Phasor estimation with finite impulse response notch filtering and half-cycle discrete fourier transform

Discrete Fourier transform (DFT)-based phasor estimation is widely used in protective relays. During a transmission line fault, half-cycle DFT (HCDFT)-based phasor estimation can achieve faster protection of the power system than full-cycle DFT (FCDFT)-based phasor estimation. However, HCDFT-based phasor estimation is not immune to even harmonics, which gives difficulty to identification of a decaying dc component in a fault current of the power system. This paper proposes a phasor estimation method which uses HCDFT and a finite impulse response (FIR) notch filter for elimination of even harmonics. The FIR notch filter is designed on the basis of the waveform symmetry of even harmonics in the half cycle of the fundamental frequency. The decaying dc component is identified by using three consecutive HCDFT coefficients of the notch filtered output. The proposed method can use far less samples of a fault current in the phasor estimation than FCDFT-based methods. This method is simulated on time constants of the decaying dc component, harmonics, frequency deviation, noise, and a fault transmission line.