This final paper analyzes the induction transformer, its equations, the vector diagram of an ideal and real transformer, and explains the properties of transformer oils. Special emphasis is placed on oil-immersed transformers and oil. Power transformers are most often oil-immersed transformers whose windings, together with the core, are placed in a boiler with oil. Whether it is mineral or synthetic oil, they have their own requirements and physical and chemical properties that need to be well known. By checking the oil, it is possible to predict future failures and avoid major damage. The measurement method used in the paper is measuring the breakdown strength of transformer oil with a Megger OST 80PB. All results presented in the paper were obtained from field testing. This method shows that adding water (water from the atmosphere and cellulose paper) has a negative impact on the dielectric strength of the oil. The quality of the oil deteriorates and it is necessary to replace or treat the oil. The purpose of this work was to familiarize the student with equipment for testing transformer oil, distribution transformers, their application, importance in the power system and maintenance of the transformers themselves.