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.

1. Introduction

Transmission lines with faults should be isolated promptly and accurately to prevent severe loss of facilities in the power system. Circuit breakers for such isolation are controlled by estimating the fundamental frequency phasors of power system currents in protective relays. Thus, promptness of the control is directly linked to the phasor estimation speed, which can be evaluated by the number of samples used for the phasor estimation. A decaying dc component as well as harmonics arises in the power system current during a transmission line fault [1]. Its time constant is determined by a fault resistance and parameters of a transmission line. The time constant is typically 0.5 to 5 cycles [1]. The accuracy of the phasor estimation is mainly dependent on reduction of the estimation errors due to the decaying dc component and harmonics. Thus, it is necessary to considered both the decaying dc component and harmonics in the phasor estimation.