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 By Stefan M. Hoek, Rene Hummel, Alexander Kraetge, Benedikt Kästner and Ulrike Broniecki Page 1 of 13 Page 2 of 13 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. Page 3 of 13 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 Page 5 of 13 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). Page 6 of 13 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.

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