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This study presents phase-resolved observations of partial discharge (PD) formation in air in non-uniform electric field at various pressure conditions. The needle-plane equivalent electrode system refers both to external components of high-voltage insulating systems and to micro-sharpening in the internal structure of those systems. The ionisation zone, accumulation of space charge and formation of corresponding current pulses depend on electrode configuration, voltage level, pressure, temperature and humidity of air. The influence of pressure on the discharge mechanism in a non-homogenous electric field has been assessed on the basis of PD patterns at different voltage levels, electrode distance, curvature of high-voltage electrode and taking into account solid dielectric barrier in various configurations. The simultaneous occurrence of two discharge forms has been registered and separated on the phase-resolved patterns. Both corona forms, which are overlapped at normal pressure, are revealed at higher pressure and at double PD inception voltage. The obtained relationships could be used in diagnostic applications based on gaseous insulation at higher pressure. [ABSTRACT FROM AUTHOR]

The novel discrimination method of partial discharges (PDs) based on the analysis of PD pulse waveform with special focus on the rise time is presented. This parameter of PD current pulse has various values for different forms of discharges in insulation systems. The time waveforms and the rise time represent an important attribute of a PD pulse, which might be used as a distinctive factor between types of discharges. The composition of phase-resolved rise-time (PRRT) images has been described and for phase-resolved PD rise-time registration, the dedicated measuring systems have been developed. The PRRT registration of PD created a new approach to the description of PD mechanisms and their identification and discrimination for diagnostics of high-voltage insulation systems. [ABSTRACT FROM AUTHOR]

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 a 6 kV/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. [ABSTRACT FROM AUTHOR]

Nowadays, one of the main methods in diagnostics of high-voltage insulating systems is registration of partial-discharge (PD) pulses and their visualisation in the form of phase-resolved images. The paper describes the influence of high-voltage harmonics on partial-discharge phase-resolved images, which is crucial for proper identification and classification of PD patterns. The evolution of these images, depending on the total-harmonic-distortion (THD) parameter being an assessment factor, calculated by means of fast Fourier transform (FFT), is shown. Numerical simulation which illustrates the harmonic influence on PD is reported. Different harmonic-distortion instances of test voltage have been analysed along with the corresponding PD phase distributions. The measurement results were obtained on thermosetting insulation of a stator bar with the controlled harmonic content of the test voltage. The basic principles of harmonic analysis, along with the measurement set-up, are presented. It is shown that THD may only be a rough and global assessment factor. For the practical analysis of PD images, as a tool for insulation diagnostics and monitoring, the test-voltage frequency spectrum will be required. [ABSTRACT FROM AUTHOR]