Improved FFT For High Dielectric Loss Tangent Measurement Based On Nine Points' Polynomial Transform

With the development of the power grid construction, the number of various kinds of high voltage facilities and huge capacitive-type Equipment in using is increasing daily. It makes great sense of ensuring the security and stabilization operation in the electrical power system. The construction of the smart grids is carring forward step by step, On-line monitoring and diagnosis for capacitive equipment can cover the shortage of the tradition method which is short in real time detection, what's more the new way can find out the problems in early stage. The dielectric loss tangent is an important parameter to reflect the aging of insulation, so that it is indispensible part in tradition. The detection of the dielectric loss tangent is in condition of high voltage and low current, the small-signal is hard to get with so many interfering noises. All this factors make it difficult to calculate accurate results. Detection of the dielectric loss tangent has some directive significance to maintain the electric device in service.Firstly, the factors which infect the insulation aging of the capacitive electric and the equivalent circuit of the dielectric loss were introduced in the paper. By means of contrasting of methods in related papers at home and abroad, principle of measurement of dielectric loss was seriously studied. Then, all the factors that could influence the measuring results were listed. Take advantage of the calculating machine, the Fast Fourier Transform was quickly computing method of the Discrete Fourier Transformation. Two operation modes of FFT were presented in the paper, and the flow path of programming was provided, so it's convenient to realize algorithm processor.Secondly, the difficult points in practical application of the DFT were carefully analyzed, and related solutions were proposed. In the condition of non-synchronous sampling, the paper presented the unavoidable signal spectral leakage and picket-fence effect of single signal. Aimed at the all the shortages, the paper proposed an improved FFT for dielectric loss tangent measurement based on Nine Points' Polynomial Transform. The signal spectral leakage among the signals in different frequencies was obviously cut down, and the amplitude of several useful frequencies points was strengthened. The phases of voltage channel and current channel could be calculated by interpolation, the dielectric loss angle. Many simulations experiments in different conditions illustrate the correctness and feasibility of our method. Furthermore, our method is superior in precision, contrast to the Hanning windowed and the Blackman-Harris windowed interpolation harmonic analysis algorithm.Then, the design thinking of a dielectric loss angle on line monitoring system based on the Nine Points' Polynomial Transform FFT arithmetic was proposed, which consisted of function analysis, chips selection and the hardware design diagram, and software flow chart were given as well. The data acquisition was carried out by AD7862, and digital signal processing was accomplished by ADSP-BF533 Algorithm processor.Lastly, the paper summarized the achievements and deficiencies, and pointed out the direction of further work.