Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> Vibro-acoustic diagnostic of on-load tap changers (OLTC) offer numerous advantages: applicable online, sensitive to mechanical as well as electrical malfunctions, retrofitable to old equipment, simple and low-cost. Over the last few years, Hydro-Quebec has spent considerable efforts in the development of a reliable acoustic methods for both monitoring and diagnostics, this paper describes the methodology proposed for periodic testing via a portable acoustic system. First, the process to extract the OLTC signatures from acoustic recordings is briefly discussed. Then, the measurement approach is described along with the diagnostic procedure. Finally, actual field cases of diagnostic are presented to demonstrate the capabilities and the sensitivity of the acoustic method. <s> BIB001 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> The purpose of this investigation is to apply Hard C-Mean (HCM) and Fuzzy C-Mean (FCM) rules in clustering data sets that correspond to different Load Tap Changer (LTC) contact conditions. The stress exerted on the moving arm of a LTC is measured and is then converted to a voltage output signal. It is shown that as the LTC contact conditions deteriorate, the repetitive patterns of the output signal changes correspondingly. The HCM, FCM, and their validity measures prove to be suitable tools for online equipment maintenance monitoring. <s> BIB002 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> An onload tap changer (OLTC) is the most maintenance intensive subassembly on a power transformer. Vibration monitoring is an effective technique that can be used to assess the condition of an OLTC nonintrusively. The authors have developed a condition monitoring system for common types of OLTCs that enables the condition of tap changer contacts and associated drive system to be inferred from vibration signals. A number of prototype systems have been installed onto OLTCs in distribution zone substations for field trials. Particular emphasis has been given to the detection of faults in a particular type of older tap changer that had been prone to a range of faults associated with the switching contacts and drive mechanism. For this type of tap changer, it has been shown to be possible to determine not only that the tap changer is aging but also to identify the particular part that is degrading. <s> BIB003 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> On load tap changers (OLTC) are widely used for voltage regulation in electricity networks. Noninvasive vibration methods for condition monitoring of internal electrical contacts have been recently proposed in the literature. The vibration signals emitted during the tap changes are usually recorded and post-processed using spectral analysis and some pattern classifier technique. To reduce the complexity of the classifier, a new technique based on genetic algorithm is proposed in this paper. A description of the data acquisition system and the corresponding collected experimental data are presented. The proposed technique is detailed and preliminary experimental results are discussed <s> BIB004 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> On-load tap changers (OLTCs) are electromechanical devices used in the voltage regulation of the electrical networks. A non-invasive monitoring method was developed using discrete wavelet transform (DWT) for condition assessment of on-load tap-changers using vibration analysis signals. The transient vibration signals emitted during a tap change are recorded and processed using spectral analysis and standards recognition techniques. The monitoring systems which detect failure during the OLTC operation are quite complexes. In order to reduce this complexity, in this work we demonstrated a new approach based on DWT analysis. The proposed technique is presented and the experimental results as well as its simulation are discussed <s> BIB005 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> The Condition Monitoring of On-Load Tap Changers is very important because they have proved to be the elements with noticeable failures in a power transformer. This article describes the development of a portable virtual instrument for monitoring this kind of elements. The monitoring task is based in the measurement and analysis of the vibrations that a tap change produces. In contrast with other methods that can be used, this one has the advantage of being able to do continuous monitoring because the transformer can be operated on line. <s> BIB006 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> On-load TapChangers (OLTC) arewidely usedfor voltage regulation inelectricity networks. Asetofelectrical contacts (taps) is usedtochange thetransformer input/output ratio astheinput voltage varies. Non-invasive acoustic methods forcondition monitoring of internal electrical contacts havebeenrecently proposed inthelitera- ture. A limitation ofthese analysis isthat there isnoinformation on gradual degradation oftheequipment condition. Inthis workaclas- sifier system using Self-organizing Maps(SOM)isdeveloped inorder toidentify different levels ofelectrical contacts wearThemethod proposed herewasabletoseparate theavailable signatures into four distinct operation conditions: post-maintenance (no-wear out), low, intermediate andsevere wearout. <s> BIB007 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> The suitable condition of an onload tap changer (OLTC) is essential for the operation of a power transformer. It is extremely desirable to have some indicators to assess the OLTC condition, especially if these indicators are capable of being used in an online monitoring system that does not affect the normal operation of the transformer. This paper describes a methodology by using envelope analysis based on the Hilbert transform and orthogonal decomposition of the wavelet to determine the main diagnostic parameters for power transformer OLTC condition assessment by means of vibration measurements during the tap changer operation. <s> BIB008 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> An onload tap changer (OLTC) in suitable conditions is essential for the operation of a power transformer. It is extremely desirable to have indicators that help assess the conditions of the OLTC, especially if these indicators can be used in an on-line monitoring system that does not affect the normal operation of the transformer. This paper describes a method for detecting faults in the tap selector by means of vibration measurements during tap changer operation, using envelope analysis based on Hilbert transform and wavelet decomposition. Different failures at the tap selector can be distinguished in the vibration signature as they are reflected in different parts of that signature. The diagnosis parameters allowing the most selective failure classification are found. <s> BIB009 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> 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. (Abstract) <s> BIB010 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> On-load tap changers (OLTC) are some of the main transformer elements that ::: make voltage adjustment in a power network possible. Their failures often cause ::: shutdowns of distribution transformers. The paper presents research work aimed ::: at the assessment of the technical condition of OLTCs by the acoustic emission ::: method (EA). This method makes the OLTC diagnosis possible without the need ::: of disconnecting the transformer from the system. The measurements were taken ::: in laboratory conditions. The influence on the operation non-concurrence of the ::: power tap changer contacts on the AE registered signals has been investigated. The ::: signals registered were subjected to analyses in the time and time-frequency do- ::: mains. The result analysis in the time domain was carried out using the Hilbert ::: transform and calculating characteristic times for the particular runs. A short-time ::: Fourier transform was used for the assessment of results in the time-frequency do- ::: main. <s> BIB011 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> 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. <s> BIB012 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> On-Load Tap Changer (OLTC) forms a vital part of a transformer. Acoustic emission signals have been acquired from OLTC of 230 kV 3 phase power transformers for signature analysis. The acoustic emission signals have been analyzed to characterize the signals with reference to energy and frequency using Discrete Wavelet Transform. Case studies of AE signals obtained from healthy OLTCs have been presented in the paper for probable consideration as reference signals for healthy and normally operating OLTCs. <s> BIB013 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> Power transformers are fundamental parts of the power system. In order to keep the availability of power systems high, the reliability of a transformer, in all its parts, must be assured. This requires also paying attention to the reliability of the tap changer, as tap changer failures are major causes of power transformer breakdown. This paper focuses on the development of acoustic diagnostics for on-load tap changers (OLTC). A selector switch-type tap changer was used to detect and characterize different defects that may ultimately lead to a tap changer failure. Among other techniques, acoustic diagnostic was considered first because it is a method which can work online without affecting in any way the normal operation of a power transformer. Special emphasis was dedicated to recreate the arcing conditions in a tap changer. Arcing inside the tap changer chamber is unavoidable, but excessive arcing leads to a multitude of problems, so that efforts have been made to detect acoustically the signature of the arcing phenomenon. In order to have both flexibility and control over the experimental parameters, the tests were conducted on two sets of test objects, i.e. a model and a small-size tap changer. Focus was made to the type of defects that pose the most imminent threat to the tap changer operation, and the most appropriate methods of signal acquisition and processing were singled out. It is shown that these defects can be recognized based on their specific acoustic signature. <s> BIB014 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> An expert system for acoustic diagnosis of OLTCs and CBs is proposed.The expert system can locate the cause of each anomaly in a broader spectrum.Acoustic signature features have been extraction in both time and frequency domains.Eminent speech processing techniques are utilized for feature extraction and comparison.Repeated testing of same device may assure the accurate diagnosis. Most of the faults on power Circuit Breakers (CBs) and On-Load Tap Changers (OLTCs) on high voltage transformers are of mechanical origin. Mechanical malfunction, mechanical wear and other types of abnormal behaviors can be detected as changes in the acoustic signatures. In this paper, an Expert System (ES) with the addition of a signal processor module and factual database module to the conventional ES has been proposed for the diagnosis of CBs and OLTCs. The feature extraction of the acoustic signatures has been done by decomposing them into voiced and silent portions in time domain and through FFT spectrum analysis in frequency domain. The OLTC's motor current has been decomposed into inrush, steady-state and extended currents to locate the cause of different anomalies in the OLTCs and acoustic-current pair has been used for the synchronization testing. Each extracted feature of the testing signal has been compared to the similar feature of a reference signal to not only identify the health of the testing device but also to locate the cause of each anomaly. Several test signals have been compared against the reference signature and the proposed expert system has proved to be a suitable, effective and reliable system for the acoustic diagnosis of OLTCs and CBs. <s> BIB015 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> Partial discharge (PD) measurement provides a means for online monitoring and diagnosis of transformers. However, extensive interferences and noise can significantly jeopardize the measured PD signals and cause ambiguity in PD measurement interpretation. Necessary PD signal de-noising techniques need to be adopted and wavelet transform is one of such techniques. Mother wavelet selection, decomposition level determination and thresholding are important processes for effective PD extraction using wavelet transform. Various methods have been proposed in the literature to improve the above processes of wavelet transform. In these methods a single threshold is normally adopted at each decomposition level and a binary decision is made to indicate whether an extracted signal is PD signal or noise. However, in online PD measurements it is difficult to find a threshold, which can be used for extracting only PD signals without including any noise. As such, the signals determined by a single threshold cannot be assured as PD signals with certainty. To address the limitations caused by the single thresholding method in wavelet transform for PD signals extraction, this paper proposes quantile based multi-scale thresholding method at each decomposition level, which can thus provide probability indexes for the extracted signals evaluating the likelihood of these signals to be PD signals. To evaluate the proposed method, PD measurements have been conducted on both experimental PD models and inservice transformers at substation. The results are provided in the paper. <s> BIB016 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> The acoustic signature is a proven diagnostic tool in many industrial applications. <s> BIB017 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> The fault rate of on-load tap-changer (OLTC) has always been in a high level, and that is exactly the primary reason of transformer faults. As a result, an on-line detection method of OLTC was proposed. The embedded platform and wireless sensor network (WSN) technology was introduced into the on-line detection system. WSN was used to obtain OLTC status information. The embedded platform software, which was integrated data processing and fault diagnosis technologies, could realize the function of displaying and saving real-time data, fault diagnosis and so on. Besides, an analysis method of OLTC vibration signal was proposed on the basis of Empirical Mode Decomposition (EMD) and Fast Fourier Transformation (FFT). <s> BIB018 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> The suitable condition of an on-load tap-changer (OLTC) is essential for the operation of converter transformer due to its frequent switch for the voltage regulation of power system. This paper describes a methodology to obtain the OLTC vibration characteristics in time–frequency domain. Considering the possible aliasing effect in vibration signal processing, an improved empirical mode decomposition (EMD) is proposed with masking signals of multiple frequencies added, which has obvious superiority in aliasing reduction compared with conventional methods. Then, an effective fault-diagnosis strategy based on energy distributions variations of OLTC vibration signals according to Lorentz information measure is brought up. The calculated results under normal and typical fault conditions of model and real OLTC have shown that the energy spectrums of different conditions vary significantly so that the similarity index can measure the difference degree of energy distribution. Meanwhile, the index of contact looseness is higher than the insulated panel looseness which indicates that the contact looseness fault has greater influence on switch-over process of OLTC. <s> BIB019 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> Vibration measurement has been adopted in some utilities in Australia, Canada, and several European countries for online condition monitoring of the power transformer's onload tap changer (OLTC). By comparing measured vibration signals at different stages, it might be possible to assess changes of OLTC's condition. However, there are still considerable difficulties in correlating vibration signals to the events of OLTC operation, which may impair the capability of vibration measurement for OLTC condition monitoring. Therefore, arcing measurement is proposed in this paper to complement the vibration measurement. Arcing is provoked when the OLTC switching contact closes at a fixed tap position and can lead to electromagnetic signals flowing through transformer windings, finally reaching earth. The arcing measurement is achieved by using a high-frequency current transducer (HFCT) clamping on the transformer's grounding cable. The joint vibration and arcing measurement can provide a better means for interpreting events involved in OLTC operation and facilitating improved OLTC condition monitoring. Since HFCT measured arcing signals can be coupled with noise, a probabilistic wavelet transform is thus employed in this paper to extract arcing signals from noise. Field measurements on two different types of OLTCs are performed using the joint vibration and arcing measurement system to validate the proposed methodology. <s> BIB020 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> This paper provides an example of the application of the acoustic emission (AE) method for the diagnosis of technical conditions of a three-phase on-load tap-changer (OLTC) GIII type. The measurements were performed for an amount of 10 items of OLTCs, installed in power transformers with a capacity of 250 MVA. The study was conducted in two different OLTC operating conditions during the tapping process: under load and free running conditions. The analysis of the measurement results was made in both time domain and time-frequency domain. The description of the AE signals generated by the OLTC in the time domain was performed using the analysis of waveforms and determined characteristic times. Within the time-frequency domain the measured signals were described by short-time Fourier transform ::: spectrograms. <s> BIB021 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> As one of the key components of aerospace power grid, the performance of on load tap changer (OLTC) of power transformer is very important. So it is necessary to realize real-time monitoring of its status. At present, there is no feasible monitoring scheme to carry out on-line monitoring and fault diagnosis for detecting faults of OLTC. The current waveform of the switching process will be abnormal due to the contact fault, the transition resistance fault, and most of the mechanical faults. This paper presents a fault diagnosis method for OLTC based on the current detection in the process of switching. Through on-line acquisition of current waveform in the switching process, empirical mode decomposition and Hilbert–Huang transform is used to calculate normalized time-frequency spectrum, and extract fault characteristic quantity, which can realize the comprehensive diagnosis of the running state of the equipment, and identify the early fault characteristics, and promote the process of the OLTC’s maintenance, from regular test to condition based maintenance. Experiment results prove that the designed fault diagnosis instrument can effectively monitor the working condition of OLTC during the switching process and make effective fault diagnosis. <s> BIB022 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> Audio-based machinery fault diagnostics has a lot of particularities when compared to similar (non-invasive) diagnostic methods. These particularities are most evident in the unavoidable influence of the machine environment which is reflected through a mixture of the useful diagnostic material with surrounding sounds. On the other hand, there are several obvious advantages of this approach. For instance, a recording microphone can be installed at a certain distance from the tested object, ensuring the complete non-invasiveness of the measuring process. This paper analyses the influence of the surrounding environment in audio-based On-Load Tap Changer transformer diagnostics as well as the ability to apply statistical methods for the purpose of extracting useful diagnostic material that can be used for its condition assessment. <s> BIB023 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> 1) OLTC <s> Over the years, non-invasive vibration-based On Load Tap Changer (OLTC) diagnostic methods have become a part of standard monitoring systems. On the other hand, audio-based diagnostics did not receive much attention. Despite the above, since signal-processing techniques are constantly evolving, audio-based machinery diagnostics is getting more and more interesting. The obvious advantage compared to the vibration based techniques lies in the fact that the audio signal can be recorded without any physical contact with the tested object. The aim of this paper is to show that there is a high linear correlation between vibration and sound signals that are produced by the OLTC, which then means that the experiences gained in the field of the vibration analysis can be transferred to the domain of the audio-based diagnostics. Another goal is to point to problems that may arise due to the environment in which the signal is being recorded. <s> BIB024
The first experiences in this field were described in    BIB001 . In addition to the OLTC vibration signals, the motor current was also recorded which was used for triggering the recording device. What was perceived here is that mechanical and electrical events manifest themselves through signals in completely different frequency bands-electrical events, such as the appearance of an electric arc, are associated with higher frequencies, while mechanical events, such as friction and synchronisation problems, are associated with lower frequencies. High complexity of the acoustic fingerprint was noted, so the authors resorted to envelope extraction using the Hilbert transform. To separate electrical and mechanical events, this transform was performed on two different data sets separated. It was suggested that the decision stage should be based on the comparison between new and previously obtained records (online testing) or on the comparison of records without any OLTC load (offline testing). Combination of both methods was suggested as the best practice. It is possible to draw some conclusions this way, but this method requires experienced and knowledgeable testing personnel, which is usually not the case in the field. It is precisely why such requirements have led to the further evolution of the diagnostic methods towards smarter systems that would enable easier detection of potential issues. DWT and TEO were used in    for envelope extraction to reduce complexity after collecting the vibration fingerprint. Properties derived from the auto-correlation function were used as input parameters for Self-Organising Maps (SOMs), which is a type of an artificial neural network used to classify and organise data. Experience showed that vibration fingerprint varies for each OLTC tap position and SOMs automatically classify the data according to their similarities. A priori knowledge was used to train the SOMs to carry out this automatic data classification. This principle implies that a large number of tests should be carried out on the objects that are in the healthy as well as those in the faulty state. Moreover, for objects that are in the faulty state, one should have a priori knowledge about the type of defect present. Five different OLTC conditions were observed that were used as classifiers in the SOMs: • normal condition; • weak springs; • worn moving contacts; • worn moving and fixed contacts; • weak springs plus worn contacts. Moreover, it was noticed that an alteration of OLTC condition is most often a gradual process rather than a sudden change. Crossing the defined threshold value of OLTC condition criterion activates an alarm that something is wrong. The main purpose of the design was to avoid false alarms. This design was successfully tested on a system that was in operation for three years at the time. In  , experiments were conducted on a transformer that was removed from the operation, so the authors were able to record vibration fingerprints for all 10 OLTC tap positions. Hilbert transform and a low-pass filter with a cut-off frequency based on Kolmogorov's complexity measure were used for signal envelope extraction. As there was no reference vibration fingerprint for the tested OLTC, one way to perform diagnostics is to compare different tap positions. The goal is to find a tap position that is most correlated with all the others. Finding such a position allows the comparison of the vibration fingerprint for each individual tap positions with that one. By doing so, the authors concluded that there are some differences between tap positions caused by their frequent use. This approach can be used to perform diagnostics in cases when measurement history is not available. However, it should also be noted that some OLTC tap positions are used more than others, which also implies that their wear will be more pronounced and all this will cause the differences between their vibration fingerprints. Secic and Kuzle  proposed a diagnostic method based on signal fractal properties. Higuchi's algorithm was used on a signal envelope of a simulated OLTC vibration model from BIB004 to estimate the fractal dimension of the signal. By changing the model parameters, contact ageing was simulated and the results showed increasing differences between the estimated fractal dimension of a new and old OLTC contacts. This method can be used for diagnosing contact degradation and to distinguish between different mounting methods. However, no validation on OLTC contacts whose condition is a priori known has been conducted nor was the proposed algorithm tested for OLTC failure detection. In the frequency domain analysis of vibro-acoustic fingerprints of an OLTC, three papers have been found which differ in the type of decision making process utilized for the detection of the OLTC condition. Rastgoufard et al. BIB002 observed sufficient differences between contacts in good and bad condition by observing the frequency spectrum of vibration signals recorded by strain gauge sensors mounted on the main drive arm of OLTC. Automatic detection of the OLTC condition was achieved with unsupervised fuzzy logic. Bhuyan et al. BIB014 measured the dynamic resistance of OLTC contacts alongside the OLTC vibration fingerprint. The authors manually identified five different regions in the frequency spectrum using FFT which are separately used to distinguish contacts in good and bad shape, and contacts with increased electrical arcing. They showed that it is possible to recognise arcing and various defects in a single phase tap changer. Hussain et al. BIB015 measured the OLTC motor current alongside the OLTC vibration fingerprint. An automatic expert diagnostic system was designed based on these signals and a knowledge database based on field experience. The main purpose of this system is to solve complex problems through reasoning based on a set of rules which minimises errors that can be caused by human factors in the decision making process. CWT was used in  BIB003  BIB006 BIB017 for the feature extraction from the OLTC vibration fingerprint. Kang and Birtwhistle  did a field evaluation over a period of three years with SOMs for data classification. They identified the establishment of reference vibration fingerprint as a practical problem in order to apply the monitoring system on a different OLTC. Viereck and Saveliev BIB017 showed several time-frequency images of the OLTC switching operation where different phenomena were clearly marked such as: preselector and diverter switch operation; transformer buzzing and 50 Hz disturbances of an energised transformer; impact of sensor mounting method; tap selector squeaking anomaly. DWT was used in BIB008 and BIB005 BIB009 BIB007 BIB013 BIB020 . Based on the research conducted in BIB005 and BIB009 , several accomplishments were pointed out by the authors: • ability to differ between new and old (used) contacts; • distinguishing vibration fingerprints of different tap positions; • distinguishing between the following types of failures: loose contact springs, broken contact bar, worn out tap selector contacts and damaged tap selector contacts. Filho and de Almeida BIB007 used SOMs for data classification to group data into 6 different clusters based on the wear level, faulty condition, etc. The main advantage of this method is that it provides information on the gradual degradation of the contacts condition, and that such info can be used by an online monitoring system. In BIB020 , besides OLTC vibration, a high frequency current transducer was used for measurement of the arcing signal for improved diagnostics. This signal was used as an additional diagnostic information that helps to clearly identify events in the OLTC switching operation that are manifested in the vibration fingerprint. Modified form of wavelet transform, called Probabilistic Wavelet Transform (PWT) BIB016 was used for segmenting arcing signals and noise. Besides the obviously popular wavelet transform, several papers were found in which time-frequency domain was used but in which the authors approached the matter differently BIB018 , BIB010 , BIB012 , BIB019 BIB011 BIB021 BIB022 -the following methods or their combinations have been used or mentioned: EMD, Hilbert Transform, HHT, HSA, EEMD, BREMD, MFEMD. These methods were used to tackle the problems of highly non-stationary nature of the signals and to improve the precision of the diagnostics. CWT, DWT and STFT have also been used in BIB010 , BIB012 , BIB011 , and BIB021 for the extraction of the diagnostic material. Li et al. BIB022 reported the shortcomings of the existing vibro-acoustic diagnostic methods when electrical faults without obvious mechanical vibration are being diagnosed, such as contact terminals fault and contact resistance anomaly. They proposed a method based on measuring the current waveform of an OLTC, for which they state it contains rich diagnostic information. Two faulty OLTCs have been analysed and their time-frequency spectrum shows rich frequency content compared to a healthy OLTC whose waveform is mainly composed of 50 Hz component. So far, all the surveyed research used accelerometers or AE sensors which require direct contact with the transformer or must be put inside the tank in the case of hydrophones. From our experience, the audio-based diagnostics is a relatively unresearched area which in our opinion is worth exploring. A method of recording the audio signal of the OLTC using an array of condenser microphones (Fig. 6 ) was tested in BIB023  BIB024 . These papers deal with the extraction of the useful diagnostic material from a mixture of different audio signals and correlating audio signature with a vibration signature. Fast Independent Component Analysis Blind Source Separation (FastICA-BSS) was used to separate the useful diagnostic material from the surrounding noise. Some distortion in the estimated signal is unavoidable, and the real question is if this distortion will prevent diagnostic information extraction method in performing its task and how much it will affect the results. Using proper data extraction techniques and comparing original and estimated signals or using the OLTC reference audio fingerprint recorded in the de-energised state are some ways to tackle this issue.