The harmonic research of power system in foreign countries started earlier, which can be traced back to the 1920s-1930s. In the era of automation, rectifiers are widely used. Since the 1950s, we have ushered in the electrical era. The harmonic pollution of power grid is more and more serious. More and more researchers begin to pay attention to the harmonic problem.<br>Harmonic analysis is a part of digital signal processing technology (DSP). According to the development of the times, the detection principle is mainly divided into the following: Fast Fourier transform, instantaneous reactive power theory, modern spectrum estimation, wavelet transform, support vector machine, neural network.<br>In 1965, J.W. Cooley and T.W. graph base proposed fast Fourier transform (FFT). This algorithm can greatly reduce the number of multiplication required when the computer performs the operation of discrete Fourier transform. Moreover, when the number of sampling points transformed is more N, the calculation amount of fast Fourier algorithm is significantly reduced.<br>In 1983, Chi Mu Tai Wen put forward the theory of instantaneous reactive power of three-phase circuit, breaking the traditional definition of power based on the average value, which has been used in many aspects. Based on this, active power filter (APF) was developed to detect harmonic and reactive current in real time, which has attracted great attention in engineering practice.<br>Modern spectral estimation can overcome the shortcomings of shorter data length and lower spectral resolution, which can be divided into parametric method and non parametric method. The model parameter method of modern spectral estimation is based on the maximum entropy spectral analysis method proposed by Berger in 1967 and the autoregressive (AR) spectral estimation method proposed by pazeng in 1968; the non parameter method of modern spectral estimation is based on the maximum likelihood spectral estimation method proposed by J. Capone in 1969 and the power spectral estimation method extended to time series by Lucas in 1971.<br>In 1974, J. Morlet, an engineer in oil signal processing in France, put forward the inversion formula of wavelet transform. Until 1986, mathematician y. Meyer constructed a real wavelet base, and cooperated with S. Mallat to establish a multi-scale analysis method that can unify the wavelet base, making wavelet analysis begin to flourish. Wavelet analysis inherits the development of short-time Fourier transform, and overcomes the disadvantage that the window size does not change with frequency. Through the local or analysis of time-domain and spatial frequency, it achieves adaptive time-domain signal analysis, which is a major breakthrough of fast Fourier transform.<br>In the early 1990s, Vapnik et al. Proposed the concept of support vector machine (SVM), and derived a series of algorithms, which are widely used in pattern recognition problems such as human image recognition and text classification, and later applied to harmonic analysis. SVM is a machine learning algorithm based on the principle of structural risk minimization, which has a lot of abnormal noise interference Because of its high accuracy, it is often used to analyze power system harmonics and interharmonics.<br>Neural network algorithm is a research hotspot in the field of artificial intelligence since 1980s. In 1943, psychologist w.s.mcculloch and mathematical logician w.pitts established neural network and mathematical model. Neural network algorithm is widely used and can also be applied to harmonic analysis. At present, the harmonic detection algorithms based on neural network mainly include the harmonic detection method based on adaptive linear neuron, the harmonic detection method based on multilayer BP network, the harmonic detection method based on multilayer feedforward neural network, and the harmonic detection method based on radial basis function network.<br>
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