Abstract
This work presents a scientific study on Short-Time Frequency Transforms (STFT) with different windows, also called Windowed Fourier Transforms, applied to power quality signals. Additionally, it deals with S transforms, with its frequency-dependent window. The disturbances related to energy quality have non-stationary nature, in which the spectral content varies over time. So, the Fourier Transform is not appropriate for such analysis, because it doesn’t show time locations, only information about existing frequencies in the signal. Therefore, the spectral analysis by windowed transforms helps to identify and detect a series of defects associated to these power signals. The motivation behind this document is to verify which window will provide a more precise identification of the characteristics of the disturbances in time-frequency domain. For this work, synthetic signals were generated for some of these disturbances, and their spectra were compared considering Gaussian, Hann and Blackman windows, as well as the S transform. Based on the obtained results, it was verified that each transform presents different behaviours acording to the input signal, except for the ones with Hann and Blackman windows, that showed similar spectra. For all of them, there is always a tradeoff between time and frequency resolutions. Therefore, the choice of the window must be done according to the desired outputs. The Dev-C ++ ® IDE was used for C ++ programming, and the Gnuplot ® program for graphics generation.
Highlights
The subject Electrical Power Quality (EPQ) includes several topics, such as: reliability, provided power quality and provision of information
A definition which relates the aplications of signal processing and the system ability to operate loads without damaging them and the capacity of the loads to operate on the system without disturbing or decreasing the eficiency of the electrical system is: “Electrical Power Quality is an arrangement between voltage quality and energy quality
The Discrete-Time Fourier Transform (DTFT) is a form of Fourier analysis that is applied to functions that operate on discrete data
Summary
The subject Electrical Power Quality (EPQ) includes several topics, such as: reliability, provided power quality and provision of information. The power quality is directly related to the defects contained in power signals, named disturbances. These disturbances may cause changes in the duration, frequency, and the interruption of such signals. There are several analytical methods developed in literature that use transforms to evaluate the signal in time-frequency domains. The disturbance signals, of non stationary-nature, can be analyzed through various time-frequency transforms, each one with a specific window, and a spectrum comprised of several parts is generated.
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