Identification of transformer magnetizing inrush current by use of dynamic impedance characteristics

To distinguish magnetizing inrush currents more effectively from internal faults in transformers, an identification strategy was proposed on the basis of dynamic impedance characteristics, which possesses very straightforward physical meaning. Theoretical analysis indicates that, the dynamic impedance presents unique features in extremely different style while inrush currents or internal faults occur in a transformer. In the case of inrush currents, the amplitude variation of the dynamic impedance is large together with average power density of each order harmonics. However, the internal faults are with relatively small amplitude variation in the dynamic impedance, and the power density of the odd-order harmonics is of very small values. The effectiveness to identify inrush currents through specific dynamic impedance features is verified by both numerical simulations and physical modeling tests, and concrete identification methodology of convenient implementation and easy computation is thereby given. Due to large tolerance ranges of the set valve values in both the time and frequency domains, high reliability and accuracy of transformer differential protection scheme are achieved.