Fitting Combined Cosine Window And Its Applications In MMC-HVDC Harmonic Analysis

Discrete Fourier transform is a common method for harmonic detection in power systems.When discrete Fourier transform is used to analyze harmonics under asynchronous sampling,spectrum leakage and barrier effect will occur in the discrete spectrum,and the detection accuracy of harmonic parameters will be reduced.The window-difference analysis method based on Fourier transform can effectively suppress the problem of spectral leakage.Selecting the right window function for harmonic characteristics can greatly improve the accuracy of harmonic detection.Based on the windowed difference analysis method based on Fourier theory,this paper proposes a window function construction method that fits the non-cosine window to the combined cosine window by least squares method,which will be suitable for harmonic analysis of power system.The cosine window function is constructed in the form of a combined cosine window and the Kaiser window and the Parzen window are taken as examples.The Kaiser window with ?=11 is fitted to the five-element combined cosine window.The signal analysis is performed in combination with the combined cosine window bispectrum interpolation FFT algorithm.Compared with the existing Kaiser window-based two-spectral FFT interpolation algorithm,Kaiser The combined cosine window FFT bispectral interpolation algorithm of the window is one to two orders of magnitude more accurate than the Kaiser window bispectral interpolation algorithm.Then,the Parzen window is fit into six combined cosine windows.The frequency spectrum of the fitted window function is compared with that of the original window.The simulation results show that the Parzen window's combined cosine window FFT interpolation algorithm has high accuracy in extracting harmonic parameters.Suitable for harmonic analysis of power systems.The flexible DC transmission system based on modular multi-level converters is a research hotspot in recent years.This paper analyzes the harmonic generation principle of modular multi-level converters and summarizes the current modular level converters.The research status of the harmonics of the flexible system,the characteristics of the back-to-back soft system,the back-to-back softness system model,the application of the Kaiser window's combined cosine window FFT interpolation algorithm and the Parzen window's combined cosine window FFT interpolation algorithm were studied.When the two different frequency grids are interconnected in a back-to-back MMC-HVDC system,the two AC grids operate synchronously and asynchronously,and the changes in the typical electrical quantities of the system under the AC-side balance and the AC-side three-phase unbalanced conditions,respectively.Kaiser window's combined cosine window FFT interpolation algorithm and Parzen window's combined cosine window FFT difference algorithm's Harmonic amplitude and fundamental frequency energy can be extracted with high accuracy in the harmonic detection of a back-to-back soft system,suitable for flexible systems Harmonic detection.