The role of this thesis is building up a test environment with ATP-EMTP network simulation program, wherewith the automatic differential relay protections of main power transformers can be off-line tested. In preamble, it gives a comprehensive statement about the significance of power transformers and a description of differential relay functions.Thereafter the problems and problem derivations of relay protecting are particularized with the models implemented in the simulation environment. Through the network simulations the thesis details the phenomenon of inrush current of power transformers, the saturation of current transformers, demonstrates the effects of specified faults on network transformers and the consequences of a generator block transformer excitation as well as the occurence of sympathetic inrush current. The transient waveforms computed with simulations and the network conditions that contributed to these transients are evaulated from the point of view, certain events, such as breakers controlled switching in which case cause the least optimal inrush current and in this way how can be unexpected relay operation created.The evaluation of transient events is terminated with the description of playing back waveform testing method with OMICRON relay test set. In the further part of the thesis, a static test of a transformer differential relay is presented and finally the plans of a student laboratory measurement is sketched, based on the developed network model solutions.