Fourier and Walsh digital filtering algorithms for distance protection

Filtering requirements for distance relays are very critical, because they must estimate precisely and quickly the electrical distance to the fault, even with highly distorted input signals. A number of digital filtering algorithms for distance relays have been proposed and some of them are in use in practical relays; however, power system evolution increases the corruption level of signals and imposes the necessity of continuing the research efforts in this area. In the present paper, a comparative evaluation of different digital filtering algorithms for distance protection is performed. An evaluation method is proposed, which gives a comprehensive information about filter transient behavior on a wide frequency range of noise. The discussion is focused in well-known algorithms based on Fourier and Walsh transforms, and includes a recently proposed combined sine-cosine filter.