Identification of Magnetizing Inrush Current for A Power Transformer Based on the Magnetic Flux

During the inrush of magnetization current in a power transformer, when it is energized, differential protection with a restraining characteristic may not function correctly due to high levels of differential current. To eliminate this problem and maintain high sensitivity, modern microprocessor-based protection devices use blocking based on the ratio of second harmonic level to first harmonic, comparing it with a setpoint of 10-20%. In some cases, the level of the second harmonic during an inrush in magnetization current may be lower than the lower limit of this setpoint, or fall within the range of blocking due to an internal fault in the transformer. The paper proposes a method for detecting the mode of magnetization current inrush based on magnetic flux and differential current. The main idea of the approach is to construct a blocking zone based on the characteristic of the dependence of the magnetic flux on the differential current. Choosing the settings for the trip and reset the developed blocking are described. The proposed method for identifying magnetization current inrush has been tested on a model implemented in RTDS, in modes of internal and external short circuits, as well as magnetization current inrushes and others. Based on the results of all the tests conducted, a conclusion was drawn about the speed and correct operation of the developed method.