| Transformer is one of the most core equipment in power system and traction power supply system,and its insulation reliability determines whether the whole system can operate safely and stably.Partial discharge(PD)monitoring is an important means to master the insulation status of the transformer.Localization of the PD source can significantly improve the efficiency of transformer operation and maintenance,and the acoustic emission(AE)method is one of the most widely used localization methods.In the field application,the following problems still exist in the PD source localization using the AE method: 1)Due to the imperfect manufacture workmanship and insulation degradation,multiple insulation defects are prone to appear,resulting in multiple PD sources inside the transformer,further causing the mixing of AE signals.2)Due to the influence of the transformer internal structure,non-direct path signal with changing propagation time will increase the localization error.3)Transformer is operating with uneven and gradientdistribution internal temperature,resulting in non-linear and non-uniform propagation of AE signal to the sensor,leading to further increase in localization error.To address above problems,this dissertation carries out theoretical analyses,experimental tests and method innovations,and the main research work and results are shown as follows.1)The PD experimental platform and the discharge signal joint acquisition system of acoustic method and pulse current method have been designed,and the measurements of AE signal under typical discharge conditions are realized to analyze the influence of defect type and voltage level on the AE signal waveform characteristics and localization effect.Different signal processing methods are tested and compared,and the signal processing flow is proposed.2)Methods of identifying the number of PD sources,separating the mixed AE signal,and selecting the PD source coordinates are proposed.For the characteristics of the PD source localization using the AE method,a method that can identify the number of PD sources in single-channel mixed AE signal is proposed,and the mixing mode of the AE signal is analyzed.Using the constancy of excitation time delay of different PD sources,a method of selecting the correct PD source coordinates is proposed.The effectiveness of above methods is verified through experiments and simulations.3)A weighted calculation method of the PD source localization is proposed to reduce the influence of non-direct path signal.The AE signal characteristics when the PD occurs in different areas are tested and analyzed.The accuracy of signal propagation path identification under different combinations of parameters has been tested,and the combination of parameters with the highest recognition accuracy is specified.The residual sum-of-squares function weighting matrix assignment rules under different propagation path identification results are proposed,which can effectively reduce the errors caused by the non-direct path signal and improve the localization accuracy.4)Considering the internal gradient temperature of the transformer,a PD source localization method is proposed.Propagation velocities of the AE signal under different temperatures are analyzed and tested,and the influence of the gradient acoustic impedance on the propagation characteristics of the AE signal is studied.A calculation method of the incidence angle of the main path under the bending propagation,a PD source localization method based on dynamic distance difference,and a framework of comprehensively localizing the PD source are proposed.Experiments and simulations have verified the effectiveness of above methods,and the accurate localization of the PD source based on the AE method has been achieved. |
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