D. Zmarzly

Application of wavelet analysis to acoustic emission pulses generated by partial discharges

The subject matter of this paper refers to the improvement of the acoustic emission (AE) method when used for detection, measurement and location of partial discharges (PDs) in oil insulation systems of power appliances. The detailed subject matter refers to the issues connected with the application of modern methods of digital processing of signals obtained during technical high-power measurements. The paper presents the results of measurements and analyses of the AE pulses generated in setups making the modeling of basic PD forms that can occur in oil insulations possible. The research concentrated mainly on the following types of PDs: point-plane, multipoint-plane, multipoint-plane with a layer of pressboard, surface, generated in gas bubbles and on the indeterminate-potential particles moving in oil. In each case, the acoustic pulses generated by PDs in both positive and negative half times of the voltage supplying the spark gaps under study were analyzed. The AE pulses measured were subject to the wavelet analysis. For each of the PD forms the continuous wavelet transform (CWT) was determined and the corresponding time-frequency distributions were drawn. Moreover, for the AE pulses generated by PDs of the surface and multipoint-plane types, the discrete wavelet transform (DWT) was calculated, the runs of the particular details for six frequency levels and the approximation run were presented. In reference to the DWT analysis for the particular details, the probability density functions (PDF) and the autocorrelation functions (ACF) were determined at the particular levels of decomposition. Also columnar diagrams showing the energy value that is transferred at the particular levels of decomposition were presented. Moreover, the energy density spectrum runs determined through a FFT were presented for comparison. The Morlet wavelet was used to determine the CWT, and the symlet wavelet was used to calculate the DWT. Next, the distributions obtained for the particular PD forms underwent a comparative analysis taking into account the voltage polarization. It was observed that different frequency structures occur for the particular PD forms. Also, a different time of coherent structure duration for the particular PD forms was observed.

Application possibilities of artificial neural networks for recognizing partial discharges measured by the acoustic emission method

The genesis of the research work presented in this paper constitutes the issue of the effective and efficient recognition of single-source one-time partial discharge forms that can occur in insulation systems of power transformers. The paper presents research results referring to the use of single-direction artificial neural networks for recognizing basic partial discharge forms that can occur in paper-oil insulation impaired by aging processes. The research work results presented show the recognition effectiveness of basic partial discharge forms depending on the descriptor of the analysis of the acoustic emission signal analysis. The detailed cognitive aim was selection of input parameters and an artificial neural network which would be the best, considering recognition effectiveness and processing time, and which could be used as a classifier in an expert diagnostic system making identification of partial discharges measured by using the acoustic method possible.

The analysis and selection of the spinning disk system parameters for the measurement of static electrification of insulation oils

The paper presents results the aim of which was determining the most favorable conditions of ECT (electrostatic charge tendency) measurement taking of insulation oils in the spinning disk system. The work proved that a proper selection of the size and roughness of the disk surface makes the measured volume density of charge qw independent of the hydrodynamic conditions in the measuring system on the solid and liquid phase border. Charge qw density meets then the criterion of a material coefficient and can be used for comparing the results obtained from the systems varying in dimensions and hydrodynamic conditions. The changes suggested also ensure a high repeatability and reproducibility of the results obtained

Streaming electrification current density distribution inside pipes assuming overcharged boundary layer

In this paper the distribution of electrification current flowing across a pipe, for alternating charge near a surface, is presented. A distribution of charge density was arbitrarily assumed compatible to the Stern theory. Assuming that the velocity distribution is parabolic (in laminar regime) or logarithmic (in turbulent regime), the convective current density can be obtained. The distribution of current density shows changes of sign at different distances from the boundary. This may be the reason for the change of total electrification current sign that is often observed during experimental measurements.

Electrostatic properties of aged fullerene-doped mineral oil

In this paper the analysis of the influence of C60 fullerenes on flow electrification of mineral insulating oil during aging is presented. Three sets of samples, one fresh and two aged for 24 h and 96 h were tested for their electrification properties using an improved version of a spinning disk system. The relations between C60 concentration and electrification current were determined for different rotational velocities. It is shown, that C60 may significantly reduce or increase the electrification current in mineral oil, depending on fullerenes concentration. The possible causes of observed behavior are analyzed.

Streaming electrification measurements in swinging cylinder system

The paper presents the results of streaming electrification measurements realized in a swinging cylinder system. Classical systems for the electrostatic charging tendency evaluation are presented along with their advantages and disadvantages. The paper presents briefly the construction of the proposed system. It allows for continuous measurement of electrification phenomena. Some examples are also presented that show the current responses for inner cylinder swinging. As a result, the analysis found that the electrification process is highly nonlinear. The spectrum response in the frequency domain contains a significant amount of harmonics.

Detection of high-energy ionizing radiation generated by electrical discharges in oil

The article presents the results of multi-variant studies of high-energy ionizing radiation emitted by electrical discharges occurring in oil insulating systems. The studies were conducted under laboratory conditions using a specially designed and built testing station. The ionizing radiation was registered using a measuring device of original construction. Electrical discharges were simulated in a point-point modeling system, which was immersed in insulating oil. Example results of the analysis and their interpretation are presented in the paper. Finally, a possible explanation of the considered phenomenon is given.

Acoustic emission measurements of acoustically induced cavitation bubbles in insulating oils

Acoustic emission measurement results of acoustically induced cavitation bubbles in insulating oil are presented in this paper. Spectral analysis of acquired by a broadband transducer acoustic emission signals is proposed as a diagnostic tool for estimation of aging properties of mineral insulating oils. The article describes the main parts of the designed and built measurement apparatus used for acoustic cavitation investigation in insulating oils. In this note representative results of the experimental data taken from the apparatus are shown.

Streaming electrification current model in a round pipe in turbulent regime

The paper presents an analytically derived model of electrification phenomena in turbulent flow. Mechanisms of diffusion, convection, and conduction were taken into account. Distributions of current density where determined for the whole range of liquid conductivity and viscosity. The equations are derived assuming an isothermal incompressible Newtonian flow in a pipe in the steady state in time and space.

Diagnostics of power transformers cores using a modified vibroacoustic method

The paper presents research results referring to the development of a non-invasive method of assessment of the power transformer core technical condition based on the analysis of the registered mechanical vibrations. It characterizes the power unit under study, the measuring system used and the developed methodology of assessment of the core pressing degree using the vibroacoustic method. The original results of the time-frequency analysis of the vibroacoustic signals presented in the paper were obtained during switching on a real 630 kVA dry type power transformer under laboratory conditions. The analysis of the signals registered was carried out for three states of its operation: the core pressed by the manufacturer, the core with loose screws fixing the upper yoke and the core with separated upper yoke beams. The method proposed by authors, the so-called modified vibroacoustic method, is based on the mechanical vibrations measurements of an appliance under study in its transient state - while switching on the unit. The research work aims at developing a modern and, what is most important, non-invasive assessment method of the core technical conditions of power transformers operating in the national electric power system.