Adaptive load shedding scheme to preserve the power system stability following large disturbances

Following large disturbances, the conventional under frequency load shedding (UFLS) relays may not operate properly, as they are designed to merely preserve the frequency stability of the system, independent of its voltage stability. In this study, an adaptive load shedding scheme is proposed to maintain the power system stability when the conventional scheme fails. In doing so, the frequency and its falling rate along with a properly defined voltage falling rate are exploited to determine closeness to the disturbance area and its severity. Accordingly, a proper amount of load is shed from the locations with the highest influence on the system stability. The proposed scheme exclusively utilises locally measured quantities and does not require a communication link. The associated logic is practical and can be easily appended to the existing UFLS relays to enhance their performance. To demonstrate the efficiency of the proposed UFLS scheme, it is applied to two large-scale power systems where the exact load model is taken into account. The obtained results verify that a large number of blackouts because of inappropriate operation of conventional UFLS relays can be prevented using the proposed scheme.