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PD Acoustic Response in Oil-Filled Transformers Juraj KURIMSKÝ∗ Abstract In oil-filled transformers, partial discharges generate electromagnetic impulses which energy is transformed into mechanical energy in the form of pressure waves. For the purpose of power transformer diagnostics, several different approaches are used to partial discharges in insulating system. Theory of acoustic waves, generated by partial discharges (PDs) in power transformers, and the PD acoustic emission and methods of detection are described. Two types of waves are discussed, which can be used for non-destructive testing of power transformer insulating system. The principle of PD source localization by means of acoustic range wave detection is described. Experimental procedure applied to 110 kV power transformer and results are presented. Keywords: partial discharges, acoustic emission, power transformer 1. Introduction There are several methods developed to examine operational conditions of crucial systems of oil-immersed transformers during the life-time [1, 2, 3, 4]. They are based on specific physical phenomena which are transformed into the observable variable or parameter. The 'specific' above means that nature of these effects can be physical, chemical, electric, etc. For the purpose of power transformer diagnostics, three different approaches are used to asset partial discharges in insulating system. There are: electric methods, dissolved gas analysis and acoustic emission measurements. Electric methods are based on electromagnetic field change detection and electric current pulses measurements, respectively. The last one provides the ability to quantify the apparent charge of PD and determine the amount of degradation energy. Generally, the measurements can be performed either online or off-line. The maintenance of power transformers became very important work for utility companies [5]. Many aspects of operational parameters are analyzed to produce relevant results. Due to chemical decomposition, partial discharges produce the change chemical properties of insulating oil. Dissolved Gas Analysis (DGA) is effective tool widely used to determine operational safety of power transformers [6]. By means of DGA it can be distinguished if operation is normal or near to ∗ Juraj KURIMSKÝ: PhD., Technical University of Košice, Mäsiarska 74, 04001 Košice, Slovakia, [email protected] a fault condition. There are three stages DGA: taking a sample of oil (sampling), gas chromatography and, finally, the analysis. The state of the art overcomes classical analysis moreover they are used modern data mining methods and forecasting of gases content in transformer oil [7, 8]. The method of acoustic emission detection is useful for assessing the activity of partial discharges during operation and is based on detecting and locating sources of pressure waves caused by electric discharge in the fluid of insulating oil. The origins of the application of this method are in the sixties of the 20th century. There are several comprehensive publications on theory and practical considerations of acoustic emission, e.g. [9, 10, 11]. 2. PD acoustic emission in oil-filled transformers In oil-filled transformers, partial discharges generate electromagnetic impulses which energy is transformed into mechanical energy in the form of pressure waves. The transformation of the energy from one form to another one is not able to be described by a simple function as it depends on many parameters, e.g. the kind and intensity of partial discharge, dielectrics nature, surrounding properties, etc. Depending on these parameters, the duration of the pressure wave can vary. The wave is distributed through the oil medium until its front touches the transformer tank or a solid object, which is the part the transformer skeleton. There are several ways ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 62 (2014), nr. 1 to use this phenomenon for diagnostic measurements. The acoustic wave generated by pressure field is expressed by a second order differential formula (1) where p is the pressure field, ∇ is the Laplace operator, v is the velocity of sound in the medium and t is the time. There are five basic parameters that quantify the acoustic pressure wave are: − sound speed v (m/s) 2 − the intensity of sound waves I (W/m ) − sound pressure p (Pa) − sound intensity LI (dB) and − sound pressure level LP (dB). Acoustic waves spread through the oil medium in all directions. This type of pressure waves are also called spherical wave and location of partial discharge creates an acoustic point-source. Generally, there are longitudinal and share wave types in real environments. Whilst longitudinal waves cause the medium moving in the direction of propagation, the shear waves produce the motion transverse to the direction of propagation. Several physical mechanisms are applied: spatial attenuation, acoustic absorption, scattering, reflection and refraction. The spatial attenuation for spherical wave is given as (2) where I0 is the intensity of the generated sound pressure and Ir is the intensity of sound pressure level at distance r from the source wave-front. Acoustic absorption is caused by the mechanical energy conversion into the heat. It can be expressed as with the optical absorption, the Lambert law: 47 striking the front of pressure waves to the wall of the transformer tank, two types of waves are excited in the steel material of transformer tank. There is longitudinal vl and shear Ls waves, which have different velocities. The values are in the Table 1. Table 1. Wave speed in transformer oil and steel When analyzing the PD acoustic emission and detection in the power transformers, the next important element has to be taken into account. Pressure waves, as they impact the wall of the transformer tank, are responsible for tank wall waves generation and motion. Steel plate vibration belongs to mechanical vibrations. The analysis is more complex according the nature of steel plate. The Kirchhoff-Love theory and the MindlinReissner theory are basics of plate motion. Plates can be excited in so called modes, which can be of order zero or higher orders, too. Let us consider zero order excitation, see Figure 1, where symmetrical (case a) and asymmetrical (case b) waves are shown. Figure 1. The steel plate vibrations It should be supposed that parameters of waves are depended on many mechanical and structural parameters. That is true. For instance, when steel plate of thickness of 1 cm is taken into account, the dependency of angle of incidence on wave frequency vary for both asymmetrical (a0) and symmetrical wave of zero order (s0) (see Figure 2). (3) where β is the coefficient of absorption. Scattering — the diffraction — occurs in non-homogeneous materials. In the oil, there may be gas bubbles or coagulated particles in the volume. Scattering appears as absorption, but losses due to scattering can be much larger. Reflection and refraction of pressure waves occur at the interface of two environments. In different environments, the pressure wave spreads at different speeds. Applied here can be the principles of Huygens law as well as Snell law of refraction, which are known from optics. In the oil medium, the only longitudinal waves are taken into account. After they are Figure 2. The steel plate Steel plate excitation [12] 3. The analysis of acoustic wave motion in power transformer Travelling, the acoustic wave front can be analytically derived [12, 13, 14]. According [12], simple model of real situation is given in 48 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 62 (2014), nr. 1 locations. The most important path is this one, which wave travelling takes minimal time to reach the sensor in. the Figure 3. 4. Experiment Figure 3. The model for acoustic wave emission in power transformer The above facts show that the direct path of the acoustic wave from source to the sensor may not be the fastest, due to the different velocities of acoustic waves in two main physical environments – the oil and the steel. In fact, there are many paths with different angles of impact Φ to the transformer tank wall by which the ultrasonic signal can reach the sensor. Suppose that the angle among the paths of PD source-sensor and the path of PD-sourcetank, that is perpendicular to the plane, is Ψ. Then the time that takes the travelling of the wave-front from PD-source to the acoustic sensor is: (4) There are several methods which can be used for PD localization experiment by means of acoustic emission measurements. Let us mention two of them. The first one is simpler. It is based on AE signal measurements in several various places regarded to PD source. The magnitude of acoustic emission, signal shape and transformer design are crucial factors for source position analysis. The second method is running in real-time. Acoustic signals are recorded from several sensors. Data are processed and position of PD source can be calculated by, e.g., triangular method. Previous experimentation has shown that efficient measurement of acoustic emission due to PD activity can be done in the frequency range from 100 kHz up to 200 kHz. Therefore acoustic sensors confirming this condition have been selected with top frequency about 160 kHz. Acoustic emission signal response of used sensors is in the Figure 4. The minimal time span of acoustic wave front arrives to the sensor can be found as extreme solution for dt/dr =0. Just then from formula (4) follows: Figure 4. Acoustic sensor for PD detection response (5) where α is the critical incidence angle Φ of the wave-front impact to the transformer tank. By substituting the numbers from the Table 1 in equation (4), we obtain a value α=13.727°. Two cases of impact angle are needed to be analyzed. The first one is the case of Ψ>α. The time tl taken to wave front arrive the sensor is: The model of the transformer oil tank with PD source has been created. PD source was placed in a known position of the source of partial discharge of a known amount of apparent charge. On the wall of the tank there were placed acoustic sensors, see Figure 5. (6) If Ψ≤α, then acoustic wave front reach travels by direct path to the sensor. The propagation time for direct path is given as: (7) The real situation is more complex; moreover, acoustic sensor is receiving all interference waves incoming from all possible Figure 5. Experimental setup ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 62 (2014), nr. 1 49 The question is: how many sensors need to be used. The answer depends on object size and internal body complexity. For laboratory experiments form 2 to 6 sensors are good. In power transformers, the acoustic wave propagation is complex due to various materials and barriers. AE signal is changed by means of attenuation, reflexing, refraction, etc. In such case, the more number of sensors, the better PD source localization. The number of 12 AE sensors seems to be minimal for PD localization in large power transformers. 5. Results For the purpose of signal processing, the Spartan-AT multichannel system has been used and independent data streams from AE sensors have been recorded. It was confirmed by the AE measurements and signal analysis that the location of partial discharge sources in the model is possible when apparent charge overcomes hundreds of pico-Coulombs. The response of such PD signal is transduced into typical acoustic pulse shape. The frequency spectra depends on AE sensors parameters. Typical shape of AE signal caused by PD source and result of 3D location of PD source in transformer tank is in the Figure 6. Figure 6. Transduced PD signal in AE sensor and 3D PD source localization The concept was applied on serious task of power transformer diagnostics. It seems to be meaningful to apply the acoustic emission measurements and direct electric method of PD measurement and take advantage of apparent charge magnitude determination and significant sources of PD localization. Both methods give different measures of PD source activity. Electric method offers, e.g. magnitude of apparent charge, PD phase distributions of several parameters, etc. The AE measurements can analyze acoustic energy of pressure waves in time domain for each sensor separately and total, too, the number of above-threshold indications, dominant magnitude versus time characteristics of AE waves, the amplitude of the signal sensor by sensor. Acoustic counts are a measure of the number of threshold crossings per AE event. More interesting details are in [11]. 6. Discussion Typically, in a power transformer the PD source can generate acoustic signals from 20 to 30 dB above ambient background noise level. Then, the level of AE signals is detectable so the timing information for the location of the discharge source was recorded. When signal rate and acoustic energy of signals are high, source location can be done. The locations were done by multi-channel analysis of acoustic wave arrival times. The crucial for sufficient precision are right positions of AE sensors. The strategy of sensor positions should be based on transformer design and construction knowledge. An example can be seen in Figure 7, where sensors positions on transformer wall are shown. 50 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 62 (2014), nr. 1 8. Acknowledgment "This publication is the result of the Project implementation: University Science Park TECHNICOM for Innovation Applications Supported by Knowledge Technology, ITMS: 26220220182, supported by the Research & Development Operational Programme funded by the ERDF." 9. References Figure 7. AE sensors on power transformer for PD localization The situation was used for PD test. Simultaneously, the electric PD measurements were done. The magnitude of apparent charge of 3500 pC was measured in the circuit of phase L1 winding. This was sufficient magnitude for successful PD localization. 7. Conclusions Known the limitations of electric PD testing in a substation environment can be overcome by applying acoustic emission PD source location, mainly wanted test for gassing transformers. By analyzing the acoustic emission the faults can be detected and localized in the insulating system, moreover, faults in the magnetic circuit of high voltage transformers with oil medium can be unleashed. Discharges in the internal partial discharge sources can be unstable, what can be solved by means of the long-term measurements and analysis of acoustic emission. When PD AE signal is more than 20 dB above the noise background, the location is done in several minutes after test start. In real large systems the accuracy of several centimeters can be achieved. Experimental results can uncover serious defect development. Detection of partial discharge acoustic emission opens up renewed possibilities in the diagnosis of high voltage oil-filled transformers. The main advantages are: immunity to electromagnetic interference, possibility to monitor the transformer during the operation and location of partial discharges sources however signal from PD toroid sensor is well suited for acoustic system triggering with respect to overvoltage circuit protection [15]. The method is particularly useful in solving the critical conditions in HV equipment; moreover it is great tool for condition based monitoring. [1] BOGGS SA, “Partial discharge: overview and signal generation”. in Electric Insulation Magazine, IEEE. 1990; vol. 6 (no. 4): pp. 33– 39. [2] CIMBALA R, “Dielectric spectroscopy of HV insulation material in time and frequency domain”. in EEEIC 2011, 10th International Conference on Environment and Electric Engineering, Conference proceedings, 8-11 May, 2011; Rome, Italy. [3] DOLNIK B, Harmonic analysis of the leakage current in surge arresters. Ph.D. Thesis. Technical University of Košice, Slovak Republic; 1996. 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[12] LUNDGAARD LE, HANSEN W, DURSUN K, “Location of discharges in power transformers using external acoustic sensors”. in Proc. 6th Int. Symp. High Voltage Eng., New Orleans. 1989; paper no. 15.05. [13] SKUBIS J, “A state of development of the acoustic method for assesment of partial discharges and directions of its improvement”. in Proc. of 2nd I.C.D.I, High Tatras, Slovakia. 2000; pp. 144–152. [14] PHUNG BT, BLACKBURN TR, LIU Z, “Acoustic measurements of partial discharge signals. http://www.itee.uq.edu.au. 51 [15] DOLNIK B, GULAS R, “Sledovanie zmien elektrických parametrov ZnO varistorov pre siete nízkeho napätia počas urýchleného starnutia”. in Starnutie elektroizolačných materiálov. 2010; vol. 5 (no. 8), pp. 4–13. 10. Biography Juraj KURIMSKY graduated the Technical University of Košice, Faculty of Electric Engineering in Košice (Slovakia), in 1990. He received the PhD degree in electric engineering from the Technical University of Košice, Faculty of Electric Engineering in Košice (Slovakia), in 2003. He is assistant professor at the Technical University of Košice, Faculty of Electric Power Engineering (Slovakia). His research interests concern: high voltage technique, diagnostics of high voltage devices, physics of materials and the material technology.