Research On Inrush Current And Its Identification For Transformer

The main protection of the power transformer is the differential protection, the theoretical basis of which is Kirchhoff's current law. However, the characteristics of power transformers make this kind of protection based on Kerchief's current law less effective and reliability. Along with the development of the modern power system, the requirements of transformer differential protection are becoming higher and higher. However, to a large extent, the problem of inrush current and sympathetic inrush restricts the performance of transformer protection. After the analysis on the problems of the differential protection, inrush current and sympathetic inrush, which are influencing the characteristics of transformers differential protection, are deeply researched in this paper.For a long time, inrush current is the key reason that makes transformers differential protection incorrect action. How to correctly and quickly identify inrush current and internal fault current is always very important and difficult. In this paper, the mechanism of inrush current occurring in power transformer is well researched; the advantages and disadvantages of several methods for identifying inrush current are discussed. Based on the instantaneous reactive power theory, a new method is proposed to identify the inrush current and the internal fault current by collecting current and voltage signals in both side of transformer to calculate instantaneous power and comparing the variation of DC component ratio of active power to reactive power of transformer differential power. It avoids the interior of transformer complex flux linkage relations, and reveals essential difference between the inrush current and the internal current from the viewpoint of energy conservation law. In this paper, the simulation of inrush current, internal fault and inrush current associated internal fault is done by using MATLAB. The simulation results show that the new principle is simple and feasible, and it has clear identification ability. Instances of sympathetic inrush resulting in the malfunction of transformer differential protection are emerged in power system in recent years. To solve this problem, the mechanism of sympathetic inrush is deeply analysis, and the simulation model of sympathetic inrush is developed by using MATLAB in this paper. Through theoretical and simulation analysis, various of factors which affected sympathetic inrush are studied, the impacts and dangers of sympathetic inrush for transformer differential protection are discussed, and the corresponding preventive measures are proposed.