Design and evaluation of a directional algorithm for transmission-line protection based on positivesequence fault components

A directional relay algorithm for EHV transmission lines using positive-sequence fault components is presented. By comparing the phase relationship between the voltage and current measured at the relay point, the algorithm can determine correctly whether a fault is in the forward or backward direction. Specially designed techniques and logic are adopted to solve the difficult problems that exist in a real system. The signal-processing procedure for extracting the required fault components is provided in detail. Extensive simulation studies were conducted on a 500 kV system model using EMTDC. Theoretical analysis and simulation results show that the proposed algorithm provides adequate sensitivity, reliability and a fast operating response under a variety of system and fault conditions. In addition, it provides significant advantages over conventional directional relays, and these are discussed in the paper