Apparent impedance calculations for distance-protected transmission lines employing series-connected FACTS devices

The presence of flexible AC transmission systems (FACTS) devices significantly influences the trip boundaries of a distance relay in a two-terminal system connected by a double-circuit transmission line. This paper provides the apparent impedance calculations for line-to-ground fault at different locations of the FACTS elements on one of the circuits, taking are resistance in the fault path and infeed into consideration. Three important FACTS devices, namely the unified power flow controller (UPFC), the thyristor-controlled phase shifting transformer (TCPST), and the thyristor-controlled series capacitor (TCSC), whose presence in the line modulates the trip characteristics, are considered in this paper. Although the case studies for a single line-to-ground fault are included in this paper, extensions to includes other types of faults, such as line-to-line and double line-to-ground, are straightforward and involve simple manipulations.