Diagnostics of Power Transformers Cores Using a Modified Vibroacoustic Method

Sebastian Borucki Institute of Electric Power Engineering

Opole University of Technology Opole, Poland

s.borucki@po.opole.pl

Paweł Frącz Institute of Electric Power Engineering

Opole University of Technology Opole, Poland

p.fracz@po.opole.pl

Tomasz Boczar Institute of Electric Power Engineering

Opole University of Technology Opole, Poland

t.boczar@po.opole.pl

Dariusz Zmarzły Institute of Electric Power Engineering

Opole University of Technology Opole, Poland

dzmarzly.@gmail.com

Abstract—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.

Keywords-vibration, power transformer, core, diagnostics, vibroacoustic method, acceleration, power spectrum density

I. INTRODUCTION The measurable effect of an innovative approach to the

management of the electricity network, and thereby the reduction of the costs of its operation, is an effective diagnostics of the devices of strategic importance. The group of such units include high power transformers which are statistically characterized by a high reliability, however their failures carry very serious technical and economic consequences [1-2]. Given the fact that a large groupe of transformers operating in the world has already exceeded the 30-year exploitation period [2-4], a slow but gradual increase in their failure frequency can be expected in the near future. Therefore a natural consequence of this situation may be a more frequent occurrence of the so-called catastrophic failures.

This situation may cause important economic losses which, in extreme conditions, will exceed several times the value of the purchase and installation of a new transformer [2, 5]. An emergency shutdown of a high power unit having strategic meaning for the power system may also cause a disturbance in the security of electricity supply in a given region and eventually the occurrence of a local blackout. Currently an effective way to prevent such situations is to develop, implement and then to use effectively modern diagnostic methods which will allow to switch off the transformer, then to upgrade or replace it at the early stage of the defect development.

The main aim of the research carried out, whose the results are presented in this paper, is an attempt to develop a modern and non-invasive method for evaluating the technical condition of mechanical structure of the transformers’ core.

II. IDEA OF THE DEVELOPPED METHOD Numerous research tests have been conducted for many

years concerning the development of effective methods for evaluating the technical condition of transformers [2, 6-8]. An element of their complex diagnostics is the evaluation of the degree of the core compression which is mainly performed on the basis of the chromatographic analysis results [9], measurements of magnetizing currents [2] and measurement results of the corrected sound pressure level [10]. Unfortunately, all these methods generally identify only the cases with extreme loosening of the magnetic circuit components and so far an effective detection of defects of the mechanical core structure has been performed during a direct inspection of the inner part of the unit, which is a complicated and time-consuming procedure.

Apart from the above methods, the vibroacoustic method (VM) is more and more commonly used to identify mechanic defects of the core. It consists of measuring mechanical vibrations of the transformer under test during its steady

The work financed with the means from the National Research and Development Center within LIDER program

179978-1-4673-0487-0/12/$31.00 ©2012 IEEE

operating conditions and then of analyzing the rms values of accelerations and amplitudes of frequency components of the spectrum [11-12]. Unfortunately, the uncritical use of diagnostic criteria adopted in the literature appears very often to be ineffective and difficult to apply. This problem is mainly due to a large variety of transformer constructions. Another problem of VM application in the evaluation of technical conditions of the cores is a variable load degree of specific units during measurements [13]. Next diagnostic limitation of the discussed method is the operation of the devices with forced circulation of insulation oil and air which generate vibrations overlapping on the useful measurement signal.

On the basis of the experiments conducted at Opole University of Technology, the so-called modified vibroacoustic method (MVW) proposed by the authors may be free from numerous disadvantages of the diagnostic methods discussed above. It is based on the measurements of transfomer vibrations under steady operating conditions – during switching on of the device. The main advantages of this methodology include: immunity to the current load degree of the unit; no significant correlation between the vibrations being measured and the type, power and construction technology of transformers; as well as the elimination of interference resulting i.a. from functioning of forced oil and air circulation devices. Additionally, the place where the measuring transducer is attached to the tank surface does not significantly affect the determined parameters of registred vibrations, and then the anlaysis of the results obtained.

III. CHARACTERISTICS OF THE TRANSFORMER UNDER TEST The research aimed at determining possible application of

the MVM to evaluate the technical condition of transformers’ cores was carried out in the Laboratory of Isolation System Diagnostics of Opole University of Technology. The diagnostic measurements were performed on a dry isolation transformer of medium voltage with 630 kVA of power. The monitored unit, whose general view is presented in fig. 1, was characterized by the following rated parameters: type – T3Gb 630, power – 630 kVA, primary voltage – 6300 V, secondary voltage – 400 V, vector group – Dyn 5, cooling – AN, gross weight – 2800 kg, year of production – 1973.

Figure 1. General view of the tested transformer of 630 kVA of power

The scope of experiment performed included measurements and time-frequency analysis of the tranformer vibrations for three cases of its functioning. The first case was related to measurements of vibrations while starting the unit with the properly twisted core. The second case was connected with the

vibration registration while switching on the unit with loose screws of the upper yoke (Ist degree of defect) whereas the third stage of measurements was carried out during a total loosening of the upper yoke and a simultaneous mechanical pressing of the magnetic circuit sheet pack (IInd degree of damage). In the course of the conducted tests, the simulation of the core defect was limited only to loosening its upper part. That methodology was adopted to allow the analysis and evaluation of the impact of the modelled defects on vibroacoustic signals’ parameters registered with use of an accelerometer placed on the upper as well as on the lower yoke beam, while the screws of the lower beam were not loosened.

The mechanical vibrations were measured with use of two accelerometers of 4514-B-002 type from Brüel & Kjær which were attached to the yoke beam with magnetic holders (fig. 2). The transducer Acc1 was placed on the upper part of the core and the Acc2 on the lower one. a) b)

Figure 2. Location of the transducers on the core of the transformer under

test: a) pictorial drawing, b) real points

The signal received by the accelerometers was transmitted on the inputs of the measuring and registering apparatus which consisted of the system Pulse Dyn XI from Brüel & Kjær. The total registration time of vibroacoustic signals from switching on the transformer to the steady operating conditions with no load was determined as t = 10 s. The analized frequency band of the measured vibrations was adopted in the range between 5 – 8192 Hz. The analysis of the vibrations was carried out in time-frequency domain in the Matlab environment with use of power density spectrum of vibration acceleration which was determined using the Short Fourier Transform.

IV. ANALYSIS OF THE RESULTS OBTAINED The analysis of the mechanical vibrations registered during the start-up of the 630 kVA transformer was performed on the basis of three-dimensional time-frequency images which present changes of power density spectrum of the measured vibroacoustic signals in time. The spectrograms presented in time-frequency-power density spectrum domain were described with use of a colour scale where each colour corresponded with a value of power expressed in decibels (dB). In order to accentuate the structures obtained a treshold function was applied due to which the low-value components were cut off. In each of the analyzed cases a treshold determined at the level of PD = -25 dB was adopted.

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Fig. 3 presents three-dimensional images of power density spectrum which were determined on the basis of the analysis of vibroacoustic signals registered while switching in no-load operation state of the transformer with a properly functioning core. Figure 3a shows a spectrogram of the unit vibrations which was determined on the basis of the measurements performed with use of the accelerometer Acc1 whereas fig. 3b illustrates the analysis results of measurement data which were measured with the transducer Acc2. a)

b)

Figure 3. Exemplary spectrograms of power density spectrum of vibration of

a transformer without a core defect: a) sensor Acc1, b) sensor Acc2

From the results of the three-dimensional STFT analysis presented in fig. 3 it results that during switching on the transformer with an efficient magnetic circuit, the frequency band of the measured vibrations is characterized by a wide range, practically from 5 to 7000 Hz. This situation occurs for the registration performed with the sensor Acc1 as well as the one perfomed with Acc2 whereas a slight increase in the amplitude of vibroacoustic signals in the range between 1800 – 3400 Hz (fig. 3b) results in all probability from the vibrations of the supporting construction of the tested unit which was placed on metal bases. The spectrograms of power density spectrum determined for the device with properly twisted core are also characterized by a periodic and constant in time frequency band of registred vibrations. At the same time in the range between 5 – 1200 Hz even components of the basic 50 Hz harmonic, which are characterized by the highest amplitude, are subject to a significant accentuation. The three-dimensional time-frequency images shown in fig. 3

should be considered in this case as a “finger print” of power unit with no defect of the mechanical construction of its magnetic circuit. The results shown in the figure above provide then a reference point for further comparative analyses associated with evaluating the impact of the modelled core defect complexity on the modifications of the proposed vibroacoustic descriptor.

Fig. 4 shows spectrograms of power density spectrum of mechanical vibrations which were obtained for the case of switching the diagnosed transformer with loose screws pressing the upper yoke (defect of the Ist degree). Figure 4a shows the results of STF transforms of the signals registered on the upper pressing beam (Acc1), whereas fig. 4b – vibrations measured on the lower beam which was not modified (Acc2). a)

b)

Figure 4. Exemplary spectrograms of power density specrum of vibrations of

a transformer with a core defect of the Ist degree: a) sensor Acc1, b) sensor Acc2

The comparative analysis of the indicated images of power density spectrum of vibrations of the device without a defect (fig. 3) and with spectrograms determined for the measurements in the unit with a defect of the Ist degree (fig. 4) has shown distinct differences in the bands of dominant frequencies. These differences are mainly visible in fig. 3a and 4a, hence for the registrations performed on the upper yoke beam. The detail indicating the existence of the damage to the transformer core, in which a defect was modelled, is an increase in the vibration amplitude in the range between 5 – 1200 Hz. Such a high power share did not characterize the

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vibrations measured while switching a unit with a properly twisted magnetic circuit. In the case of a device with a defect of the Ist degree a completely different value characterizes also the amplitudes of harmonic and interharmonic vibrations in the band between 3000 – 7000 Hz. For the operation of a transformer with a loose upper yoke, occurs a visible limitiation of the component value of the vibrations from this band – their amplitudes decrease by about 5 dB. A separate comparative analysis of vibroacoustic signals measured on the lower beam has shown a relatively similar share of the modifications of frequency components in time (fig. 3b and 4b). It means that the measurments performed with use of the accelerometer Acc2 do not directly evidence damage to the upper part of the core of the unit under test. The existence of a defect may be evidenced in this case by a slighlty increased value of power density spectrum in the range between 5 – 1200 Hz and the decreased power share of components from the band between 1800 – 3600 Hz.

In the last stage of the tests, the signals which were registered during the start-up of the transformer with loose screws of the upper yoke beams and mechanical pressing of the sheets of its core were subject to the analysis. Fig. 5 illustrates three-dimensional spaces describing modifications in time and frequency of spectrum power density of the measured mechanical vibrations for such a modelled defect (IInd degree of damage). a)

b)

Figure 5. Exemplary spectrograms of power density spectrum of vibrations

of a transformer with a core defect of the IInd degree: a) sensor Acc1, b) sensor Acc2

On the basis of the results shown in fig. 5a and their comparison with images indicated in fig. 3a and 4a it should be ascertained modelling of the IInd degree of damage to the core caused further decrease in power share of frequency components from the range between 3000 – 7000 Hz. In comparison with the results obtained for the case of operation of a transformer with a defect of the Ist degree, the amplitude of power density spectrum from the band over 3000 Hz decreased by about further 5 dB. Correlating this result to the measurements obtained for the unit without defects, this value is lower by about 10 dB. In addition, in the spectrograms obtained during the tests on the device with a completely loose and mechanically pressed upper part of the magnetic circuit, the shares from the band between 1200 – 3000 Hz (over 5 dB) also decrease. The comparative analysis of three-dimensional images of spectrum power density determined on the basis of the measurements of vibroacoustic signals on the lower yoke beams (fig. 3b, 4b, 5b) has shown therefore significant similarities in the range and values of the amplitude of individual frequency components of vibrations. The results obtained in this manner confirm then no damage to the core at its base. The possible identification of the defect of the magnetic circuit in its upper part may be evidenced in this case by a slightely reduced value of spectrum power density in the band between 1800 – 3600 Hz.

V. SUMMARY On the basis of the research carried out at Opole University of Technology and related to the analysis of the vibrations registered under the steady operating conditions of transformer distinct differences have been demonstrated in three-dimensional spectrograms of power density spectrum (fig. 3 – 5). The dimensional time-frequency images determined for three cases of the magnetic circuit operation differ from each other as to the dominant frequencies’ amplitude and range. These differences are visible mainly for the measurements performed with use of the accelerometer Acc1 which was placed within the modelled core defect area (fig. 3a – 5a). The STFT images determined in the band range between 5 – 8192 Hz, for the normal operation (without defects) and for two modelled degree of mechanical damage to the core of the transformer under test differ from each other as to the amplitude as well as to the dominant shares of harmonic components of the registered vibrations. In relation with the above, the analysis results in tendency and dynamics of changes in time and frequency of particular frequency components of mechanical vibrations being measured in the unit under test can be used as a diagnostic indicator of the method being developed of the evaluation of the technical condition of the energy power transformer cores. Regarding the results shown in this paper, its authors have presented a potential possibility of using the proposed method (modified vibroacoustic method) to evaluate technical condition of power transformer cores. The results of the research carried out obtained within the experiments conducted show then a possibility of developing a non-invasive and effective method of evaluating the technical

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condition of the magnetic circuit mechanical structures of the transformers used, especially the ones of strategic meaning.

ACKNOWLEDGMENT The work financed with the means from the National Research

and Development Center within LIDER program.

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