Impulse testing of power transformers using the transfer function method
TL;DR: In this article, the transfer function of a transformer winding is deconvoluted in the frequency domain from the digitally recorded neutral current and high voltage applied during impulse tests, and the integrity of the winding insulation is determined by comparing the transferred function obtained at full and reduced test voltage.
Abstract: The transfer function of a transformer winding is deconvoluted in the frequency domain from the digitally recorded neutral current and high voltage applied during impulse tests. The integrity of the winding insulation is determined by comparing the transfer function obtained at full and reduced test voltage. Differences between the transfer function plots reveal local breakdowns in the winding that can be dissociated from partial discharges. Thus the method permits unambiguous acceptance or rejection if the transformer and, since the transfer function is theoretically immune to changes in the applied impulse, also allows evaluation of the chopped-impulse test. Some 100 windings of large HV power transformers have been tested using the transfer function method, which on several occasions has revealed transformer faults as well a test setup problems that would have been missed or misinterpreted by conventional techniques. >
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TL;DR: In this paper, a model to simulate the high-frequency behavior of a power transformer is presented based on the frequency characteristics of the transformer admittance matrix between its terminals over a given range of frequencies.
Abstract: A model to simulate the high-frequency behavior of a power transformer is presented This model is based on the frequency characteristics of the transformer admittance matrix between its terminals over a given range of frequencies The transformer admittance characteristics can be obtained from measurements or from detailed internal models based on the physical layout of the transformer The elements of the nodal admittance matrix are approximated with rational functions consisting of real as well as complex conjugate poles and zeroes These approximations are realized in the form of an RLC network in a format suitable for direct use with the ElectroMagnetics Transient Program (EMTP) The high-frequency transformer model can be used as a stand-alone linear model or as an add-on module of a more comprehensive model where iron core nonlinearities are represented in detail >
244 citations
TL;DR: The transfer function concept is well known as an additional method of evaluating the impulse test of power transformers in the test laboratory and another application for this method is monitoring of power transformer in service as discussed by the authors.
Abstract: Summary form only given as follows. The transfer function concept is well known as an additional method of evaluating the impulse test of power transformers in the test laboratory. Another application for this method is monitoring of power transformers in service. According to the method of how to measure transient signals for the calculation of transfer functions, two kinds of monitoring can be distinguished: off-line and on-line monitoring. Both kinds of monitoring as well as their influencing factors are discussed with on-site measurements on power transformers in service.
175 citations
TL;DR: In this paper, a synthetic spectral analysis is proposed to deal with wideband frequency responses of each phase, which augments low and medium-frequency components, and equalizes the frequency intervals of a resulting combined curve by a log-frequency interpolation.
Abstract: A transformer is one of the most important units in power networks; thus, fault diagnosis of transformers is quite significant. In this paper, the frequency-response analysis, deemed as a suitable diagnostic method for electrical and/or mechanical faults of a transformer, is employed to make a decision over a defective phase. To deal with wideband frequency responses of each phase, a synthetic spectral analysis is proposed, which augments low- and medium-frequency components, and equalizes the frequency intervals of a resulting combined curve by a log-frequency interpolation. Furthermore, for discriminating a defective phase through computing overall amounts of deviation with other phases, the two well-known criteria and three proposed criteria are examined with experiment data. The overall diagnosis results show that the proposed criterion discriminates a defective phase with the highest average hit ratio among all of the provided criteria for selected faults.
150 citations
Cites methods from "Impulse testing of power transforme..."
...corresponding to mechanical deformations are notably observed in those frequency responses, much research has been carried out, during the last decade, on FRA [1]–[6] or the transfer function method (TFM) [7]–[11]....
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TL;DR: In this article, the authors present the current status and future trends in the application of the frequency response analysis (FRA) technique with the transformer in service (online) through bibliographic review and analysis.
Abstract: This paper presents the current status and future trends in the application of the frequency-response analysis (FRA) technique with the transformer in service (online) through bibliographic review and analysis. As a result, three basic stages of the online FRA test have been identified and defined: injection and excitation signal measurement; recording, filtering and processing of measured signals; and curve analysis and interpretation. This work presents an overview of the online FRA technique, useful for subsequent research in this area.
114 citations
Additional excerpts
...During the last two decades, measurement of uncontrolled transients was a procedure proposed to determine possible deformations of the windings, as presented in [16] and [17]....
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TL;DR: In this article, a novel frequency response analysis (FRA) method is described, which is being used on power transformers at the KEMA laboratories, and is applied to digitally recorded time signals, is easy to use, and provides high-resolution frequency spectra for both magnitude and phase.
Abstract: A novel frequency response analysis (FRA) method is described which is being used on power transformers at the KEMA laboratories. This frequency domain method is applied to digitally recorded time signals, is easy to use, and provides high-resolution frequency spectra for both magnitude and phase. This tailor-made FRA method uses a low voltage special impulse source to avoid aliasing. Results from several measured transformers are presented. >
112 citations
References
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TL;DR: In this article, a frequency response analysis (FRA) method was used to measure wind deformation in power transformers, and the results indicated that benchmark reference data was not always necessary to identify certain kinds of mechanical damage.
Abstract: Winding deformation in power transformers can be measured externally using a new frequency response analysis (FRA) method Field experience since 1975, on five separate transformers up to 550 MVA rating, 230kVclassindicatesthatthismethod hasadvantagesover the low voltage impulse (LVI) method as a practical maintenance tool. Results on suspect transformers indicate that benchmark reference data is not always necessary to identify certain kinds of mechanical damage.
314 citations
TL;DR: In this article, the normal functions of a coil or winding are obtained from the solution of the Fredholm integral equation (FIE) in the case of a single coil or a winding.
Abstract: THE NORMAL FUNCTIONS of a coil or winding are obtained from the solution of the Fredholm integral equation
26 citations
TL;DR: In this article, the importance of failure detection and means available at present are discussed at length and a new method is proposed which is more sensitive than the older methods and appears to be less subject to individual interpretation.
Abstract: Impulse testing of transformers has resulted in improvement of design and manufacturing methods Not only should the test methods now given in American Standard C572 be simplified to effect a reduction in cost of tests, but the test code should be modified to give an adequate assurance of failure detection The importance of failure detection and means available at present are discussed at length A new method is proposed which is more sensitive than the older methods and appears to be less subject to individual interpretation
23 citations
TL;DR: In this paper, the authors analyze the "equivalent bits" characteristic which is frequently employed as a criterion of the digitizer dynamic performance, and an additional test of the digitization error distribution is proposed.
Abstract: Requirements on measuring accuracy of high-voltage (HV) impulses employed for testing of self-restoring and non-self-restoring insulation are reviewed in order to select a suitable digitizer for the impulse recording. Certain models of fast digitizers are characterized by an apparently high time resolution, but their actual dynamic performance in case of measuring nonrepetitive, irregularly shaped impluses cannot be deduced from the manufacturer specification. The paper analyzes the "equivalent bits" characteristic which is frequently employed as a criterion of the digitizer dynamic performance, and an additional test of the digitization error distribution is proposed. Such a test results in another characteristic of the "equivalent bits," which is based on the maximum (rather than rms) error and which reflects the actual deformation of the recorded impulse form.
23 citations
01 Apr 1958-Transactions of The American Institute of Electrical Engineers. Part Iii: Power Apparatus and Systems
TL;DR: Many theoretical and experimental studies have been made since the beginning of this century on the problem of the surge performance of transformers, rotating machines, air coils, current-limiting reactors, etc as discussed by the authors.
Abstract: Many theoretical and experimental studies have been made since the beginning of this century on the problem of the surge performance of transformers, rotating machines, air coils, current-limiting reactors, etc. This work has led to a clear understanding of the effect of lightning, switching surges, and impulse voltages on stations and substations and on the apparatus included in them. It has also led to the introduction in all countries of insulation co-ordination, standard impulse tests, and rational protective practices. This, in turn, has achieved the result that properly designed and properly protected apparatus are now practically immune to the abnormal voltages arising during service conditions. Despite these achievements, certain aspects of the problem, both theoretical and experimental, yet remain to be solved, and work is actively progressing in this field in many countries. This paper presents a historical survey of the problem, the solutions which have been devised, and the present status of the art. It analyzes the work done in various areas through a comprehensive review of significant contributions to literature. It is hoped that this review will enable interested engineers to gain a full understanding of the problem, to be used as a basis for future original contributions in this important and fascinating field of electrical engineering. (All superscripts refer to the references listed in reference 1. The first two digits refer to the year of publication, with 19 omitted.)
23 citations