Pawel Fracz

Measurement of optical signals emitted by surface discharges on bushing and post insulator

The paper presents the measurement methodology and result analyses of optical signals emitted by surface discharges occurring on bushing and post insulators, recorded using camera during tests under laboratory conditions. The performer analyses were carried out to determine the impact of the supply voltage on the intensity of the optical radiation emitted by partial discharges according to red, green and blue colors.

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.

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.

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.

Detection of defects in on-load tap-changers using acoustic emission method

The on-load tap changer (OLTC) constitutes one of the main elements of an energy power transformer which ensures the possibility of voltage regulation in an energy power net. Its break-down is a common cause for the transformer cut-off. The paper presents results of scientifically investigative works which aim is the OLTC technical condition estimation by use of the acoustic emission (AE) method. The AE method allows for OLTC diagnosis without the necessity of exclusion the power transformer from operation. The measurements were performed under laboratory conditions. The influence of the degree wear and tear of the OLTC head on the AE signal was evaluated. Gathered signals were analyzed within the time and time-frequency domains. Results analysis in the time domain was performed by use of the Hilbert transform. Furthermore the characteristic times for the particular runs were calculated. Evaluation of the time-frequency domain results was performed by use of short-time Fourier transform.

Nonlinear modeling of streaming electrification measured in swinging cylinder system

This paper is related to the phenomenon of streaming electrification. The selected results of nonlinear identification of the electrification process are presented. We described the signals used for identification and validation and then two structures of the models used: the nonlinear autoregressive model and the Hammerstein-Wiener model. We then conducted parametric identification and compared the obtained results. We used the Gauss function as the input signal. In the case of the autoregressive model, we used the sigmoid network to estimate the nonlinearity. In the case of the Hammerstein-Wiener model, we tested various types of input nonlinearities. The piecewise linear nonlinearity estimators turned out to be the most effective. Results for the most effective models of both structures: the results of the nonlinear ARX model and the Hammerstein-Wiener model were similar and had varying fit values in the range of 1 to 3 percent. They also had similar generalisation features. In all cases, we also obtained similar shapes of the processes of residues and correlation functions. They indicate a relatively faultless representation of the responses of models as well as certain differences, especially in the end part of the processes representing the given process.

Analysis of streaming electrification nonlinearity in swinging cylinder system

In the paper some results of experimental works on streaming electrification in swinging cylinder system have been shown. Mineral transformer oil is used as dielectric liquid in which charge generation occurs. A shuttle system with swinging cylinder is proposed to study the flow electrification phenomena. Designed system allows for modulation of liquid flow and thus gives the possibility to determine both frequency and phase characteristics of the electrification process. The system is a modification of the Couette system. It uses the cylinder, which oscillates around the specified location. This system retains the advantage of Couette cylinders on the flow hydrodynamics. Construction of proposed system is described. Few examples are presented in different conditions. The results confirm the nonlinearity of streaming electrification.

High voltage power transformer diagnostics using vibroacoustic method

Processing and analysis results of vibroacoustic signals generated during the switching process of electric power transformer are presented in the paper. Three cases are considered: (1) the transformer has no defects, (2) the pressing screws of the upper yoke were slackened and (3) the pressing screws of both the upper and lower yoke beams were slackened. Further the measuring transducer was placed at four different locations on the transformer tank surface. The gathered signals have been analyzed in the frequency domain by use of the power spectral density function. The magnitude of the first eight harmonics, included in the measured signals, is subjected for comparative analysis.

Streaming electrification phenomena generated in the swinging system

The paper presents the results of experimental investigations on streaming electrification in a swinging cylinder system. Mineral transformer oil is used as a dielectric liquid in which the electrification process occurs. A shuttle system with a swinging cylinder is proposed to study the flow electrification phenomena. Some measurements and analyses are presented in the shuttle system. The analysis includes a determination of the influence of the frequency and amplitude of pendulum oscillation on the electrification signal. The power spectral densities, spectrograms and continuous wavelet transform were also used to assess the frequency and time-frequency characteristics of the measured signals.

The analysis of the impact point of the power transformer core of torsional load on the measured parameters of the vibroacoustics signals

The research, whose results are presented in this paper, was conducted in the Institute of Electric Power Engineering of Opole University of Technology and focused on the development of the vibroacoustic method used to evaluate the condition of the mechanical structure of the magnetic circuit units operating in the national transformer system. The paper presents results of research work related to the development of the vibroacoustic method to evaluate the technical condition of power transformer cores. The paper presents the results of laboratory measurements and analysis of mechanical vibrations in the transformer core according to the variable torque twisting the screw yoke of the monitored unit. The aim of this work was to evaluate the torque twisting the yoke of the diagnosed object on the registered parameters of vibroacoustic signals which are used in the diagnosis of power transformers, magnetic circuits.