Research on identification between inrush current and internal faults of power transformer based on H-S transform

In order to solve the problem of misoperation of transformer differential relay due to the inrush current, inrush current and internal faults must be discriminate effectively. This paper presents a novel approach by using hyperbolic S-transform (HST) which is a very powerful tool for non-stationary signal analysis giving the information of transient currents both in time and in frequency domains. The signal was transformed to phase space by using HST and the features extracted, the time-frequency contours are obtained after HST, it is found that the time-frequency contours in case of inrush current are different from that in case of internal faults. By calculating the energy and the ratio of energy level 1 and level 9 we found that ratio 6 is the threshold between inrush current and fault current.