Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <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> BIB001 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <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> BIB002 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <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> BIB003 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <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> BIB004 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <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> BIB005 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <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> BIB006 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <s> Affected by various shortness shocks, the windings of transformer is loosening and deforming. As a result, power transformers' ability is decreased to withstand short circuit current and a big run accident is planted. Therefore, it is of great importance to conduct the research on the simulation test of winding especially wingding looseness of the power transformer. A method for power transformer winding simulation test is proposed in this paper based on the research on the vibration characteristics of transformer windings, which aims to carry out the test of vibration characteristics of transformer windings looseness in a limited test case that the power capacity is not enough. On the one hand, the test has verified the theoretical study of characteristics. On the other hand, new vibration characteristics have been found through the test. Technical achievements are as follows. 1) The method put up with in this paper can monitor the fault of the windings accurately under the circumstance that the power capacity is limited. The test operation is simple and it is easy to operate and measure the parameters. This method is useful universally for the same type of transformers. 2) It can reduce the demand for power. The operation is safe and economic. It is easy to carry out the operation. 3) The voltage of the high and low sides is so low that the influences are reduced brought by the vibration of core. As a result, it can improve the accuracy of the test. 4) Some vibration characteristics of transformer windings are found. We find that the linear relationship between some vibration signals and the square of current still persists during the process that the vibration signals pass to the tank surface and the changes of the 100Hz feature amplitude of different measuring points are different with the current changing. <s> BIB007 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <s> Abstract Transformer tank vibration has been used for the condition monitoring and fault diagnosis of power transformers, as well as for the evaluation of radiated sound power from such power transformers. In both applications, seeking a correlation between transformer tank vibration and the vibration of the transformer windings and core becomes a challenge for fault identification and noise control at the sources of vibration and noise. The purpose of this paper is to experimentally investigate the winding vibration of an electrically live power transformer and characterize the changes in the spatial and frequency features of the vibration as various mechanical faults are introduced to the transformer winding. To avoid the effects of transducer loading and electromagnetic fields on the measurement results, a laser Doppler vibrometer was used to make non-contact measurements of the winding vibration. <s> BIB008 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> VOLUME 7, 2019 <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> BIB009
Some guidelines regarding the core/windings vibration measurements were also defined in several papers BIB001 , BIB002 , BIB004 : • reduction of the external noise can be achieved by: insulation between the sensor shell and the transformer's tank, shielding of the transmission cables and grounding of the measurement system; • noise from the power supply lines can influence the measurements; • measurement points should have a high correlation with the load current to better reflect vibration characteristics of the windings; • when arranging the measurement points, one should take into account the structural features and flat surfaces of the transformer for better signals; • stiffeners or other complex structures should be avoided; • the sum of first four harmonics, the total harmonic distortion and the ratio between the amplitudes of the 50 Hz and 100 Hz harmonic are sufficient to establish good diagnostics and the estimation of the remaining service life of the transformer; • best mounting position is at the top of the transformer oil tank with auxiliary measurement point at the side of the tank. Li et al. BIB007 tackled the issues concerning laboratory testing due to the restrictions on the available power in the laboratory, but also the inability to dissipate large amounts of energy. They suggested a setup consisting of a current limiting resistor, a voltage regulator and a universal transformer whose LV side is connected to the LV side of the tested transformer. Hong et al. BIB009 presented several health metrics based on DFT decomposition in the frequency domain. As it was mentioned earlier in this paper, mounting method of an accelerometer impacts its frequency response. Additionally, the accelerometer is in direct contact with a high voltage objects and therefore it must be properly shielded from electromagnetic interference. This is why Zheng et al. BIB008 used a laser Doppler vibrometer to study the vibration of the windings. Four cases were studied: normal operating condition, different clamping pressures, loose insulation blocks and loose core clamping. Several conclusions have been drawn: • radial vibration pattern of a healthy transformer is approximately bilaterally symmetric, while the radial vibration pattern of a transformer with faulty windings becomes asymmetric; • looseness of winding clamping pressures or insulation block looseness are manifested as a change in vibration patterns at dominant frequencies; • looseness in core clamping pressure doesn't affect the vibration pattern as much so the spatial variance cannot be used to determine the severity of the core looseness, but it is still detectable by observing the winding vibration distribution. Very interesting observations were given in BIB005 , BIB003 , and BIB006 . These papers analyse the impact of geomagnetically induced currents and coupling effects between nearby AC and HVDC transmission lines on transformer vibrations. The Sun emits solar wind, which is basically low-density plasma that consists mostly of electrons, protons and alpha particles. These charged particles travel through the earth's atmosphere, causing variations in the intensity of the electrical field and creating a DC that can flow through the transformer's neutral. A similar effect is created by surrounding single-pole HVDC systems with the earth as a return conductor. The flow of DC causes increased magnetostriction, and may also lead to saturation of the transformer. Increased magnetostriction causes an increase of vibration intensity and additional losses through the occurrence of higher harmonics and reduction of reactive power. The conclusions of these studies are the following: • 50 Hz component is present in the measured signal in the case when there is a DC flowing through the transformer neutral. This component is not present when there is no neutral current; • amplitudes of the second and fourth harmonics increase as the DC increases; • the shape and the intensity of the vibration fingerprint is significantly influenced by the DC; • frequency analysis has shown shifting of the dominant harmonic and appearance of higher harmonics when the DC exists; • authors propose monitoring additional inputs for condition assessment of the transformer: transformer load and DC through neutral.