,000-bus CPF computation is about 3 times faster than that of the outer loop-based method.