A New Model for Analysis of Unusual Mal-Operation of Differential Protection Due to the Ultra-Saturation Phenomenon during the Loaded Power Transformer Energization

In this paper, a new model based on the thevenin equivalent circuit for investigating the ultra-saturation phenomenon during the energization of a loaded power transformer is presented and its effect on the differential protection of the transformer is considered. The ultra-saturation phenomenon causes the mal-operation of the power transformer differential protection. So, the description and control of the ultra-saturation phenomenon is necessary for preventing of the false trip of the differential protection.  In this paper, to model the ultra-saturation phenomenon, the nonlinear characteristic of the transformer core, the effect of current transformer, and the core losses are taken into account. It is assumed that the load of the transformer is a resistive and inductive load. Also, the effect of the residual flux and inception angle on the ultra-saturation phenomenon are investigated. The results show that the ultra-saturation phenomenon poses a great problem for protective relaying of power transformers. The outcomes of this research can further be used as hints for substation operation personnel as well as for the development of new protection stabilization criteria, which is not discussed further in this paper. The explanation of the ultra-saturation phenomena is the first step toward developing new ideas and criteria for more reliable transformer protection that would better handle such abnormal cases than currently employed relaying equipment. In this paper, the fourth-order Runge-Kutta method is used to solve the equations and simulation of the ultra-saturation phenomenon is done by MATLAB program.