Distance relay over-reaching due to UPFC presence on second circuit of a double circuit line

This paper discusses the distance relay over-reaching in the presence of unified power flow controller (UPFC), one of flexible alternating current transmission system (FACTS) devices, on the far end of the second circuit of a double circuit transmission line. This is done by presenting the measured impedance at the relaying point and the ideal tripping characteristic. The measured impedance at relaying point is greatly influenced in the presence of UPFC on the protected line or even in the case of UPFC presence on the far end of the second circuit. The measured impedance depends on many factors including power system structural conditions, the pre-fault loading, the ground fault resistance, and UPFC structural and controlling parameters.