Differential protection improvement of power transformers using wavelet analysis of power system frequency

This paper presents a new technique for inrush current detection of power transformers. The method is based on wavelet analysis of power system frequency which is estimated by Prony method. The Prony method is utilized to estimate power system frequency with three consecutive samples rapidly. After that, Daubechies wavelet family uses the estimated frequency to investigate the unbalanced conditions in power system. Meanwhile, a new time based criterion is introduced to determine internal fault currents from inrush currents by using the changes of estimated power system frequency. Extended simulations for various faults and inrush conditions demonstrate precise operation of the proposed algorithm also in CT saturation.