Amplitude comparator based algorithm for directional comparison protection of transmission lines

When there is a source present at both terminals of the transmission line, the relays protecting the line are subjected to fault current flowing in both directions. Directional relays operate only when the fault current flows in the specified tripping direction. Using the quasi steady state components of the locally measured deviations of the voltage and phase shifted current from their prefault values, the direction to a fault is determined. This directional information is exchanged with the relay at the opposite end of the protected zone. If the combination of the locally detected direction and the information from the remote end indicates a fault inside the zone of protection, a trip signal is issued. The deviation signals of the voltage and phase shifted current are determined by subtracting the previous values from the corresponding values in the present cycle. The voltage and current deviation signals contain exponentially decaying dc and high frequency transient components in addition to the steady state component [1]. The exponentially decaying dc component in the current signal is eliminated by phase shifting the current signal by a replica impedance. The current signal is lowpass filtered in the analog part of the processor before phase shifting it. A 1st order digital lowpass filter is used to filter the voltage deviation signal. An amplitude comparator method is used to determine the direction to a fault. An amplitude directional discriminant value, ADIS, as given by the equation below is evaluated from the filtered deviation signals.