T. Boczar

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.

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.

Indicator analysis of partial discharges measured using various methods in paper-oil insulation

Results of a comparative analysis of two non-dimensional coefficients, calculated based on signals emitted by partial discharges (PD) are presented in the paper. The PD were generated using setups for modeling of three basic types of PD sources, which can occur in paper-oil insulation in power transformers. The following measuring methods were considered: electrical, ultra-high frequency, acoustic emission, and optical spectrophotometry. All measurements, for each of the PD-source configuration using the various methods, were conducted simultaneously in a laboratory under same measurement conditions. Based on the gathered results it was stated that it is possible to recognize the partial discharges form based on the non-dimensional coefficients, which differ in value for all of the considered methods except for the acoustic emission. The main contribution of the research performed lies in the simplicity of the applied signal processing procedure, which can be adopted to a device or system for diagnosis of partial discharges occurring in paper-oil insulation.

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.

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.

Acoustic emission measurements of ultrasound induced cavitation bubbles in aged insulating oils

The paper presents the behavior of ultrasound induced cavitation voids and bubbles in mineral insulating oils with different aging properties. At suitable values of signal amplitude and frequency many of cavitation bubbles were observed. Collapsing bubbles were responsible for broadband noise included in acoustic emission signal spectra. To represent the cavitation intensity a cavitation indicator based on power spectral densities of acoustic emission signal was proposed.

Aging properties of fullerene doped transformer oils

In paper the influence of thermal oxidation on nano-modified transformer oil is analyzed. The probes of mineral transformer were doped with different content of C60 fullerenes from 0 mg/l to 512 mg/l. One set of probes were then aged for 64 hours in the temperature of 110°C with the presence of copper inhibitor. Oil parameters such us: resistivity, dielectric loss tangent and permittivity were measured for each probe set. The influence of C60 additive on individual characteristics of fresh and aged oil was analyzed.

Comparative studies of partial discharge using acoustic emission method and optical spectroscopy

This paper concerns improvement of evaluation methods of partial discharge (PD), which may occur in the power transformer insulation systems. This particular study regards the next stage of research works, which aim to determine the energy share of particular physical phenomena accompanying generation of PD. The performed works were connected to comparative analysis of PDs, which were measured with the acoustic emission (AE) method and with optical spectroscopy. The applied measurement setup was specially designed for generation of surface partial discharges (SPDs) in transformer oil as insulation medium. In the introduction section elements of the experimental setup, i.e. the spark gap for modeling of SPDs and the transformer tank model filled with insulation oil will be characterized. Further the measurement equipment and the metrological parameters for both applied PDs registration methods will be described in detail. In the following sections the achieved results will be presented and discussed. In particular spectra of optical signals registered in the range from 200 to 1100 nm and frequency spectra of AE signals registered in the range from 0 to 500 kHz will be presented. In order to determine the effect of voltage value, which was used for generation of SPD, on the registered optical and AE signals, the results will be presented for three relative values: 0.6, 0.7 and 0.8 times the value of breakdown voltage of the applied test system. In addition, results of time-frequency analysis of EA signals, in the form of two dimensional spectrograms calculated using the short time Fourier transform will be presented. In the following section the achieved results of performed comparative analysis will be presented and discussed.