Study On Measures Of Avoiding Mal-Operation For Transformer Differential Ptotection

Power transformers are essential and important elements of power systems, and the safe operation of power transformers directly determines whether the power system works safety or not. At present, the main protection of the power transformer is the differential protection. Unbalanced currents generated by transformer inrush current and current transformer (TA) saturation are two major factors. In addition, the increase of power system short-circuit fault capacity and time constant results in TA saturation more seriously. Two core issues caused transformer differential protection to mal-operate are studied in the paper.Firstly the basic principle of transformer differential protection is elaborated in this paper, several methods of identifying inrush current and TA saturation are introduced and sophisticated analyses and evaluations are given.This thesis is focused on the two critical problems of transformer inrush and TA saturation. On one hand, the generation mechanism and characteristics on the transformer inrush current is analyzed in detail, based on simulation model for transformer switching-on without load. On the other hand, the TA saturation is studied in this paper by MATLAB simulation. Based on this, some conclusions of TA saturation features are summarized. Moreover, major factors that affect current transformer saturation are analyzed.Difference method is simple and easy to implement, but the poor stability. Relative to the lack of difference, a method of area comparison is proposed in this paper. On the basis of combination of both, a new method of detection for transformer differential protection against CT saturation is proposed. The method can effectively prevent the current differential protection external mal-operation at the time of CT saturation. The simulation results clearly indicate that the proposed method is correct and effective.