Study On A Method For Transformer Differential Protection Based On Fractal

Nowadays, with the construction of global energy interconnection, the transformer of higher voltage and larger capacity put into operation and higher requirement on the transformer relay protection device is to be called. Longitudinal differential protection can identify the inner fault and outside fault accurately.However, under the circumstances of inrush current and saturation of current transformer, differential protection will produce unbalanced current which can lead to maloperation of differential protection device. Therefore, the identification of inrush current and anti-saturation algorithm of current transformer is still the research focus of transformer differential protection.The thesis studies the influencing factors of differential protection from inrush current of transformer and saturation of current transformer. To establish the model of current transformer saturation, inrush current and the inner fault and outside fault of transformer by PSCAD/EMTDC software, then acquire differential current of different circumstances.Through the mechanism analysis and simulation results of transformer inrush current, the thesis come up with two algorithms based on box dimension and grille fractal. Based on the essential difference between inrush current and inner fault in waveform, difference of box dimension value of the signals can identify inrush current. If the box dimension value is less than threshold then the signal is inrush current or else the signal is inner fault current. And, based on the variation of box number, if the signal have only one large changing point in box number variation, it's inner fault current or else it's inrush current. By the simulation of MATLAB software,it proves that the algorithm can identify the inrush current quickly and accurately and can simplify calculation and easy to realize.Based on the theoretical analysis and simulation results of current transformer saturation, the outside fault current waveform present discontinuity under the circumstance of current transformer saturation while the waveform of inner fault is continuous. Using the grille fractal can locate the singular point, the outside fault current own two singular point in each period. The simulation proves that the algorithm is reliable and accurate.In conclusion, the thesis study the algorithm of transformer differential protection creatively based on fractal and prove the accuracy, reliability and rapidity of algorithm by simulation experiment.