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Computation of the network harmonic impedance with Chirp-Z transform

Duda K., Borkowski A., Bień A.

Metrology and Measurement Systems

2009

Vol. 16, nr 2

299-311

An innovative technique for power system harmonic impedance computation is presented in the paper. We present a brief introduction to harmonic impedance estimation problems as well as a detailed description of the proposed method. Frequency analysis with DFT is strongly affected by frequency leakage and picket fence errors. A number of methods was developed to reduce the impact of these phenomena including hardware solutions like PLL-based sampling synchronization, or software solutions like time- or frequency interpolation. Our work presented in the paper concerns new software-based method for reducing frequency leakage and picket fence errors and an application of this method of power system harmonic impedance estimation. The proposed method of harmonic impedance computation is based on Chirp-Z transform (CZT) instead of discrete Fourier transform (DFT). CZT enables arbitrary sampling of the frequency axis when computing spectra of discrete signals. We apply CZT to fundamental frequency estimation and for evaluating voltage and current frequency bins for higher harmonics. The spectral leakage is reduced by a standard Hanning window. The accuracy of the proposed method was verified by laboratory experiments and compared with the results of DFT-based computation. The proposed method is robust against non-synchronous sampling, thus it is dedicated for widely used data acquisition systems with a fixed sampling frequency. The proposed method is more accurate then DFT-based computations in case of voltage and current signals sampled at a fixed frequency. The advantages of the proposed method are evident for signals with off-nominal frequencies especially for higher harmonics.