Continuous monitoring, timely and high-quality diagnostics are basic conditions for ensuring reliability and uninterrupted availability of power plants. The security of supply of electrical energy depends on energy transformers that work efficiently and safely. The focus of this thesis is the diagnostics of the power transformer. The topic is examined from both empirical and theoretical aspects and in the introductory part, energy transformers are categorized and a technical description of the structure of the transformer is provided. In the empirical part of the paper, presentations are given of (1) functional tests and adjustments of equipment for control, monitoring and protection of transformers; (2) diagnostic testing of the transformer and (3) continuous monitoring of the transformer. The contribution of this final paper is reflected precisely in the empirical part in which the author himself is an active participant as a test engineer. Namely, the sixth and seventhchapters describe in more detail the transformer diagnostics and monitoring system that is part of the ATR3 autotransformer in TS Konjsko, on which the diagnostic tests, which are described in detail in the sixth chapter, were carried out. At the end of the final paper, provided as attachments, the results of testing the autotransformer ATR 3 in the field before the first commissioning of the transformer as well as the results obtained by diagnostic methods in April 2024 are presented. To summarize, monitoring and diagnostic methods are reliable tools development of which is certain, especially in the atmosphere of new technologies and upcoming artificial intelligence. The latter can be the direction for continuing this professional work within scientific field.