This final paper describes the differential protection of power transformers and its application in the transformer protection system. The basic requirements for differential protection are given, so that it can function as such in the system in the event of a fault or dangerous condition in the power system, and without interrupting the power supply to the end consumer, if possible. In the second chapter, in order to better understand the importance of the power transformer in the system, a basic description of the transformer is given. The classification of transformers, types of transformers are briefly described, and a characteristic diagram of the transformer with the corresponding markings is given. In the third chapter, dangerous operating conditions of the power system and types of failures of the power system are described, which in any case represent a danger to the safe functioning of the transformer. Both the power system and the transformer are exposed to various atmospheric discharges, mechanical damage, insulation damage, etc. on a daily basis. In the fourth and fifth chapters, the importance of differential protection of power transformers is described. The specifics of power transformer differential protection, static and numerical differential relays, and when and why stabilized differential protection is used are briefly presented. In the fifth chapter, multifunctional relays from Koncar, ABB and Siemens are described and some of their possible functions are shown.