Research On The Longitudinal Differential Protection Of Transformer And Magnetizing Inrush Current Characteristics

The fast development of the national economy and more and more serious energy and environmental issues accelerate the construction pace of the power grid. As one of the most important part of the power industry, the substation is key of electric power transmission and the power transformer is the most important equipment in the substation. The reliable opera-tion of the transformer is essential for the stable operation of the power grid. Differential pro-tection is the main protection of the transformer, which needs to remove kinds of faults selec-tively, rapidly, reliably and sensitively. It has great application value and practical significance to study the operating characteristic performance of the transformer protection. The main dif-ferential protection and the magnetizing inrush current characteristics are researched, modeled and simulated in this paper.Firstly, the current situation of energy and grid development and the importance of the reliable operation of the transformer in the power system are analyzed. The necessity of stud-ying the differential protection and magnetizing inrush current characteristics are put forward. Varieties of inrush current identification methods are summarized. Secondly, the basic prin-ciple of differential protection is introduced. The treatment methods of phase compensation, magnitude compensation and the elimination of zero sequence current are applied to the CT secondary current to realize the "equivalent" effect based on the Yndll type three-phase transformer. The conversion methods are given based on the Y side and the A side respec-tively. The reasons of the unbalanced current in the steady state are summed up. The steady ratio restraint protection which is used to avoid the unbalanced current on differential protec-tion performance is discussed. Thirdly, the generation mechanism of the magnetizing inrush current and its characteristics are deep studied based on the transformer working principle. Mathematical model of the relationship of the inrush current amplitude and no-load closing angle based on the single-phase two-winding transformer is obtained. A transformer differen-tial protection simulation system is modeled in PSCAD/EMTDC software to study the influ-ences of no-load closing angle, transformer capacity, transformer knee voltage, primary vol-tage to the magnetizing inrush current. Lastly, a case of the transformer magnetizing inrush current in Jiaxing Dongzha substation is analyzed using the simulation model.