Courant continu tolerable dans les transformateurs de puissance

This research concerns the overheating of power transformers in the presence of direct current.;The temperature rise in power transformers subjected to a dc source of excitation was measured on core-form 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. The results show that the tie plates of this particular type of transformer are the components most susceptible to rapid temperature rise. Unfortunately, an irregular reading of the thermocouple measuring the mid-point of the tie plate was observed experimentally.;Therefore, tests on smaller 100-kVA transformers were performed to take a specific look at these tie plates under the effect of a temperature increase. A sufficient number of thermocouples were installed on the tie plates.;Then, finite-element simulations on a magnetic steel plate in a magnetic field similar to that created by the magnetizing current of a saturated transformer were conducted in order to establish a method of calculating the eddy current losses in the tie plates of large power transformers. Once it had been validated by the tests on 100-kVA transformers, this method was applied to the large transformers and the local losses were calculated as a function of the direct current level.;The results of the tests on the large transformers are used as a basis of comparison for analyzing the influence of direct current on the temperature rise in three-phase transformers (3-legged core-form, 5-legged core-form and shell-form transformers). The comparative analysis deals with the temperature-rise of structural parts inside the windings. The principle of duality was applied so as to obtain equivalent electric circuits of the magnetic circuits of the three-phase transformers. The electrical circuits thus obtained were validated by experimental results on small three-phase transformers.