Effects of instrument transformers location on measured impedance by distance relay in presence of TCSC

Distance relays are widely used as the main and back-up protection for transmission lines. They operate based on measured impedance at the relaying point, which is affected by several factors including pre-fault line loading conditions and short circuit levels at the line ends. Introduction of flexible AC transmission system (FACTS) devices imposes further changes on the apparent impedance measured at the relaying point. This paper studies the effect of one of the FACTS devices, i.e. thyristor controlled series capacitor (TCSC), on the measured impedance at the relaying point. In the presence of TCSC on a transmission line, the measured impedance at the relaying point is affected. In addition to TCSC compensation degree, the installation point of the instrument transformers is also effective on the measured impedance. The variation of the tripping characteristic is shown for the different compensation degrees and the various instrument transformer connection locations.