A Novel Algorithm to Extract Exact Fundamental Frequency Components during Faults for Digital Protection of Power System

Whenever fault occurs in power system, the operating quantities contain serious harmonics and decaying dc in addition to fundamental frequency component. In order to ensure better protection to the power system, protective relays must filter their inputs to reject unwanted quantities and to retain desired signal quantities. In addition, the decaying dc and higher order harmonics seriously decreases the precision and convergence speed of extraction of fundamental frequency signal. Accuracy and convergent speed of filter algorithms of protective relays had received great attention. A novel approach is proposed which combines the appropriate analog low pass filter and modified full cycle discrete fourier transform (FCDFT) algorithm to remove the decaying dc from input signals for protection. The proposed algorithm was tested for line to ground faults on 345KV, 200km overhead transmission lines. Electromagnetic transient program (EMTP) and power system computer aided design (PSCAD) were used to generate fault current signals under different fault locations and fault inception angles. The results shows that the proposed technique accurately measures the fundamental frequency component regardless of the characteristic frequency component as well as the decaying dc offset components.