Location Of Partial Discharges In Transformer Windings By Using UHF Method

Windings can be considered one of the core components of power transformers. Partial discharges (PDs) in windings are not only vital cause for breakdown fault, but also the early symptom of breakdown. For improving maintenance efficiency and keeping transformers’safety, detectition and location of PDs in transformer windings are of importance. For acoustic signals attenuated sharply by windings, PDs in windings cannot be detected precisely by existing acoustic emission (AE) system. Because of higher sensitivity, PDs can be located by using UHF method without metal obstacles. However, it is very difficult to locate PDs in windings for four key problems caused by windings:low detection sensitivity, serious error in estimated arrival time of UHF signals, algorithm failure caused by arrival time errors and invalid sensors’arrangement. To improve UHF location method for PDs in windings, the propagation of electric-magnetic (EM) wave are studied systematically and a novel methods are proposed to overcome the difficultis caused by windings.The time-domain propagation of EM waves radiated by PD pulse in windings is simulated using Finite Difference Time Domain (FDTD) Method. The attenuation, polarization model and time deley of EM waves caused by windings are analyzed. Three propagation features are revealed. Firstly, the attenuations of EM waves radiated by three PD current pulse are different. For instance, the attenuation reaches to 50dB when current pulse is perpendicular to the surface of windings, while the attenuation is only 0-10dB when current pulse is parallel to it. Sencondly, it is shown that the EM waves radiated by PDs in windings are linearly polarized. The direction of electric field strength is almost parallel to the pitch angle direction. Thirdly, it is found that the propagation route of EM waves through windings is a polyline. Arrival time along the polyline is later than the straight line between PDs and viewpoints. Lastly, these conclusions are verified by tests.For the linear polarization feature of EM waves radiated by PDs in windings, linearly polarized time-domain wide-band antenna is suggested as UHF sensor for location. Tie shape antenna with teminal load is developed. Experiments are carried out in laboratory to test this antenna. Tie shape antenna is more sensitive for detecting head wave passed through windings. The amplitude of head wave by tie antenna is 2.25 times of that by planar spiral antenna.For the attenuation and distortion caused by windings, the signal to noise ratio (SNR) of the head wave is too low to be found out. To solve this problem, the new arrival time calculation method based on multi scale analysis is proposed and experiments are carried out to verify it. In this method, one dimension wavelet transform for UHF signal is done firstly, the scales with bigger SNR are choosen, then the arrival time are calculated under these scales. Experiments carried out on a 220kV transformer show that the accuracy of TOA using this metohd is much better than that using other methods. The mean value of errors is just 0.21ns, while it is 1.02ns by accumulated energy method.To avoid algorithm failure caused by tiny time error, a innovative algorithm is raised, which is named curved solid intersection method based on Newton Iteration method in complex domain. Firstly, time delay compensation interval is constructed. Secondly, four arrival times are compensated by using time delay in this interval. Thirdly, PD locations are calculated using newton iteration method in complex domain, the part of real results are reserved. The 3-D space surrouned by these results is taken as location result. Experiments carried out on a 220kV transformer verified that 6 PD sources can be located more precisely using this method than that using mesh searching algorithm. The location errors are smaller than 0.3m and the mean value of it is 0.12m. When using the mesh searching algorithm, the biggest location error is 0.57m and the mean value of it is 0.21m.A novel sensor arrangement simulation method is suggested, which is used for analyze the feasibility of sensor arrangements. By using the curved solide intersection method, the location results include many points. The validity of location result is decided by the error of central point and the dispersity of these points, which is represented by standard deviation. The smaller the standard deviation is, the sensor arrangement is more efficient for this points. By simulation, it is concluded that star shape sensor arrangement is more efficient than rectangle and diamond shape sensor arrangement, what’s more, the longer distance between sensors leads to higher location pericision and 3-D array is better than 2-D array. Experiments carried out on a 35kV transformer verified that the 3-D star shape array has a higer precision than 2-D diamond shape array, which the former is 0.16m and the latter is 0.32m.Finally, a location system for PDs in windings are developed. Location tests are carried out on a new 220kV transformer and a high voltage reactor. Espcially, a real PD source is located on the top of the reactor. The dismantling examination agrees with the location result. The validity of UHF method were verified by the tests.