Online vulnerability assessment of Zone-3 distance relays against static load Encroachment: A novel approach based on fault chain theory

Abstract

One of the main factors in triggering cascading outages is the undesired operation of zone-3 vulnerable distance relays under static load encroachment. In this paper, to avoid the contribution of vulnerable relays in cascading outages, a novel online vulnerability assessment method based on fault chain theory is proposed to identify the critical relays. The main idea is to use the fault chain theory to model the process of valid successive line outages that occur step by step and lead to load encroachment into zone-3 distance relay. A novel normalized prediction index based on relay margin change, margin sensitivity variation, load encroachment factor and overload tolerance of the line is proposed to predict the next possible outage. The distance relays involved in most fault chains are identified as vulnerable relays requiring to be controlled. A suitable index is presented to detect the critical relays. In this regard, an optimal control strategy is also proposed to restore the acceptable operating margins of the identified vulnerable zone-3 distance relays in the system’s emergency condition. The validity of the proposed approach is evaluated on New England 39 bus system to confirm the effectiveness and benefits of the proposed method. The obtained results are exciting.