W. Jiekang, L. Jun, L. Ying, and H. Fen


Power system, digital protection relaying, nonsinusoidal signal, frequency estimation, low-pass filter, numerical differentiation Nomenclature v(t) voltage signal in time domain t time index i, j point index M derivative number ∆t difference of t p central point for derivative * Department of Electrical Engineering, Guangxi University, P.R.China, 530004; e-mail: wujiekang@163.com, lj161@gxu.edu.cn, hefen68@163.com ** Department of Electrical Engineering, Zhejiang University, P.R. Chin


The analysis and design of the low-pass filter system is presented for the fundamental frequency tracking system of the digital protection relaying. Three algorithms, sinusoidal signal frequency tracking (SSFT), nonsinusoidal signal frequency tracking (NSSFT), and filter-based frequency tracking (FBFT), are designed for the implementation of digital protection relaying based on DSP in hardware and on numerical differentiation and central Lagrange interpolation with seven points in software. FBFT-algorithm-based fundamental frequency tracking system employs a low-pass filter to filter high- order harmonics of the nonsinusoidal signals, and FBFT algorithm is realized in the same steps as that of SSFT algorithm. Using SSFT algorithm and FBFT algorithm, the frequency of power systems is estimated at an error of 0.001% over 1 Hz to 100 Hz, with the amplitude varying from 160 V to 280 V and the phase angle varying from 0 to 360 in at most a half cycle. Some examples are given to illustrate the proposed algorithms in Matlab.

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