A novel unblocking function for distance relay to detect symmetrical faults during power swing

Abstract

Distance relays face the difficulty of differentiating a power swing from the three phase faults leading to their unsecured tripping. This paper proposes a novel unblocking algorithm to detect the symmetrical faults during power swing using only the local current measurements. During power swing, the current signals resemble an amplitude modulated waveform; the characteristics of which are dependant on the power swing slip frequency. Conversely, the current signals during a three phase fault are composed of a steady state ac component and the decaying dc content. Thus, the rate of change of current magnitude phasor will be non zero during the power swing condition; whereas it will be zero during a three phase fault when the dc content in the current signal is eliminated. The scheme is tested on the IEEE-PSRC EMTP reference model. The proposed unblocking algorithm justifies its reliability for transposed/untransposed, compensated/uncompensated transmission lines. The rigorous simulations show that the proposed unblocking algorithm is immune to system impedance, power swing slip frequency, in-feed effect, mutual coupling, current inversion, voltage inversion effects; and is computationally efficient.