Impact of short-time fourier transform parameters on the accuracy of EMI spectra estimates in the 2-150 kHz supraharmonic interval

Methods based on time-domain sampling and successive tapering and Fourier analysis, used in the “harmonics” frequency interval up to 2.4kHz, cannot be automatically extended to the 2-150 kHz interval, in particular considering the emissions of Switched-Mode Power Supplies (SMPSs) in a modern smart grid or low-voltage distribution scenario. Emissions are substantially impulsive and oscillatory superposed to the mains signal, possibly featuring multiple switching fundamentals and their harmonics with unstable or purposely changing fundamental frequency. Fourier analysis (namely Discrete Fourier Transform) is commonly used to characterize the spectrum of emissions from time-domain data, but the influence of its settings should be verified for accuracy and systematic errors, so to establish the uncertainty of spectral estimates. After an initial theoretical and practical discussion of characteristics, objectives and weak points, a verification is carried out with the help of several measurements performed in different operating conditions on a set of six different SMPSs, evaluating systematic error and uncertainty, and defining a set of recommendations.