The Effect of the Harmonics, the Fault Location and the Fault Resistance on the Performance of the Impedance-Type Distance Relay

One of today's trends is continuously increasing the number and the power ratings of the non-linear loads connected to the electric power systems. These non-linear loads, such as power-electronic converters, are the main source of the harmonics that affect the performance of the impedance-type distance relays. It was found that, in the presence of harmonics, undesired operation of such type of distance protection will occur, as the distance relay can react improperly and sense faults at zones other than the desired ones and the selectivity of the distance relays will be violated as improper relays will react while those concerned will not. It has been also shown that the fault location and the fault resistance were affecting the impedance seen by the distance relay. The data obtained from the analysis of a three-phase-to-ground fault, in a sample four-bus practical system, was used to study the behavior of the distance relay and to estimate the impedance seen by it. An algorithm, that can be used to calculate the system quantities in the presence of a harmonic source, was suggested.