Enhancing distance relay security during power system stresses: An apparent power approach

The economic and environmental factors have forced the modern day power system to operate very close to the security margins. The undesired operation of distance relays during these system stresses have contributed to the catastrophic failure of the power system in the past. An algorithm for the secured operation of distance relays during system stressed conditions is proposed in this paper. The per-phase instantaneous values of active power and reactive power are calculated from the local current and voltage measurements, and the apparent power trajectory is plotted in the P-Q plane. A restrain region is developed based on the loadability of the distance relay. During the system stressed conditions the apparent power trajectory stays within the restrain region; whereas after the three phase fault inception, it shifts into the operating region. The proposed scheme is tested on the IEEE 39-bus New England system and the results obtained demonstrate the reliability of the scheme.