The quasi direct currents, which flows through the winding of power transformer, could cause the migration of working point from the knee of the magnetizing curve to the zone of saturation. In that way, the operational mode of power transformer changes. To account the effect of quasi direct current on the operational mode of power transformer was described the model of winding of power transformer with endless magnetic core and cylindrical magnetic cover. Compare the results of accounting model with the model of winding without effect of cylindrical magnetic cover the 20% difference in the characteristics of inductance from magnetizing current in the area of magnetizing currents under 100A was achieved.

Keywords: power transformer, saturation, modeling, the power transformer winding, electric power supply system, quasi direct current, power line, geomagnetic storm, space weather