Microprocessor-based distance relay

This paper presents the design and operation algorithm for the protection of short transmission line using microprocessor-based distance relay. The characteristic of a distance relay is realised by comparing voltage and current at the relay location. The ratio of voltage (V) to current (I) gives the impedance of the line section between the relay location and the fault point. The signal will be taken from the transmission line and it will be converted to digital signal then goes to the microprocessor which has a program written in assembly language to estimate the apparent impedance of the protected line, the calculated impedance is capered to the replica set impedance inside the relay to decide a trip command. The decision will be made according to this signal whether there is a fault or not.