07_wavelet_denoisin_pd - Wavelet-based partial discharge...

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Wavelet-based partial discharge image denoising M. Florkowski and B. Florkowska Abstract: An application of wavelet-based denoising to phase-resolved partial discharge images is presented. The basic principles of wavelet denoising analysis, with a special focus on image decomposition, as well as examples of hard- and soft denoising thresholding are reported. For the purposes of decomposition, the Deabuchies wavelet and wavelet packets at different levels were applied. Simulations are discussed and the results obtained during online measurements on a6kV / 200 kW motor are presented. The method described is especially suited to cases in which an external additive noise uncorrelated with a partial discharge (PD) signal is present during acquisition, for example, in cables, transformers, rotating machines and gas-insulated switchgears. The fundamental issue in image recovery using wavelet denoising seems to be the choice of the threshold value and the type of the wavelet. Proper preprocessing is crucial prior to pattern recognition on the basis of a correlation with predefined PD forms. In addition, wavelet decomposition could be treated as lossy image compression in applications such as image internet transfer to / from external databases, in which only wavelet coefficients could be sent discarding the ones below a certain threshold level. The method presented can be applied during PD acquisition, for example, in high voltage cables, transformers, rotating machines and gas-insulated switchgears. The wavelet denoising processing will definitely find future applications in PD analysers, besides the boxcar accumulation method and spatial or FFT-based digital filtering. 1 Introduction Modern constructions of high voltage (HV) electric- insulating systems must meet high requirements of electrical, thermal, mechanical and chemical endurance. This is made possible by using new electro-insulating materials, especially synthetic dielectrics such as thermoset- ting resins, thermoplastics, high quality ceramic materials and liquid dielectrics. The condition of an insulating system depends on its manufacturing process and oper- ational hazards influences, which then determine the dielec- tric structure homogeneity in the system. This determination means the possibility of the occurrence of various defects in the dielectric structure and deterioration in their volume. In practice, they are mostly voids, pores, gaps and inclusions of other materials, even metallic inclusions. These various defects, causing electric field distortion, become centres of partial discharges (PD), which sooner or later result in the insulation system deterioration or even breakdown. Therefore techniques based on signal and image processing are intensively applied to this research field to improve the diagnostic process.
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07_wavelet_denoisin_pd - Wavelet-based partial discharge...

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