Distance Relay Tripping Characteristic in Presence of UPFC

Distance relays are widely used as main and back-up protection for transmission lines. Their operation is based on impedance measurement at the relaying point, which can be affected by several factors, including pre-fault line loading and short circuit levels at the line ends. The problem of growing loads and limited paths for new transmission lines, has been somehow overcome by introduction of flexible AC transmission system (FACTS) devices into the network, which imposes further changes on the measured impedance at the relaying point. This paper presents the effects of unified power flow controller (UPFC), one of FACTS devices, on the measured impedance at the relaying point. In addition to UPFC controlling parameters, its installation point also affects the measured impedance. The variation of the tripping characteristic for different installation points and controlling parameters of UPFC will also be studied.