Power transformer is a key element of the electricity network which allows for technological and cost-effective production, transmission, distribution and consumption of electricity. The transformer transforms the voltage level of network sections in a passive and reliable manner. Power transformer through its magnetic yoke and coils, transfers huge amounts of energy, namely complete power of the built-in section, and it is necessary that transformation losses are minimal. Transformers are up to today devices with the highest utilization suitable for this task. This paper elaborates the methods of determining the parameters of equivalent models of power transformers. Starting from the basic laws that apply to the transformer, simple schemes and models are shown that are obtained by neglecting certain influence, to more complex models which are currently used in practice and contain the appropriate amount of data for calculations of transformers, network and surrounding devices that depend on these parameters. The calculation is performed of parameters for one specific power transformer of 31.5 MVA output and 110/10 kV and the models are developed as previously shown in the papers.