Research On Criteria And Improvements Of Identifying Inrush Current For Transformer Differential Protection

A new protective method and improvements of discrimination between transformer inrush current and internal fault current are brought forward in this paper. 1) A new method of fast discrimination between transformer inrush current and internal fault current based on the half-cycle moving and comparison method is presented。Based on the mathematical characteristic that the absolute value integration of the sinusoidal waveform is the same in any half-cycle, the transformer inrush current and internal fault current can be distinguished by comparing the ratio of the half-cycle integral areas before and after a selected time interval. Its operating time is less than one fundamental cycle. 2)Improvements of the theory of 2nd harmonic and waveform symmetry used in discriminating between transformer inrush current and fault current by fuzzy recognition are presented. Taking advantage of the feature of inverse time, it is in order to improve the operating speed of transformer differential protection when turn-in-turn fault happens and avoid the mis-operation when unobvious inrush current happens.