This paper explains power transformers, their application, division and function in the power system. Their principle of operation, basic laws and relationships are also explained. Special attention is paid to the isolation of power transformers and their testing and maintenance. Each transformer differs according to power, voltage and performance, and there are numerous different versions of their insulation. The condition of the insulation is very important for the reliable operation of the transformer. By checking the condition of the insulation, it is also possible to predict failures that often cause large costs. There are several types of insulation testing that can be classified into two basic types: direct current testing and alternating current testing. Testing with alternating current gives a much better insight into the state of the insulation because it enables the testing of inter-turn insulation, and the insulation itself is loaded with alternating voltage during operation. However, the equipment required for alternating tests is much more expensive and bulky, which makes it unusable in field tests. In the aforementioned paper, several results of field testing of isolation of oil distribution transformers with direct current are also listed and explained. The purpose of the work was to acquaint the student with the equipment for testing the state of insulation of electrical devices, its application, and familiarization with distribution transformers, their application, importance in the power system, and maintenance of the transformers themselves.