Saturation time of transformers under dc excitation

This paper studies the saturation time of transformers under the effect of a de voltage bias. An approximation formula developed in an earlier project has been honed and validated. Comparison with EMTP simulation results allowing the stages of development of the saturation process over time to be reproduced bears witness to the capability of this formula. Different cases studied in order to describe the saturation phenomenon by de excitation, show how the formula is applied and point out its limitations. It is stressed that the saturation time on the power system can be very long (in the order of 1 min or more), when the de voltage applied to the magnetizing inductance is very low. The presence of low-resistance windings connected in delta can substantially increase the transformer saturation time. The flow of de current is not necessarily an indication that a transformer has reached saturation. (C) 2000 Elsevier Science S.A. All rights reserved.