A New Algorithm For Distance Relaying On A Transmission Line

The impact of transition resistance on the performance of distance Protection has always been a major problem in the practical application. This paper proposes a new algorithm to calculate the distance between the relay point and the fault point on a transmission line. Based on the phase angle relationship of the fault point voltage and its variation before and after the fault moment, the algorithm can accurately complete the measured impedance calculations with reasonable compensation angle. The algorithm is completely independent from of transition resistance in principle and is not affected by the load current, featuring robustness to system impedance change, with only single-ended fundamental frequency electrical quantity is used, the algorithm is simple and easy to implement. This paper gives the theoretical derivation of the algorithm. And the simulation results show that the principle of the algorithm is correct and has a high accuracy.The measured impedance of distance relay may enter the protected zone of third-zone distance relaying during power swing or extreme over-loading, causing unnecessary trip and even severe cascading blackout of power system. To solve this problem, This paper proposes a new principle of third-zone distance relaying independent of the load current based on the different characteristics of sequent currents and the product of power factor and voltage at relay point during unbalanced faults, three-phase faults, power swings and extreme loading. The principle of zone-three distance relaying proposed in this paper can correctly identify the every kind of unbalanced faults and balanced three-phase faults from power swings and extreme over-loading. The new third-zone distance relaying don’t required to be blacked during power swings and wont malfunction during extreme over-loading. The validity of proposed principle has been verified by PSCAD simulations.