Comparing TCSC Placements on Line Ends and Mid-Point from Measured Impedance Point of View

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 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 Thyristor Controlled Series Compensator (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 not only with its compensation degree, but also with its installation point. In the case of TCSC at the line end, the connection point of the instrument transformers also affects the measured impedance. Here, the measured impedance at relaying point is compared when TCSC is installed at the mid-point of the line and once it is at the line ends for various instrument transformer connection points.