Localizing Partial Discharge in Power Transformers by Combining Acoustic and Different Electrical Methods

Localizing partial discharge in power transformers by combining acoustic and different electrical methods

Stefan M. Hoek, Rene Hummel, Alexander Kraetge, Benedikt Kästner and Ulrike Broniecki

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LOCALIZING PARTIAL DISCHARGE IN POWER TRANSFORMERS BY COMBINING ACOUSTIC AND DIFFERENT ELECTRICAL METHODS

Stefan M. Hoek1, Rene Hummel1, Alexander Kraetge1, Benedikt Kästner2, Ulrike Broniecki2 1OMICRON electronics, Austria 2Technical University Berlin, Germany

Abstract Power transformers are important nodes in the electrical power grid. The reliability of such electrical systems depends on the quality and availability of the power apparatus. Common reasons for breakdowns are problems in the insulation system. Detecting partial discharge (PD) in the insulation and windings of a power transformer at an early stage reduces the risk of total breakdown of power apparatus.

One method to detect PD is the acoustic measurement. With this technique a detection and localization of PD is possible by placing acoustic sensors on the surface of the transformer tank. The low level of electrical interferences from outside the measurement setup constitutes one of the strengths of this method. A further advantage is the ability of identifying the position of the PD source, for it is very important to estimate the risk and to solve insulation problems fast and effective.

1 Partial discharge in high voltage apparatus Partial discharge measurements on transformers are an accepted tool of quality control, in factory and on site. Different PD measuring techniques are using different physical peculiarities of the PD phenomenon, e.g. electric discharge currents (acc. to IEC 60270 [1]), gas formation (DGA - dissolved gas analysis), electromagnetic (UHF Measurement) or acoustic radiation (some tens of kHz). Partial discharge measurements according to IEC60270 standard are often the basis for acceptance tests of the insulation system of high voltage (HV) equipment [2]. The main benefits of acoustic PD measurement are the possibility of detecting PD without intervention into the device under test (DUT) and the possibility of localizing the PD source by the accuracy of some centimeters. In case of an evidence for PD, the location of the potential PD source can be important to verify PD and to estimate the risk of a complete failure. The knowledge of PD location is also crucial for the assessment of the asset and the process of maintenance or repair.

2 The propagation behavior of acoustic PD signals in transformers The acoustic response of PD inside a transformer is typically measured by a piezo-electrical sensor in the frequency range of some tens of kHz up to some hundreds of kHz [3]. Due to the resonant character of the sensors, the measured acoustic PD signal is inherent overlaid by oscillations as illustrated in Figure 1. For that reason the determination of the frequency content and proper signal form is difficult [4]. Using the difference in arrival time of the acoustic PD signal at multiple sensors, algorithms compute the location of the PD source. The complex physical processes involved in sound propagation and the large structural differences between different DUTs may be challenging during the measurement. The following parameters have to be considered:

 The PD source position and the inner structure of the transformer mainly influence the propagation path.

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 More than one propagation path from source to the sensor is possible (direct oil, reflection, steel path).  The speed of sound depends on the propagation path (crossed medium), the frequency and temperature.  Depending on the position of the source and the inner structure of the transformer, a direct oil path propagation may prevent a proper measurement by attenuating the signal too much.  The individual consideration of the measurement setup and the inner structure of the DUT, a cautious interpretation of the measurement results by an experienced person is essential.

Figure 1 Acoustic PD Signal

Speed and damping of the acoustic waves are dependent on the crossed medium, frequency range and temperature [5], [6]. Figure 2 and Figure 3 shows the variation of the velocity of sound in transformer oil for different temperature and frequency. For example, the propagation speed decreases during the heat-up period of the transformer by approximately 15%, from about 1400 m/s at 20 °C to 1200 m/s at 80 °C.

1600 1310

1500 1300 1400 1290 1300 1280

1200 1270 Velocity Velocity (m/s) Velocity Velocity (m/s) 1100 1260 1000 1250 -40 -20 0 20 40 60 80 100 120 140 0 200 400 600 800 1000 1200 Temperature ( °C) Freqency (kHz)

Figure 2 Figure 3 Dependence of sound velocity on temperature in Dependence of sound velocity on frequency transformer oil (f=150 kHz) [5] in transformer oil (T=60 °C) [5]

The propagation path is often complex. According to Figure 4 multiple propagation paths of the emitted sound wave are possible. Depending on sensor and PD location, multiple acoustic wave components of the same PD event are potentially detected by one sensor and overlay the direct oil signal as illustrated in Figure 5. The acoustic wave can be reflected by the tank wall, core, winding, flux shields and other components. Components of the reflected wave will arrive at the sensor position later than the signal travelling a direct path. Furthermore, the acoustic wave can couple into the transformer wall and propagate through the steel of the tank to reach the sensor. Due to the higher propagation speed in steel of about 3.000 - 5.000 m/s [3], the so-called steel wave signal can reach the sensor earlier than the waves

Page 4 of 13 following the direct oil path. This effect complicates the automated determination of the starting point of the direct oil signal.

Figure 4 Figure 5 Possible propagation paths in the DUT Acoustic PD signal components according to propagation paths

The measurable direct oil signal at the sensor position depends on the intensity of the causative PD event [4] and on the damping in the propagation path. Therefore, the attenuation by core, winding, transformer board, flux shielding etc. should be as low as possible. For that reason, the search for sensor positions that ensure good signal quality is essential during measurement procedure. The knowledge about the inner structure of the transformer is helpful for good positioning and repositioning of the sensors.

3 Localization of PD Different algorithms can be used to perform a time-based localization of PD. The input information used by the algorithms is the time of arrival of the signals propagating on direct oil path wave at multiple sensors. The exact time of arrival has to be determined by evaluating the measured signal. A criterion for the starting point can be found e.g. by investigation of energy steps [7] or by threshold criteria [8].

Amplitude Δ t1 , 2

t1 t2

Sensor S1

Sensor S2 Occurence of PD time

Figure 6 Absolute and relative times in a two-sensor-setup

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The relative arrival times at different sensor positions lead to time differences (Δt1,2). These time lags are the only available data in all-acoustic measurements, when the data acquisition is triggered by the acoustic signal at one of the sensors. If the time delay between occurrence of a PD and the arrival of the associated acoustic wave is available, the absolute propagation times (t1, t2) from source to sensor can be used for localization, Figure 6. The exact timing of the emission of the PD signal can be estimated e.g. by an electrical PD measurement according the IEC 60270 standard or a measurement in the ultra-high frequency (UHF) range. In the latter case, sensors within the transformer walls can be used to collect the high frequency electromagnetic wave that is emitted during PD [9]. The principle and a measurement setup are shown in Figure 7 and Figure 8.

Figure 7 Figure 8 Schema of the measurement setup for the UHF trigger UHF probe installed signal in a DUT

The distance between sensor and source is calculated using the available absolute or relative propagation times and an assumptive average propagation speed. With the determined distances and the sensor positions a geometrical localization of the PD source can be performed in several steps.

Figure 9 Principle of the acoustic localization

The arrival time at a single sensor and the timing of PD occurrence leads to the surface in the shape of a sphere around the sensor position on which the PD source is supposedly located, Figure 10. The radius r is proportional to the absolute propagation time (t1) and the propagation speed. In all-acoustic measurements the data of a single sensor does not contain meaningful information. In this case the data of two acoustical sensors - the relative time Δt1,2 – delivers a distance difference (Δd1,2) and therefore a hyperbolic sphere (Figure 11).

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Figure 10 Figure 11 Spatial information from one absolute time (t1) Spatial information from one relative time (Δt1,2)

The position of the source can be specified with the information of more sensors. For this purpose multiple of the described geometrical shapes are intersected. The absolute propagation time of the signal at a second sensor leads to a second sphere, the resulting intersection shape is a circular. In a further step the absolute coordinates of the source can be estimated by intersecting the circulars of three sensors. This procedure is shown in Figure 12. Figure a)-c) shows the spheres around three acoustic sensors, in section d) the resulting intersection circulars and the estimated point of the acoustic source are displayed. In an all-acoustic measurement environment the approach is in principle identical. In this case a fourth sensor delivers the necessary information to estimate a point.

a) Sensor A.1 (blue) b) Sensor A.2 (green)

c) Sensor A.4 (black) d) Resulting circulars by use of Sensors A.1, A.2 and A.4

Figure 12 Source localization with three sensors using absolute times

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The depicted method is based on a direct propagation path for the acoustic wave from source to sensor. As described above, the transformer must not be considered as an empty box and the propagation speed is highly dependent on the travel path. For that reason, the model is always a simplification of the real setup. Thus, an inaccurate or wrong localization of the source position is possible. To ensure reliable measurement results, a workflow is proposed that is based on an iterative relocation of the sensors with the intention to find positions with a minimal and undisturbed path between sensor and source. At the same time, the assembly of the sensors among themselves is restricted by a minimal distance and a proper placement in multiple axes.

4 Improving sensitivity and accuracy Due to the inherent inaccuracy of the assumed model, all results should be verified. This can be accomplished by multiple measurements with identical and varied sensor setups and software parameters. The outcome for different sensor setups must not be entirely congruent but the resulting cluster of points leads to the identification of incorrect or ambiguous results and to the proper localization of the affected part of the DUT, Figure 13.

Figure 13 Plot of multiple localization results to identify mislocation

In addition a verification of the recorded pattern and the identified failure location often can be done with potentially available data from the DUT manufacturer (inner structure) and other investigations like electric PD measurement.

a) Unfiltered Signal b) Filtered signal c) Filtered signal (zoom)

Figure 14 Effect of high-pass filtering of an acoustic PD signal

The signal quality and hence the sensitivity of the measurement can be enhanced by digital signal processing. High-pass, low-pass and band-pass filters can be used to reduce the impact of acoustic disturbances like core noise (Barkhausen effect), oil pumps or fans, Figure 14. A preliminary selection

Page 8 of 13 of the considered frequency range is made by the choice of the sensor type [3] as piezo-electric sensors have limited bandwidth. Figure 14 shows the effect of a 45 kHz high-pass filter on a measurement which was recorded with a sensor with 75 kHz resonance frequency. Another opportunity to raise the signal-to-noise ratio of recurring PD events is provided by the averaging technique, Figure 15 and Figure 16 [6]. The sample values of multiple PD events following each other are added and in further consequence divided by the number of iterations. The PD signal is constructively superposed with each addition. The superimposed noise is statistically distributed and therefore the corresponding sample values are different for each occurrence of PD. For that reason, an appreciable reduction of noise can be achieved with a sufficient number of summands (typically 10 to some hundreds) because of the partially cancellation of sample values. Due to the required correlation of the time windows this technique is limited to applications with a sufficiently stable electric or acoustic trigger source.

Figure 15 Figure 16 Principle of signal processing through averaging Outcome of the averaging algorithm at different by using an electrical trigger (acc. IEC or UHF) numbers of runs

5 Study case of localization The described investigation was performed on a 150 kV/20 kV three-phase power transformer with a nominal capacity of 16 MVA. Figure 17 shows the spatial computer model and the electric-acoustic test setup. The electric multi-phase PD measurement pattern (Figure 18) clearly indicates a fault nearby phase W.

Figure 17 Figure 18 Test setup (el. and acoustic PD test setup) Electric PD pattern

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A first measurement was performed with the sensor setup shown in Figure 19 to acquire a large-scale scan of the windings and the high voltage bushing of phase W. Positioning of sensors was substantially limited by cooling facilities and stiffeners on the housing. Signal quality on all sensors was insufficient for reliable determination of arrival times and therefore a localization of the discontinuity was not possible for this setup. Still, the high amplitude and the short absolute delay lead to the assumption that the PD source may be located near to the sensor A.3 (red). Therefore, all sensors were relocated to the relevant area.

Figure 19: Figure 20: Initial sensor setup Result of measurement for the initial sensor setup, sensors A.1-A.4

After sensor relocation the signal form on all sensors indicated propagation paths with little disturbance, although the signals on sensor A.1 (blue) and A.3 (red) contained clear steel wave components (Figure 21 and Figure 22). This characteristic was probably caused by the sensor positions on the top of the DUT that were unavoidable due to the construction of the transformer.

Figure 21 Figure 22 Final sensor setup Result of measurement with automatically computed arrival times for the final sensor setup, sensors A.1-A.4

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The computed spheres at each sensor were checked separately to be located inside the DUT with plausible radii. Subsequently the circular intersections of all possible sensor triples were reviewed, see Figure 23. The sensor combinations in a) and b) immediately provide nearby intersection points, c) and d) suggest a comparable area.

a) Sensors A.2,A.3,A.4 b) Sensors A.1,A.3,A.4 c) Sensors A.1,A.2,A.4 d) Sensors A.1,A.2,A.3

Figure 23 Circular intersections calculated with the automatically computed arrival times for all sensor triples

The starting times were calculated by using the energy criterion as well as the overall propagation velocity. These starting times were manually adapted to consider the steel wave characteristics on sensors A.1 and A.3, see Figure 24. Using the adapted parameters, identical intersection points for all sensor triples were found, see Figure 25.

a) Sensors A.2,A.3,A.4 b) Sensors A.1,A.3,A.4

c) Sensors A.1,A.2,A.4 d) Sensors A.1,A.2,A.3

Figure 24 Figure 25 Final results of measurement with manually Final results for all sensor triples adapted arrival times, sensors A.1-A.4

5 Conclusions This paper describes the basic idea of the method of time based acoustic localization. The PD signals are captured using three or more piezoelectric acoustic sensors, magnetically mounted on the tank at

Page 11 of 13 different locations. For localizing the source, the time delays between the recorded acoustic signals or between an electrical signal and the acoustic signals are used to get information about the propagation of the acoustic signal inside the transformer tank and the distances between signal source and sensors. The received sensor signals are filtered and processed to obtain the difference between the signal arrival times at each sensor. In addition to the acoustic measurement, electrical measurement can be used in parallel to obtain a trigger signal. This can be essential for the success of localization. Alternatively to the electrical measurement according the IEC 60270, unconventional measurement techniques e.g. in the UHF range, can be used to gain a trigger source for an acoustic measurement. A case study of PD localization is shown. The procedure and successes of an acoustic measurement with electrical trigger has been proved. The results are analyzed and visualized by using a 3D model.

References

1. IEC 60270 (2000) „High-voltage test techniques - Partial discharge measurements” International Electrotechnical Commission, Publication 60270, 2000

2. Tenbohlen, S.; Denissov, D.; Hoek, S.M.; Markalous, S.M. “Partial Discharge Measurement in Ultra High Frequency (UHF) Range” IEEE Transactions on Dielectrics and Electrical Insulation, Vol 15, No 6, Dec 2008

3. C57.127 (2007) “IEEE Guide for the Detection and Location of Acoustic Emissions from Partial Discharges in Oil-Immersed Power Transformers and Reactors”, The Institute of Electrical and Electronics Engineers, Inc. New York, USA, 2007 4. Bengtsson, T.; Leijon, M.; Ming, L. “Acoustic Frequencies Emitted by Partial Discharges in Oil”, 8th International Symposium on High Voltage Engineering, 1993, p.113-116

5. Howells, E. ; Norton, E.T.: “Parameters affecting the velocity of sound in transformer oil” IEEE Transactions on Power Apparatus and Systems (1984)

6. Lundgaard, L.E. „Partial Discharge – Part XIV: Acoustic Partial Discharge Detection – Practical Application“, IEEE El. Insulation Magazine Sep 1992 Vol.8,No.5

7. Große, C.U.; Reinhardt, H.-W.; (1999) „Schallemissionsquellen automatisch lokalisieren“ MP Materialprüfung, Jahrg. 41, pp.342, Carl Hanser Verlag, München, Germany, 1999

8. Wagenaars, P.; Wouters, P.A.A.F.; van der Wielen, P.C.J.M.; Steennis, E. F. “Algorithms for Arrival Time Estimation of Partial Discharge Pulses in Cable Systems” IEEE Vancouver, Canada, 2008

9. Coenen, S.; Tenbohlen, S.; Markalous, S.M.; Strehl, T. (2008) “Sensitivity of UHF PD Measurements in Power Transformers” IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 15, No. 6, pp. 1553-1558

Biography Stefan Hoek is with OMICRON electronics in Austria since May 2008. He works as product management with main focus on partial discharge analysis. He studied electrical engineering at the University Stuttgart (Germany) and worked as research assistant at the Institute of High Voltage

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Technologies (IEH). His research focus was on partial discharge localization in GIS with help of measurements in the UHF range. Rene Hummel is with OMICRON electronics in Austria since 2009. He works as an application engineer for partial discharge products, conducting trainings and giving on site support. He studied electrical engineering at the University of Technology in Berlin (Germany) and did research concerning the method of pulse sequence analysis (PSA) of partial discharges. Alexander Kraetge is with OMICRON electronics in Austria since 2006. He works as product manager for partial discharge products. He studied electrical engineering and received his PhD degree at the University of Technology in Berlin (Germany). Ulrike Broniecki studied electrical engineering at the TU Berlin (Germany) and works as research assistant at the department of High Voltage Engineering at TU Berlin since July 2008. Her research is focused on acoustic partial discharge measurement and localization. Benedikt Kästner is studying electrical engineering with focus on high voltage engineering at TU Berlin. Besides, he is with OMICON electronics as a student worker.

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