Digital Differential Relay for Transmission Line Protection Using a Correlation Method

Power system protective relays must be as reliable and fast as possible even though input singals contain fault induced exponentially decaying DC-offset and damped high frequency oscillations. A digital differential relaying system for primary protection of transmission line using travelling wave has been suggested.[1] It was shown that the fault current appears as a difference of two signals - the travelling wave at one terminal and the time delayed travelling wave at the other terminal. The amplitude of fault current was conventionaly computed from each interpolated signal. In this paper signals are transmitted to the other end and the phasor of these quantities are extracted by a correlation method and the fault current are obtained from the phasors. The phasors are computed by correlations of rectagular signals which have half-cycle window and input signals. The correlation integral is equal to the sum of sampled values which are obtained by averaging the input signal during each sampling interval. In the proposed method the phasor of fault current is insensitive to high frequecy transients because it is computed from an analog integral. And the effect of exponentially decaying DC-offset is reduced by using another two sample values. The proposed method has been tested by numerical simulations and the results found to be satisfactory.