<p>Connecting of electrical devices with different resistances leads to voltage deviation in power transmission lines, which is considered as one of the important elements of a supply power system. Increase in voltage deviation may cause tank heating and excessive loss of active power. Preventing of these situations and reducing the damage to power transformers as a result of voltage deviations in the power transmission lines as well as minimizing the power loss in the power transformer systems are regarded as one of the relevant tasks nowadays. The purpose of this study is to determine additional power loss that occurs in power transformers as a result of voltage deviations. The magnetic core and coils of an oil power transformer in operation were selected as a subject of the research. An experiment carried out on the TM-100/10/0.4-U3 oil power transformer with a connection scheme D/⅄0 -11, with voltage deviation in its upper and lower clamps as an additional factor in its active parts, enabled calculating the power dissipation, using the developed software. Moreover, it was found that the power dissipation in this power transformer, operating in different load modes, depends on its load factor.</p>