Study of the acceptable DC current limit in core-form power transformers

The temperature rise in power transformers subjected to a dc source of excitation was measured on coreform single-phase 735-kV autotransformers rated 370-MVA and 550-MVA. The measurements were compared to the temperature rise values obtained on the same transformers operating in overexcitation at 1.95 T. The results show that the tie plates of this particular type of transformer are the components most susceptible to rapid temperature rise. Smaller-scale tests on 100-kVA transformers were performed to take a specific look at these tie plates under the effect of a temperature increase. Finite-element simulations combined with analytical studies of temperature rise were performed in an attempt to determine a tolerable dc current limit based on permissible temperature standards.