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

The presence of flexible AC transmission systems (FACTS) devices significan tly influences the trip boundaries of a distance relay in a two-terminal sy stem connected by a double-circuit transmission line. This paper provides t he apparent impedance calculations for line-to-ground fault at different lo cations of the FACTS elements on one of the circuits, taking are resistance in the fault path and infeed into consideration. Three important FACTS dev ices, namely the unified power flow controller (UPFC), the thyristor-contro lled phase shifting transformer (TCPST), and the thyristor-controlled serie s capacitor (TCSC), whose presence in the line modulates the trip character istics, are considered in this paper. Although the case studies for a singl e line-to-ground fault are included in this paper, extensions to includes o ther types of faults, such as line-to-line and double line-to-ground, are s traightforward and involve simple manipulations.