Integrated Impedance-Based Pilot Protection Scheme for the TCSC-Compensated EHV/UHV Transmission Lines

This paper analyzes the characteristics of equivalent dynamic power frequency impedance of the thyristor-controlled series capacitor (TCSC) and the integrated impedance, which is defined as the ratio of the sum of the voltage phasors across the two ends of the transmission line to the sum of the current phasors through the two ends of the same transmission line. The integrated impedance of the transmission line is used to determine whether there is a fault inside the protected line section or not. When an external fault occurs, the integrated impedance reflects the capacitive impedance of the protected line, its imaginary part is negative, and the absolute value of its imaginary part is large. When an internal fault occurs, the imaginary part of the integrated impedance on the fault phase is either a positive value or a small negative value. According to such characteristics, internal faults can be distinguished from external faults. Based on integrated impedance, a novel pilot protection scheme for the transmission line with TCSC and controllable shunt reactor is presented. Digital simulations based on 750-kV transmission systems show that the presented scheme offers high sensitivity and reliability with many advantages over conventional current differential protection.