Voltage inversion due to UPFC presence on second circuit of double circuit transmission line causing distance relay mal-operation

This paper discusses distance relay maloperation in the presence of unified power flow controller (UPFC) on the second circuit of a double circuit transmission line. This is done by presenting the measured impedance at the relaying point in the case of UPFC presence at the near end of the second circuit. The measured impedance at the relaying point is greatly influenced in the presence of UPFC on the transmission line or even in the case of installing UPFC on the near end of the second circuit. The measured impedance at the relaying point depends on many factors including power system structural conditions, the pre-fault loading, the ground fault resistance, and UPFC structural and controlling parameters.