A. Cichoń

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.

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 conductive layers short circuit in HV condenser bushings using Frequency Domain Spectroscopy

Frequency Domain Spectroscopy (FDS) is mainly used to determine the moisture in the paper-oil insulation systems in power transformers and Oil-Impregnated-Paper (OIP) bushings. Another application of this method is detection of puncture developing in the paper layer between the adjacent conductive foils in the condenser core. The paper presents theoretical calculations of tanδ frequency response in the case of short circuit of conductive layers in bushings condenser core and their experimental verification, with the help of paper-oil insulation capacitor. Furthermore, examples of measurements of real bushings in operation that agree with the proposed idea are described.

Diagnostics of technical condition on load tap changers by acoustic emission method using different types of measuring transducers

Results of scientific research works considering selection of measuring transducer applied for registration of acoustic emission (AE) signals generated by on load tap changers (OLTC) are presented in the paper. During the coupling process AE signals are generated by: the mechanical setup of tap changer, contacts operation and partial discharges. For most OLTC types, the working medium is insulation oil in which the acoustic wave being formed propagates to the steal tank. There exists a possibility to register AE signals with a piezoelectric transducer attached to the outer tank surface or with a hydrophone immersed in the oil. AE signal obtained in such manner contains information which describes operation of the power tap changer and the selector. A comparative analysis of AE signals generated by OLTC working under laboratory conditions in three setups, each applying measuring transducers with different transmittance, is presented in the paper. In measurements three types of transducers were applied: wide band transducer type WD AH-17 from Physical Acoustics Corporation, hydrophone type TC4038 from Reson and accelerometer type 4514 - B - 001 from Brüel & Kjær. The comparative analysis was performed in order to determine their suitability for OLTC technical condition diagnosis. Based on the results achieved one transducer which allows the registration of acoustic events generated by the mechanical setup as well as by electric phenomena occurring during OLTC operation was selected.

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.

Possibility of using acoustic emission method for testing load tap changers during normal operation of the transformer

High voltage power transformers are one of the most important devices in modern electrical grids. Power transformers are directly responsible for continuous power supply as they are located in crucial points of electrical energy transmission and distribution system. This fact makes maintenance of such a transformer especially important. In order to properly manage reliability of those significant devices proper routines are required but constant increase in power demand makes turning-off and inspection increasingly difficult. This causes that in many situations diagnostic procedures must be performed during operation of transformer.

Application of multi-comparational algorithms to classification of acoustic emission signals generated by partial discharges

This article concerned on the mechanism of functioning of two multi-comparational algorithms, that is: CMCA (Classic Multi-Comparational Algorithm) and OMCA (Optimised Multi-Comparational Algorithm). The range and possibility of application of frequency descriptors that explicitly characterise the acoustic emission (AE) signals from partial discharges (PD) were described. The method for selection of quantity of the standard waveforms for the purpose of the classification process, carried out using the tested algorithms was presented. In addition, there were showed results of the simulation, performed using the proposed algorithms and two databases containing the AE signal waveforms generated by basic forms of PD, which can occur in insulation systems of transformers. The efficiency of CMCA and OMCA algorithms in application for AE signals generated by PD, as well as the reason to complete the developed database by another class of waveforms due to the defect of insulation system that not being catalogued previously was demonstrated in the study.

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.

Characteristic of acoustic emission signals generated by electric arc in on load tap changer

Results of scientific research works considering technical condition diagnosis of on load tap changers (OLTC) by use of acoustic emission (AE) method are presented in the paper. OLTC are crucial part of electric power transformers. They enable for on load voltage regulation in the network. Reliable operation of these devices is important especially for distributive transformers but also for units working in mills and industrial plants vulnerable on occurrence of voltage value changes in the network. Operation of OLTC is connected with formation of electric arc which occurs on main contacts and also on resistor contacts. Intensity of this phenomenon depends among others on: scale of current flowing during switch over, wear and tear level of contacts, character of receptions etc. The aim of research works, results of which are presented in the paper, was to determine characteristic parameters, so called “finger prints”, for AE signals generated by ignition of electric arc and in particular determination of the corresponding time-frequency structures. The measurements were performed in a modeling assemble equipped with tap changer including separated selector and power changer that were supplied with single phase voltage, 50 V, forcing current flow, nominal value equal to 150 A. The measurements were performed for each phase separately each time forcing current flow through the contacts accruing to a single phase. Electric arc created during coupling and uncoupling of contacts generated AE signals which were registered with a measuring apparatus equipped with wide band transducer type WD AH-17. Analysis of achieved results was performed in the time-frequency domain by use of short time Fourier transform (STFT), continuous (CWT) and discrete (DWT) wavelet transform.

Parameters of acoustic emission signals constituting the knowledge base of a computer diagnostic system of power transformer insulation systems

The paper presents the range and possibilities of using expert systems at diagnostic measurements of insulation systems of power appliances taken by the acoustic emission (AE) method. Multi-parametric descriptors characterizing the AE signals, which can be used in the knowledge base as criteria making the identification of basic partial discharge forms (PDs) possible, have been characterized in detail. The paper shows descriptors describing the AE signals registered in time, frequency, time-frequency domains and statistical data.