Transformer Inrush Current Problems

How to discriminate between internal fault and magnetizing inrush current is the difficulty of transformer protection when apply differential relay.In this thesis, the mechanism of inrush current occurring in transformer is well researched, the advantages and disadvantages of several methods for checking inrush current are discussed, the theory of applying second harmonic current to lock the tripping operation of the circuit breaker during inrush current and its problem are analyzed. The principle of Au is analyzed in this thesis, and a new method to discriminate between internal fault and magnetizing inrush based onAu is bring forward in this paper. In this article, the simulation of inrush, internal fault and inrush which internal fault occur on at the same time and exterior fault is done by using PSB of MATLAB. The result of these simulations demonstrates that these two methods are easy, effective, and adapted to practical application.Based on the analysis of sympathetic inrush in operating transformers, the principle and characteristics of sympathetic inrush and influencing factors are studied. Then the influence on transformer protection caused by sympathetic inrush are discussed. Finally, several countermeasure to sympathetic inrush are proposed.The transient induced by the isolation of the external faults is analysised at last in this paper. It is pointed out the transformer differential protection and the current protection possibly maloperates due to the recovery surge.