Control of unusual mal-operation of three-phase power transformer differential protection due to ultra saturation phenomenon based on Clarke׳s Transform and Discrete Wavelet Transform

The ultra-saturation phenomenon is one of the transient phenomena that may lead to the false trip of the power transformer differential protection during the energization of a loaded power transformer. This paper presents, at first, a model of a loaded three-phase power transformer by using and combining magnetic and electrical circuits and then, a new algorithm is presented to control the unusual false trip of three-phase power transformer differential protection due to ultra-saturation phenomenon based on Discrete Wavelet Transform (DWT) and Clarke׳s Transform. In this algorithm, the transient phenomena of three-phase power transformer including the magnetic inrush current, the ultra-saturation phenomenon, and the external and internal faults of power transformer are simulated. To distinguish between these phenomena, appropriate criteria using Clarke׳s Transform and DWT by the use of energy coefficients and standard deviation of coefficients are presented. Also, to model the ultra-saturation phenomenon, the nonlinear characteristic of the magnetic core and the effect of the saturation of the current transformers are taken into account. It is assumed that the load of the transformer is a resistive and an inductive load. The results show that the control of the ultra-saturation phenomenon is necessary for preventing the false trip of the differential protection.