Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> Knowledge of the health of power transformers is important to prevent high costs caused by failures and to maintain the quality of the service. On-line monitoring and diagnosis seem to be the right way to reach this objective. In this paper a set of experiences related to the installation of vibration and temperature sensors inside a test transformer are described. Different types of sensors have been tested, and some of them discarded because of induced electromagnetic noise. Some of the main results and conclusions are shown in this paper. These experiences are included in a more general project called TRAFES, whose main objective is the continuous monitoring and diagnosis of large power transformers. <s> BIB001 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> Electric utilities are concerned about the availability and reliability of power transformers. The ability to diagnose the integrity of the transformer windings and core through the nonintrusive vibration tests would be useful to accurately assess transformer condition. In this article, the principle of monitoring the power transformer core and winding' conditions via measuring the tank vibration is described. By reasonably choosing the vibration sensor and the charge-amplifier, a set of measuring system was set up, also some techniques of anti-interference were adopted. Using this measuring system, the tank vibration signals of transformer were picked up in the lab. From the frequency spectra of the vibration signals, it can indicate that it is feasible to monitor on-line the core and winding condition of the power transformer by measuring the tank vibration. <s> BIB002 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> This paper deals with new procedures based on vibro-acoustic techniques to diagnose power medium-voltage/low-voltage (MV/LV) transformers. The normal operating machines can be monitored and checked from a remote, dedicated diagnostic center where information is received directly from the field. The diagnostic method adopted and the experimental test results are reported. Tests have been performed either in the laboratory or directly in the field on transformers in normal operating conditions. The laboratory tests related the transformer vibrations to internal anomalies. The suggested monitoring and diagnostic system allows one to usefully schedule preventative maintenance, reduce costs, and improve the quality of power distribution. <s> BIB003 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> This paper presents a new technique based on vibration measurement to diagnose power transformers, which is called the onload current method (OLCM). It can acquire the fundamental frequency component of the core vibration signal without running the transformer at the open-circuit condition. The diagnostic method adopted and the experimental test results are reported. Tests have been performed at normal operating conditions in both the manufactory and the laboratory. With the tests performed in the manufactory, the vibration characteristics of transformer windings and core are described, and then the principle of OLCM is introduced. To verify the validity of OLCM, the laboratory tests are conducted which relates the transformer vibrations to the simulative fault. But the presented method was verified on a 5-kVA transformer which is a very far cry from an actual power transformer. It needs to be developed further before OLCM can be used effectively on the transformer in the field <s> BIB004 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> The vibration signal of transformer's core is correlative with the conditions of core's compression and insulation, therefore, vibration monitor of transformer's core is an effective method for on-line monitoring the power transformer delitescent faults. A new method of obtaining the energy distribution with time and frequency of transformer's core vibration based on Hilbert-Huang transformation (HHT) was developed. Firstly, empirical mode decomposition (EMD) separated the vibration series into a finite and small number of intrinsic mode functions (IMF). IMF was directly decomposed from vibration series and could reflect the intrinsic physical property of vibration series more clearly. Secondly, Hilbert transform was applied to every IMF. Then, three dimensional amplitude(energy)-frequency-time spectrum designated as Hilbert spectrum was got, where both amplitude and frequency were functions of time. The Hilbert spectrum eliminated the spurious harmonics without physical mean and provided high-resolution time-frequency characteristics of core's different work states, the test results show core vibration characteristics. It is demonstrated that the new detection method is one effective way for fault diagnosis of transformer's core <s> BIB005 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> Nowadays electrical utilities or industries are trying to monitor their electrical apparatus with online monitoring, without shutting the apparatus down. We are in PLN has taken this monitoring concept by applying vibration monitoring. This vibration monitoring has been applied to some of the transformers as a pilot project in PLN Region Jakarta and Banten. As voltage applied to the transformer, vibration is produced. This vibration level depends on the transformer construction and design. Vibration level shall increased by through fault current, phase to ground or phase to phase fault. This electrical fault will change the transformer core or winding construction by mechanical force produced. The effect of the fault can be found by measuring the vibration level before and after several faults on low voltage side. In this method, vibration level is measured by putting twelve magnetic vibration sensors on the transformerpsilas body in the same height, six sensors on each side. By using this configuration, the transformer vibration can be measured confidently from different sides. Further investigation is still needed to determine the transformer condition by measuring the vibration level after fault on low voltage side. <s> BIB006 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> DC biasing current through the neutral point of ac power transformer would lead the power transformer to generate more vibration and sound, this will do harm on power transformerspsila core pieces or windings and finally will threaten the safe operation of the transformers. This paper described the principle of DC biasing, designed a new DC biasing experiment to analyze how the experimental transformerpsilas vibration and sound change along with the value of DC biasing current or windings voltage and finally some curves were given in order to show the direct connection between vibration peak value, sound pressure and DC current. <s> BIB007 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> The success of health monitoring and condition assessment for power transformer with vibration analysis lies on proper extraction of vibration features. The extraction of features in turn depends on appropriate signal processing methods. In this paper, a WPT and HHT based time-scale-frequency analysis is developed to extract the vibration features from power transformer tank. In this method, the WPT is adopted firstly to decompose the signal into a set of narrow band signals. Prior to HHT, the decomposed components are screened with a CTS technique. Finally, HHT is applied to these components to obtain both frequency and amplitude characteristics. By comparing with the single WPT and HHT methods, the results show that the time-scale-frequency analysis can overcome the intrinsic shortcomings of these two methods, which will lead to unreliable vibration features for ignoring frequency information in wavelet-based method, mode mixing and meaningless IMFs generation resulting from EMD in HHT. Therefore, the developed method can provide a better alternative to implement vibration feature extraction from vibration signals of power transformer. <s> BIB008 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> The ability to diagnose the integrity of the transformer windings and core through online diagnostic method would be useful for electric utilities who are concerned about the availability and reliability of power transformer. This paper presents results of a study to identify the condition of power transformer windings and core by measuring the vibration signal of outdoor transformer in normal operating conditions. The obtained vibration signal which is recorded from various places on the transformer tank is analyzed by matching the signal frequency spectrum with transformer load. From this study, mechanical integrity of windings and core inside the transformer is determined by comparing the correlation between each main frequency of vibration signal and the load among similar transformer. <s> BIB009 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> This paper aims to study the sensitivity of 500kv power transformer tank to winding vibrations. Through full analyses of winding vibration characteristics, tank mechanical characteristics and vibration differences in different positions on tank surface, the authors propose some evaluation criteria and principles on vibration measuring positions on the tank. Based on the principles, the relevant experimental studies and analyses are conducted for 500kV power transformers with a single phase structure. Besides, the methods are also proposed for selecting sensitive measuring positions that can effectively reflect the winding vibrations of 500kV power transformers. The study results show that the principles and methods proposed will provide good foundations and support for on-line condition monitoring and fault diagnoses of power transformers. <s> BIB010 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> When the HVDC system works in monopole ground return mode, dc current flows through the neutral of ac power transformer with the neutral grounded, which will enhance its vibration and audible noise. In this paper, dc bias experiments on ac power transformers carried out on 500-kV power transformers both in the transformer factory and the substation are described; the experimental results show how the vibration and audible noise of the transformer change under the dc bias current or windings voltage. The frequency characteristics of the vibration and audible noise of the dc-biased transformer are discussed under the conditions with and without loads. The direct connections between the vibration peak value and the dc bias current, and between the sound pressure and the dc bias current are revealed. Long-term online monitoring shows that the vibration of the transformer changes seriously even if there is not a dc system operating in monopole earth return mode, so when we analyze the influence of dc bias on the vibration and audible noise of the transformer, other influential factors should be expelled. <s> BIB011 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> The issue of an effective assessment of the technical condition of the mechanical structure of the power transformer core and windings originated the research work discussed in this paper. This publication presents the results which indicate the possibility of developing a new and noninvasive diagnostic method of the transformer active part. The method developed is based on the time-frequency analysis results of the mechanical vibrations measured of the monitored appliance. The registration of vibrations is performed during switching the transformer monitored into idle operation. This paper characterizes the power object under study, the measuring system applied, and discusses a new methodology of assessment of the technical condition of the core and windings using a modified vibroacoustic method. The original measurement results presented in this paper were obtained during switching on a 200-kVA transformer in laboratory conditions. The analysis of the signals measured was carried out for four operating conditions of the transformer under study: 1) with the core pressed by the manufacturer; 2) with the core with loose screws fixing the upper yoke; 3) with the core with loose screws fixing the upper and lower yokes; and 4) with the core with loose screws fixing the upper and lower yokes with a simultaneous unpressing of the windings of the upper voltage side. <s> BIB012 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> This paper makes in-depth research on the vibration of distribution transformer in the operation. It points out the vibration of the distribution transformer mainly from the iron core and winding, and sets up the mechanical model of the iron core and winding. The ANSYS software is used to conduct the finite element simulation for the enclosure vibration affected by the vibration of internal transformer. The results show that the most serious vibration occurs in the central of the distribution transformer tank's top part, so this part should be considered as the most important for the vibration test of distribution transformer. This experimental study on the physical model of distribution transformer, and arranges several acceleration sensors in the distribution transformer tank according to the results of the simulation calculation is completed. Meanwhile, the acceleration sensors are arranged in the iron core after swing the core of distribution transformer, in order to analyse the spread characteristic of the distribution transformer's internal vibration. The real-time data from sens or are collected by NI PXIe - 4499 data acquisition card, which are effectively controlled by NI PXIe - 8133 controller, and NI Labview is used to develop background software for operation analysis and diagnosis. Through constructing test loop, it simulates the different operation conditions of the distribution transformer, such as no-loading, part-loading and full-loading. It analyses the influencing factors of distribution transformer operation vibration from the perspectives of core and winding, e specially the fastening degree of the core and winding and the dc bias in the operation process. The variation test of distribution transformer in operation brings about several conclusions. If the vibration amplitude of tank top center increases sharply, and the vibration frequency is mostly odd times to power frequency, then it may be the problem of loose core. If vibration amplitude of tank top center increases considerably, and most of the vibration frequency is even times to the power frequency, it may be the problem of loose winding. Moreover, in the fastening condition, the dc bias is easier to intensify the vibration of distribution transformers than the load current. <s> BIB013 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> This paper aims to separate the sources vibration from the transformer tank vibration, which is normally collected by sensors. Blind source separation consists in the recovering of different physical sources of a system given only a set of external measurements, and makes successes in the field of machine vibration analysis and health monitoring. However it has made few contributions to the vibration analysis for a power transformer, since vibration sources in a vibrating transformer are highly correlated with each other. As a result, the assumptions of statistical independence between sources normally held in conventional BSS (Blind Signal Separation) methods cannot be satisfied for transformer vibration separation. In this paper, a TIFROM (Time-Frequency Ratio of Mixtures) -BSS method is adopted to separate the sources vibration from the transformer tank vibration, on the basis of the diversity of STFT (Short Time Fourier Transform) ratios in time-frequency (TF) domain. The experimental results indicate the effectiveness of the application of TIFROM-BSS for transformer vibration separation. <s> BIB014 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> In this paper, a winding condition assessment model using vibration signals is presented, which can be used to diagnose power transformers online. The basic principle of this model is based on the correlation analysis of winding vibrations. In the model, the fundamental frequency vibration analysis is used to separate the winding vibration from the mixed signal. Then, a health parameter is proposed via principal component analysis. Another parameter is also proposed to detect the fault locations for suspected faulty transformers. In laboratory tests, the model is validated on a specifically designed 110-kV transformer. During the tests, man-made winding deformations are simulated to compare the vibrations under different conditions. The model has also been tested on several in-service power transformers. The preliminary study shows that the proposed model is feasible to assess the power transformer winding condition. <s> BIB015 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 2) CORE/WINDINGS <s> Abstract Health and usage monitoring (HUM) systems are critical for power transformers, and should provide accurate diagnostic information without delay. Diagnostic methodologies are the core of HUM systems. In this paper, we propose four complementary diagnostic techniques for power transformers based on vibrations that provide health metrics from different perspectives. During the field experiment, the vibrations of several in-service transformers under different conditions are collected, and the corresponding diagnostic results are obtained. Next, a health grade system is defined, and the suggested thresholds for each technique are also presented. The effectiveness of the health grade system is illustrated experimentally on three representative transformers. The consistency between the results of the proposed techniques and the actual condition of the test transformers show that the proposed techniques can distinguish transformers under different conditions. <s> BIB016
A diagnostic technique based on time domain analysis of vibration stationarity was proposed in BIB016 . Vibration stationarity analysis is based on recurrence plot analysis where recurrence means a time in which the trajectory returns to a location it has visited before. System determinism is chosen as the transformer health criterion which is defined as the ratio of recurrence points that form diagonal structures to all recurrence points. The authors state that the system determinism value of a healthy transformer should be close to 1. Significantly more papers deal with transformer core/ windings vibration analysis in the frequency domain BIB001 , BIB002 , BIB003 , BIB009  BIB004 BIB006 BIB010 BIB013 BIB007 BIB011 . Here, their conclusions and contributions are summarised. Core/windings vibrations appear at multiple frequencies up to several kHz and the magnitudes of those frequency components are dependant on the external temperature. These frequencies appear as even, odd and rational multiples of the fundamental frequency and generally the most dominant components are 100 Hz and its integer multiples. Vibrations are highly correlated with the transformer loading and 100 Hz, 200 Hz and 300 Hz components are most sensitive to the loading of the transformer. Vibrations with frequencies <100 Hz are usually the product of cooling systems and oil pumps operation. Core vibrations caused by magnetostriction are non-linearly proportional to the applied voltage while the winding vibrations are non-linearly proportional to the current. Munir et al. BIB009 reported the dependency of vibrations to the square of the current, while Aidi et al. BIB010 found that the nonlinear properties of the insulation between the windings make the relation closer to the cubic representation. To separate the core (magnetostriction) and windings (Lorentz forces) vibrations it is best to do the measurements under no-load and under normal loading conditions BIB003 because the vibrations under normal loading are caused by both of those phenomena. Guo et al. BIB013 showed that even multiples of fundamental frequency harmonics appear under fastened core and odd multiples under slack core. Odd harmonics appear both under fastened and loose windings, but the intensity is different between the two cases. Classical ICA-based BSS recovers the different physical sources of a system given a set of external measurements assuming statistical independence of the sources. However, sources of transformer vibrations are highly correlated due to the mutual dependence of voltage (core) and current (windings). Therefore, Jing et al. BIB014 proposed a modified BSS algorithm named TIFROM-BSS (Time-frequency ratio of mixtures-blind source separation) which was used to separate the core and windings vibrations. HHT was also used in BIB005 and BIB008 , but in these papers it was applied on vibration signals obtained from the transformer tank. Xiong and Ji BIB005 concluded that for healthy VOLUME 7, 2019 FIGURE 7. Fundamental frequency vibration variation between two samples BIB015 . transformers dominant frequencies in the Hilbert spectrum are 100 Hz, 200 Hz and 400 Hz, while for defected transformers the intensity of LF and HF components is amplified and for medium frequencies (oscillations around 400 Hz) it is decreased. These differences increase as the transformer defect is greater-the looser the core's clamp bolt is, the greater the frequency and energy content. In BIB008 , before implementation of HHT, Wavelet Packet Decomposition (WPD) was used to minimise noise influence and to remove unwanted components from the signal. HHT is much more effective when it is preceded by WPD since in that case it is much easier to spot dominant frequencies and their time distribution. Hilbert spectrum showed dominant frequencies that are integer multiples of fundamental system frequency (50 Hz). Basically, the time-frequency domain approach shows similar frequency content to the one in papers in which the authors used pure frequency approach. However, this method probably has an advantage over the pure frequency approach as the time dependent frequency content can be observed along with the load variations in time. In BIB012 , a transient method was proposed in which the vibration signals are collected during switching the transformer from offline to idle mode. Aside normal operating condition, three more cases were studied: core with loose screws fixing the upper yoke, core with loose screws fixing both upper and lower yokes and core with loose screws fixing both yokes with simultaneous reduction of HV windings' clamping pressure. The advantages of this method are: immunity to loading current magnitude, significant independence between the vibro-acoustic signals and the type, power and construction technology of the transformer, elimination of disturbances from oil and air circulation appliances, insignificant influence of transducer mounting position. The downside of this method is that the transformer must be out of service, but this time is only around 10 minutes compared to several hours needed for offline diagnostics. The aforementioned defects can be easily discerned in the time-frequency spectrogram since each defect has idiosyncratic amplitudes and dominant harmonics.