Assessment of line stability indices in detection of voltage stability status

During the last decade, several line indices have been proposed to assess voltage stability status of power systems based on transmission lines capability. In these methods, it is assumed that both the active and the reactive powers flow in the same direction from the sending bus to the receiving one. Then, the distance from existing operating point to the operating point in which maximum power transfers is detected by the line index. Since these indices can be implemented easily and have low computational burden, they are extensively used in the literature to detect stability status of power systems in various applications. In this paper, four line stability indices (i.e. Lmn, FVSI, LQP, and NVSI) which are extensively used in the literature have been comprehensively analyzed and their drawbacks and advantages are shown. It will be shown that the performance of these indices is not always good and highly depends on the power system structure and operating point. Therefore more attention should be paid to select a proper index to assess voltage stability status. Moreover, a suitable criterion is proposed to determine sending and receiving buses when the directions of the active and reactive powers are not the same.