Researches On Noval Principles Of The Transformer Differential Protection Based On Waveform Similarity

With the development of the EHV/UHV,the application of large-capacity power transformer is more and more extensive.And the reliability of protection actions configured by transformers is of vital importance.As the main transformer protection,the correct operation rate of the differential protection has been affected by the inrush current and CT saturation.In order to improve the performance of transformer differential protection,this paper has carried out the following research work:Based on the analysis of the similarity between the transformer differential current waveform and the sine wave waveform under various working conditions,two new principles of the transformer differential protection based on waveform similarity identification algorithm are proposed.The first is a new principle of the transformer differential protection based on the Bhattacharyya coefficient.This method uses the histogram technology to extract the waveform distribution characteristics of the differential current,and uses the Bhattacharyya coefficient to characterize the matching degree between the differential current and the predicted sine wave waveform distribution histogram,so as to distinguish the fault differential current and inrush current.The second is the new principle of the transformer differential protection based on the Hausdorff distance algorithm.Combined with the advantages of the Hausdorff distance algorithm in calculating the similarity of graphs,this method uses the Hausdorff distance value to quantify the similarity between the standardized differential flow sequence and the standard sine wave sequence,so as to effectively distinguish the inrush current and the fault current.Based on the PSCAD/EMTDC software platform,an accurate simulation model of the transformer system was established to verify the effectiveness of the two new principles under the operating conditions including the closing switch without load,the internal faults and CT saturation.Then,an 800 V double-power three-winding transformer dynamic mode test system was established in the AC-DC-Flexo Hybrid Power Grid dynamic mode laboratory of the three gorges university to further verify the performance of the two principles.