The Research On Metering Technology Of Three-phase Harmonic Energy

The widely use of nonlinear and impact loads in electric power systems have led to severe distortion of current and voltage waveforms. The existence of these harmonics greatly affects the quality of electric power and deteriorates the safe operation of electric power systems. The accurate harmonic analysis and measurement can provide a scientific basis of achieving harmonic governance. This paper takes the three-phase harmonic energy as main study object and starts a series of studies about its metering technology:Firstly, the basic concepts and the danger of harmonics in the power system are introduced and the harmonic sources and existing detection technology of harmonic are pointed out. Then the advantages and disadvantages of the existing testing methods are analyzed and the basic principles of harmonic measurement are discussed. Also the influences of harmonics to the energy measurement are discussed and the significances of different harmonic power algorithm are analyzed.The fast Fourier transform (FFT) is the most commonly used method for power systems harmonic analysis for its simplicity and easily implementation in embedded system. However, because of the spectral leakage and picket fence effect produced by using FFT under noncoherent sampling conditions, the signal parameters, including frequencies, amplitudes, and especially the phases, cannot be obtained accurately. In the literature a windowed interpolation FFT (WIFFT) algorithm was proposed, and the precision of harmonic analysis can be improved to some extent. However, the calculation of the rectification formulae were complex and the calculating burden was great, especially when dealing with complex harmonic signals. In this paper, the spectral characteristics of Rife-Vincent window are analyzed, and an approach for harmonic analysis based on the five term Rife-Vincent (I) window double-spectrum -line interpolation FFT (DSLIFFT) is proposed. Then the applicable rectification formulas of the interpolation are obtained by using polynomial curve fit functions, and subsequently calculating burden was dramatically reduced. The simulation results indicate that the approach presented in this paper is adapted for the extraction of weak signals components and the accurate analysis of complex harmonics. The five term Rife-Vincent (I) window with a better curb ability to the spectral leakage, and the picket fence effect can be modified by the DSLIFFT algorithm. Compared with the WIFFT algorithm, when using the approach presented in this paper under the noncoherent sampling conditions, the parameters of 21 orders harmonics can be estimated accurately, even the fundamental frequency varies and the signal with white noise.Simulations results show that, by using the proposed harmonic analysis method based on the five term Rife-Vincent (I) window DSLIFFT, the precision of harmonic power measurement, especially reactive power of the complex harmonic signal, can be estimated accurately.The tranditional induction and electronic ammeter can not measure the harmonic energy accurately, which can affect the fairness of charge. In this paper, a three-phase harmonic ammeter (TPHA) based on ADS8364+ADSP-BF533+M30624FGPFP is proposed. The working principle of the ammeter is introduced firstly, then the data acquisition circuit and the harmonic analysis and harmonic energy metering circuit are illustrated in detail. Then the software flow of harmonic energy measurement and the realization of the interpolation algorithm based on the five term Rife-Vincent (I) window are presented. At last the solution of the power-down of the system is proposed.With the design of the TPHA, the interference sources and the printed circuit board (PCB) transmission line effect of the instrument are analyzed, and the electromagnetic compatibility (EMC) designed approach of the instrument are illustrated in detail. The EMC test results of the instrument are accord with the national standard.In allusion to the error sources of harmonic energy metering, the system error models are established and the correction methods of the static error, ratio error and phase error are proposed. According to the test results by the electric power measurement center of Hunan province, the proposed TPHA can measure both fundamental energy and 2nd～21st harmonic energy respectively, errors of either the current and voltage measurement or the active power measurement are less than 0.1%, the accuracy of the fundamental energy measurement reaches 0.2s class, and the measurements of the harmonic energy accord with the Class A standard requirement of GB/T-14549-1993.