Adjusting third zone distance protection to avoid voltage collapse

Undesirable distance relay operations are possible to be initiated under heavy stressed system conditions leading to voltage instability. This can result in cascaded outages in the transmission system that may cause severe system deterioration. A number of recent blackouts have been reported to be initiated from undesired distance protection operations. This paper proposes an adaptive distance relaying scheme that avoids undesirable third zone tripping during the evolution of voltage instability phenomena. The relaying scheme is based on local measurements and provides the necessary time for other emergency controls like undervoltage load-shedding to be applied, if needed, in order to mitigate system collapse.