Three-terminal transmission line protection using synchronized voltage and current phasor measurements

This paper presents a new relaying algorithm for EHV three-terminal transmission line protection using synchronized voltage and current phasors. The development of the scheme is based on distributed transmission line model and the mutual coupling effect of lines is decoupled using Clarke transformation. Then, a robust fault detection index composed of voltages, currents, and modal line parameters is derived. The proposed detection index can quickly identify faults and correctly distinguish between internal faults and external faults with respect to the protected-zone. In particular, the index can easily detect a high impedance fault. A faulted-phase selector is also designed to provide single-pole tripping for single-phase to ground fault and to issue three-phase tripping for other fault types. The relaying algorithm offers complete protection function including fault detection, faulted-zone discrimination, and fault type classification for three-terminal lines. The simulation studies verify that the scheme provides excellent performance.