Removal of decaying DC in current and voltage signals using a modified Fourier filter algorithm

Protecting transmission lines frequently involves applying distance relays. Protective relays must filter their inputs to reject unwanted quantities a nd retain signal quantities of relevant interest. Accuracy and convergent s peed of filter algorithm are essential for protective relays. A widely appl ied filter algorithm, the Discrete Fourier Transform (DFT) can easily remov e integer harmonics using simple calculation. However, the voltage and curr ent signals contain serious harmonics and decaying de during the fault inte rval. In addition, the decaying de and higher order harmonics seriously dec rease the precision and convergence speed of fundamental frequency signal f rom DFT. In this investigation, we derive a novel algorithm which combines the appropriate analog low pass filter and modified Full Cycle DFT (FCDFT) or Half Cycle DFT (HCDFT) algorithm to remove the decaying de in a voltage or current signal, Using the Electromagnetic Transients Program (EMTP) simu lates the transient responses of transmission lines during the fault period . Applying the proposed algorithm in distance relays effectively suppresses the decaying de and quickly decomposes the accurate fundamental frequency components.