A new algorithm to improve the operation of distance relays zone 3 by using Synchronized Phasor Measurements

Cascaded tripping of power lines due to mal-operation of zone 3 distance relays was one of the main causes of many previous blackouts worldwide. Encroachment of load into zone 3 characteristics during stressed system operation conditions was a basic factor for such mal-operation of the relays. By improving the operation of zone 3 it is possible to prevent mal-operations so that cascaded line tripping can be avoided. This paper proposes a new algorithm that utilizes Synchronized Phasor Measurements (SPM) to enhance the operation of distance protection zone 3 in many aspects. For instance, the relay can differentiate between actual system faults and load encroachment and be blocked. In addition to this, the tripping decision is not affected by the value of fault resistances from zero to High Impedance Faults (HIF) values. The paper discusses the basic idea of the new algorithm and explains with the aid of a simple application how to apply it.