New algorithm for distance protection of high voltage transmission lines

Calculated reactance between relay point and fault represents an accurate estimation of fault distance when the fault resistance or the R/L ratios of the equivalent impedance at line ends are very small. When these conditions are not satisfied, a large error results in locating fault distance due to back-feed effect from the remote end. A new recursive algorithm for distance protection is developed, which accurately determines the fault location by predicting the back-feed current. The prediction needs estimated values of the remote end source impedance to be passed to the relay via a communication link. Trip decision is based on checking the evaluated fault location and its variation with time. The algorithm is validated by digitally simulating transmission lines with different fault conditions taking into consideration the reduction in the generator reactance after fault inception. Simulation results show a fault detection time of about one half-cycle