Digital technique for impedance protection of transmission lines

A digital scheme for impedance protection of transmission lines is presented in this paper. The scheme is based on the vector representation of fundamental components of voltage and current waveforms. The 60 Hz impedance is calculated using 16 samples per cycle over a time window of one cycle. The one cycle window and high sampling rate have the advantage of less than 3/4 cycle fault detection as well as high dc and harmonic rejection. An algorithm, its implementation and real-time test-results on a laboratory model for various faults are included. Theoretical analysis is also provided to illustrate the behaviour of the digital impedance algorithm.