A New DFT-Based Phasor Estimation Algorithm Using High-Frequency Modulation

This paper proposes a new fast algorithm to estimate the fundamental phasor of fault current signals, based on discrete Fourier transform (DFT). Since fault currents include decaying dc component (ddc), DFT-based algorithms have inaccuracy in phasor estimation. The proposed algorithm consists of four steps. First, a new auxiliary signal is introduced based on a simple high-frequency modulation of the fault current. Then, DFT of the fault current and sample-summation of both fault current and auxiliary signal are calculated, for each one-cycle-length data stream. Next, DFT estimation error due to the ddc is calculated. Finally, by removing this error, accurate fundamental phasor is obtained. This paper uses an innovative and accurate formulation based on notations of signal processing literature. To validate the proposed algorithm, several computer-simulated signals and an electromagnetic transient program-generated signal are employed. The proposed algorithm is compared with recent phasor estimation methods, using wide variety of test signals. Standard indices of rise time, settling time, and percentage overshoot are used for comparison. These comparisons show that the proposed algorithm has robust performance in off-nominal frequency condition, and presence of harmonics, noise and multiple ddc components. Moreover, it provides faster convergence speed and lower computation burden.

I. Introduction

Computer based protection has attracted many researches because of its applications in wide area monitoring, protection and control (WAMPAC). In this type of protection, accurate estimation of voltage and current phasors plays an important role. Therefore, several researches investigate various algorithms for phasor estimation. The presented algorithms for this purpose are: discrete Fourier transform (DFT), least square error (LSE), cosine filters, discrete wavelet transform (DWT), Kalman filters and intelligent techniques such as artificial neural networks (ANNs) [1]–[8]. The DFT is widely used for phasor estimation because of its simplicity, high accuracy and speed [9].