Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <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> FIGURE 3. <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> FIGURE 3. <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> BIB003 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <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> BIB004 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <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> BIB005 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <s> In order to take advantage of the merits of WPT and HHT in feature extraction from vibration signals of power transformer, a time-scale-frequency analysis method is developed based on the combination of these two techniques. This method consists of two steps. First, the desirable wavelet packet nodes corresponding to characteristic frequency bands of power transformer are selected through a Correlation Degree Threshold Screening (CDTS) technique for reconstructing a time-domain signal that contains useful information of power transformer. Second, the HHT is then conducted on the reconstructed signal to track the instantaneous frequencies corresponding to natural characteristics of power transformer. Experimental results are provided by analyzing a real power transformer vibration signal. Compared with the features extracted by directly using HHT, the features obtained by the proposed method reveal clearer condition pattern of the transformer, which shows the potential of this method in condition monitoring of power transformer. <s> BIB006 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <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> BIB007 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <s> SUMMARY Hydro-Quebec’s transformer reliability statistics show that on-load tap-changers (OLTCs) and bushings are at the origin of two-thirds of transformer failures. Consequently, Hydro-Quebec’s Research Institute (IREQ) has been working for a number of years to develop technology to monitor those two components. This new development has been undertaken in conjunction with HQ’s IMAGINE (Implantation de la Maintenance Automatisee et Gestion des Informations Numeriques des Equipements – Automated equipment maintenance and digital information management) project in an aim to improve equipment maintenance and project efficiency. This project will fully leverage digital technology and make the transition from systematic maintenance to conditional and predictive maintenance using remotemonitoring best practices. A new technology for bushing monitoring using a decentralized measurement system has been validated in field condition by a comparison of the measurements using a standard galvanic system. HQ’s field experience with OLTC and bushing monitoring emphasized some important aspects that have to be considered for interpretation of the data in monitoring mode: - The OLTC vibro-acoustic signature is significantly influenced by temperature; a timerealignment technique can be used to compensate this effect; - Rain can have a significant influence on tanδ measurements; it is possible to filter data in this condition by analyzing the standard deviation of several successive ∆tanδ values. The diagnostics from these new systems are integrated into the HQ’s information technology model to complete the existing data and improve the global transformer condition assessment. The development of those new monitoring technologies for bushings and tap changers will help prevent major failures, optimize maintenance costs and improve the safety of field personnel. It will also contribute to HQ’s efforts to cope with an aging transformer population and improve condition-based transformer maintenance. <s> BIB008 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <s> 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 Bruel & Kjaer. 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. <s> BIB009 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <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> BIB010 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <s> The acoustic emission (AE) is a group of phenomena involving generation of transient elastic (acoustic or vibro-acoustic) waves, resulting from liberation of intermolecular bond energy (deformation, cracking, phase transitions). This fact contributed to elaboration of the acoustic emission method, which is widely used in: (i) diagnostics of cutting tools and machine cutting process, (ii) machinery and civil structures condition assessment, (iii) fatigue and fracture materials research, (iv) detection of material defects, (v) orogenic and earthquake prediction research, (vi) rockbursts prediction in deep mines and (vii) in chemical reactions and phase transitions research. This method is also applied in electrical power engineering for detection and location of partial discharge (PD) sources in insulation system of large power transformers and GIS/GIL (Gas Insulated Substations/Gas Insulated transmission Lines). <s> BIB011 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <s> This paper discusses the use of vibrations for power transformer surveillance. Therefore, basic research of transformer vibration is presented including the physical background of vibrations and methods of measurement in terms of sensor techniques comparing in-oil and outside tank wall measurement. A case study of an outside tank wall measurement performed on an aged generator step-up transformer presents first experiences in terms of long term measurement evaluating data of a 2-years period. A use case for vibration measurement is drawn for direct current (DC) detection in power transformers. DC can couple into 3 phase AC systems from nearby HVDC systems. The core's subsequent static magnetic field leads to increased core power losses and higher stress for the entire transformer. <s> BIB012 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <s> Classic testing techniques give very little information on mechanical problems occurring inside an on load tap changer (OLTC) while in service. The vibro-acoustic method is based on the principle that all mechanical operations produce a vibro-acoustic pattern while operating. This pattern is recorded by an accelerometer and converted into a vibro-acoustic signature that is unique for each tap changer, and stable in time in the case of a healthy mechanism. The functioning of an OLTC is complex since a lot of mechanisms are operating at the same time. To facilitate the comprehension of the vibro-acoustic signature, the following instruments have been used at the same time during the execution of a test: - A high speed camera (to see) - A vibration transducer (to listen) - A dynamic resistance accessory (to capture the electrical currents) - A current clamp (to record the motor current, determine the operation synching, and identify the operating range of the taps) Each of these tests provides a piece of information that, coupled with the other tests, creates a complete picture of the condition of the OLTC. Therefore, the final target of all the tests is to be able to easily detect vibration problems in the future. In fact, the simultaneous correlation of all the signals, images and videos allows a fast understanding of the internal movements of an OLTC. It also improves the knowledge about the vibration for the operator as a reliable interpretation tool to find problems on OLTCs while in service, and before they create an outage. <s> BIB013 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <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> BIB014 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> FIGURE 3. <s> Winding distortions can be mechanically induced during transportation, by large transient currents caused by lightning events or short-circuits, or as a result of gradual clamping looseness as the transformers age, after many loading and unloading cycles. Winding distortions lead to increased electromagnetic forces between the windings and the core, giving rise to changes in vibration signatures that could be useful as online condition monitoring tool. Unfortunately, this method is difficult when using electrical sensors mounted on the tank walls, i.e., vibration signals are highly dependent on mounting positions, and contaminated by noise from oil pumps, cooling fans, or the oil tank itself. For the first time, a thin fiber optics sensor has been developed to fit the 2 mm gap between the winding and the slot for direct winding vibration measurements. This sensor is an extension of the Fabry-Perot fiber optics vibration sensor — The VibroFibre — developed by QPS 5 years ago and already deployed around the world for monitoring Stator End Winding (SEW) vibration. The same hermetic junction plate developed to isolate the hydrogen cooled generator from the outside ambient is now conveniently adapted to become the feedthrough into the transformer tank, thus preventing any oil leaks to the environment. This paper discusses the successful showcase developed between QPS and Xian E-Sys Automation Engineering Co., Ltd in China. The proposed sensor, which is 2 mm thick, 50 mm long, and 20 mm wide, can measure vibration between 20 Hz to 1000 Hz. This work has been inspired by an extensive research program at University of Western Australia performing a rigorous finite element model simulating the magnetic forces interaction and vibration inside the transformer. We will validate all their simulation results showing how it is possible to detect significant winding distortions like buckling and bulging, open and short circuits, and clamp looseness. Missing inter-winding separation can also be identified as we trend the vibration amplitudes at twice the line signal frequency. The special sensor is also capable of measuring temperature to achieve a cost effective monitoring solution to extend the life of the transformers while providing good diagnostics. <s> BIB015
Effects of mounting configurations on high-frequency response for commercial accelerometer M608A11  . Thus, for accurate diagnostics, it is crucial to choose not only the appropriate sensors but also to pay attention to the mounting method and how it affects the sensor's frequency response. Furthermore, the ability to synchronise measurements when multiple sensors are used is also crucial. Additionally, different physical phenomena can manifest themselves in the acquired signals and, as will be shown later, a lot of authors employed auxiliary measurements in their acquisition systems (e.g. OLTC motor current, temperature, OLTC motor rotor angle in BIB014 ). The location of the sensors is also very important and in many papers authors sought the optimal sensor location by trying to maximise signalto-noise ratio (SNR) or by mounting the sensor as close to the source of vibrations as possible. Since the sensors are mounted on HV objects, additional precautions in the form of electrical insulation can be taken if necessary. For example, Sanz-Bobi et al. BIB001 covered every electrical contact with silicone and every sensor cable was inserted in a fibreglass envelope to address the insulation and electromagnetic issues for the sensors installed inside a transformer. Hydro-Québec's experiences in OLTC diagnostics have been reported in BIB008 . They used a vibro-acoustic sensor mounted on the OLTC tank as close as possible to the OLTC diverter switch. OLTC motor current was used for triggering the tap change measurement. Field experience showed that the vibro-acoustic signature is significantly affected by ambient temperature, and it was reported that a switching time of a healthy OLTC is increased by 26 ms at ambient temperature of −11 • C compared to the ambient temperature of 27 • C and that this could be erroneously interpreted as a faulty condition. The recorded signature was compensated for the temperature using first-order time-realignment technique  . Alongside accelerometers and OLTC motor current measurements, Brikci et al. BIB013 also used a high-speed camera to record OLTC movement and dynamic resistance measurements. VOLUME 7, 2019 To summarise, the following issues must be addressed in order to conduct quality diagnostics of a power transformer: • sensor mounting method; • synchronisation of measurements if multiple sensors are utilised; • prediction and understanding of underlying physical phenomena which can be manifested in the collected signals; • sensor location; • sensor insulation. Experimental results demonstrate that for the proper diagnostics of transformer core and windings based on vibroacoustic measurements it is usually enough to observe the frequency spectrum in the range up to 1 kHz  , BIB006 , although harmonics up to 2 kHz were observed in BIB002 and BIB004 . On the other hand, OLTC diagnostics require observation of a higher frequency range-up to 10 kHz BIB005 , although some authors BIB009 BIB007 BIB010 observed a much higher frequency spectra (even up to several hundreds of kHz) in order to be able to apply some digital filtration techniques, or to be able to observe not only acoustic events of the mechanical origin, but also other events, e.g. emergence of electric arc or events related to PD activity. PD analysis is usually performed in the UHF spectrum (300 MHz-1 GHz). However, during the process of PD inside the transformer tank, some part of the electrical energy from the PD pulse (about 1-5%) is converted to the mechanical energy, which results in the emission of the acoustic wave BIB011 . This fact is used by acoustic emission partial discharge monitoring methods, where emitted waves are captured by piezo-electric sensors mounted on the external wall of the transformer tank. The dominant frequencies observed of recorded PD signals typically lie in the range of 70 kHz to 180 kHz  , which are quite higher when compared to the signals generated by transformer vibrations or OLTC operation. Besides accelerometers, some researchers also used other types of sensors such as hydrophone BIB009 or strain gauge sensors BIB003 . In BIB015 , a 2 mm thick fibre-optics vibration sensor was successfully installed in the gap between the windings of a power transformer to measure axial vibration. The sensor accurately registered vibration frequencies between 5 Hz and 1000 Hz. Kung et al. noted that by splicing a single mode fibre to the Fabry-Perot cavity, the whole fibre acts as a vibration sensor and can function without a ''diving board''. An interesting study was conducted in BIB009 where the authors compared samples that were gathered simultaneously from several sensor types mounted in/on the OLTC. Besides the standard accelerometer, a hydrophone and a wide-band contact transducer were used as well. Power density spectrum analysis showed a number of differences between these signals. The signal recorded with the hydrophone contained frequencies with the largest share in the band 0-50 kHz and such a signal is not reliable for establishing diagnostics because the transducer is very sensitive to electromagnetic interference in that frequency range due to the current through OLTC's contacts. On the other hand, a signal acquired with a wide-band transducer, that contains frequency components in the range 10 kHz-500 kHz is the best choice for OLTC diagnostics as it also allows analysis of acoustic emission (AE) signals during current flow and electrical discharges when higher frequency components are present. However, time domain plot shows that hydrophone captures events that are not so evident in the signal gathered with contact sensor types. Also, by using a hydrophone immersed in oil it is possible to avoid contact problems demonstrated in Fig. 3 . Beltle and Tenbohlen BIB012 also used a hydrophone immersed in transformer oil and an accelerometer mounted on the transformer's tank. Although the hydrophone shows better SNR compared to the accelerometer, the inaccessibility of insulation oil as a medium in which the hydrophone should be immersed makes this sensor less attractive compared to the contact sensor types. Signal acquired with an accelerometer contains low frequencies in the range 0-10 kHz and thus it only allows analysis of AE signals of the mechanical origin. However, the accelerometer is the most commonly used type of sensor in the vibro-acoustic OLTC diagnostics. Finally, accelerometer output is usually a voltage signal that contains some unwanted frequency content. That is the reason why this signal needs to be passed through a series of high pass and low-pass hardware or software filter. Filter designer should pay special attention to the accelerometer resonant frequency, at which its' maximum sensitivity is achieved. These sensors require external supply and their output signal is usually measured in mV so proper gain is also required in the signal acquisition stage.