Secured Zone 3 Protection During Stressed Condition

Maloperation of the zone 3 element of distance relays during stressed system conditions, such as power swing, load encroachment, or voltage stressed condition is one of the main reasons for large disturbances in power systems. The improved protection technique for zone 3 can help to prevent such maloperation and, thus, more reliable power systems can be envisaged. This paper proposes an algorithm that utilizes two new criteria: (1) the maximum value of the transient monitoring function obtained from three-phase currents and (2) the phase angle of the positive-sequence impedance to support zone 3 of the distance relay. The technique is tested for various power system events, such as the three-phase fault during the power swing, load encroachment, voltage stressed conditions, and current-transformer saturation during the three-phase fault using data simulated through EMTDC/PSCAD in the IEEE 39-bus New England system. The simulation results show that the zone 3 protection scheme using the proposed technique can correctly discriminate the three-phase fault from stressed system conditions.