Aims: The paper presents a simulation of a theoretical model of electromagnetic extreme regimes, idling and short circuit, in a three-phase power transformer. The main goal of this paper is to show in a quality way that the computer simulation of the obtained theoretical model using the software package MATLAB Simulink confirms the possibility of analyzing the extreme modes of transformers. Methodology: To solve the problem of the presence of a transformer as an element of power in the power system, the following are used: Analytical methods in which the transformer is presented as a network element - usually as a quadripole, numerical programming that includes dynamic programming and independent variables, and combined programming with integer values. In the process simulation on elements with given parameters and frequency, a comparison of the substitution schemes used so far in the theory of electric circuits is performed. The accuracy of these theoretical models can be confirmed by voltage and current diagrams in the adapted software package MATLAB Simulink. Conclusion: The presented theoretical model and the applied algorithm in the simulation are universal and can be used for different states in which a three-phase power transformer can be found. Problems that occur in the analysis of transient processes in electrical networks where the element is a transformer can be minimized using simulation methods. Changing any parameter in the electrical circuit requires a new calculation of the condition of the circuit from the very beginning due to the new initial conditions. Analytical methods combined with the simulation method were used in the research of extreme regimes, such as idling and short circuit. The verified simulation method realized using the adapted MATLAB Simulink program had a threefold purpose: to serve the calculation and simulation of quantities that can be obtained by testing or measuring, to establish the original algorithm and to verify the proposed method. Also, this type of simulation can replace the idle and short circuit test.