Filtering for protective relays

The authors identify filtering requirements or criteria for different relays. They limit the discussion to relays needing precise measurements of the system-frequency component of the signals, such as distance relays. Many different filtering methods which may meet the filtering requirements are conceived and reviewed. Filtering questions are answered through requirement-assessment, analysis, simulation and examples. CAL, cosine, Fourier, correlator, least-squares and Kalman filters are considered and compared. The differences between finite and infinite impulse response filters are examined.