The paper describes in detail the function of the differential protection of the numerical relay device type RET670 of the company ABB. At the same time, the theoretical basis of the work of (stabilized) differential protection of energy transformers is presented, with a special emphasis on the specifics of its implementation in a numerical relay device type RET670 (such as, for example, a fault locator, recognition of the transformer being plugged into idle mode, etc.). The paper also presents the influence of various design and operating factors of power transformers on the vaporized operation of differential protection. Finally, the paper presents the method of parametrizing the mentioned protective function of the numerical relay device RET670 on a characteristic example of a two-winding and three-winding (three-phase) power transformer.