Current signal processing-based methods to discriminate internal faults from magnetizing inrush current

Two new methods, current change ratio (CCR) and percentage area difference (PAD) were proposed to solve a problem of how to distinguish between internal faults and inrush condition when transformer is switched on. This problem may delay operation or may mal-operate some protection schemes like deferential protection. The methods were concluded after observing and analyzing the behavior and shape of large number of both inrush and internal fault signals that had been obtained using a model transformer in a laboratory. The methods were practically tested on a three-phase transformer with rated power of 20 kVA at Cardiff University's laboratory and the data were processed using LabVIEW and MATLAB programs. The results showed that internal faults can be correctly distinguished from inrush condition within a short time (from 5 to 10 ms), particularly the minor internal faults such as the interturn fault which is submerged to inrush current and make it is too difficult to be detected. The advantages of these algorithms are simple in design and faster than the second harmonic method which is the most popular method used for solving this problem.