Improved FFT Algorithm For Harmonic Energy Measurement

Nonlinear loads,such as electric and electronic equipments, are widely applied in power system, power harmonic pollution is becoming increasingly serious. Serious deterioration in the quality of power is affecting the user's normal work. The precise analysis and accurate measurement of harmonic energy can provide a reliable scientific basis for the power management.The fast Fourier transform (FFT) has already become a main method of the electrical harmonic analysis since it's easily realized in embedded system, but there are difficulties in performing synchronized sampling and integral period truncation in the electric power harmonic analysis, and the accuracy of analysis result will be disturbed by the frequency leakage. The window functions and interpolation algorithms are utilized to correct the measured frequency, phase and amplitude by FFT.In order to overcome some shortcomings of existing methods in realizing the interpolation algorithm, in this paper, an approach for electrical harmonic analysis based on the Nuttall window double-spectrum-line interpolation FFT is presented. The effects of harmonic analysis precision brought by the fundamental frequency, sampling frequency and the the data length of truncation are dicussed. The applicable rectification formulas of the double-spectrum-line interpolation is obtained by using polynomial curve fit functions, and subsequently calculating burden is dramatically reduced. Further, the harmonic energy measurement method based on the Nuttall window double-spectrum-line interpolation FFT is discussed. The simulation results show that, under the non-synchronized sampling and non-integral period truncation conditions, the electricity harmonic analysis method proposed in this paper has very high accuracy.To meet the urgent demand of electric power industry on harmonic energy measurement, a kind of three-phase multi-functional harmonic energy meter structure composed by AD73360L+ADSP-BF533+M30624FGPFP is presented. The operating principle of electric energy meter is described. The data acquisition circuit of six-channel synchronous-sampling A/D converter and the data processing circuit with the core of ADSP-BF533 are introduced in detail. Software process of the harmonic energy measurement method based on the Nuttall window double-spectrum-line interpolation FFT is proposed. Correction method of ratio error and phase error of harmonic energy measurement is discussed. Test results show that, three-phase multi-functional harmonic energy meter based on the algorithm proposed in this paper has its error of fundamental active power measurement lower than 0.2%, error of fundamental reactive power measurement lower than 1%, amplitude error of harmonic electric voltage lower than 2%, phase error of harmonic electric current lower than 5%, and phase angle error between harmonic voltage and electric current lower than 5°. Above data results meet the accuracy requirement of the A class harmonic measurement instruments according to GB/T-14549-93.Simulation and engineering applications show that, the proposed approach for electrical harmonic analysis based on the Nuttall window double-spectrum-line interpolation FFT is feasible, which can inhibit errors caused by the spectral leakage and fence, and is able to achieve the precise harmonic analysis and harmonic energy accurate measurement.