Title :
Temperature responses to step changes in the load current of power transformers
Author :
Nordman, Hasse ; Rafsback, Niclas ; Susa, Dejan
Author_Institution :
ABB Oy, Vaasa, Finland
Abstract :
Comprehensive load/thermal testing was performed on a 400/400/125-MVA, ONAF-cooled transformer. It was shown that the local hot-spots in the windings, core and structural parts rise much faster at load increase than what an exponential function based on the time constant of top oil would predict. This paper analyzes these fast rises more in detail for hot-spots in windings, core, yoke clamps, and tie plates. An alternative mathematical model of the winding hot-spot response is presented. Different cooling modes, like ONAF and OFAF, are dealt with. Also, different modes of oil circulation through the windings, in horizontal (zig-zag) or axial cooling ducts, are compared. The results are verified by fiber optic installations and tests on several large power transformers in the range of 250-650 MVA. Hot-spot responses predicted by the IEEE Loading Guide, Annex G, are also compared to measured values.
Keywords :
cooling; power transformer testing; thermal analysis; transformer cores; transformer windings; 125 MVA; 250 to 650 MVA; 400 MVA; IEEE Loading Guide Annex G; OFAF-cooled transformer; ONAF-cooled transformer; axial cooling ducts; core hot spots; eddy losses; horizontal cooling ducts; leakage flux; load current step changes; load testing; mathematical model; oil circulation; overloading; structural parts hot spots; temperature responses; thermal testing; tie plate hot spots; time constant; top oil; winding hot-spot response; winding hot-spots; yoke clamp hot spots; zig-zag cooling ducts; Clamps; Cooling; Oil insulation; Performance evaluation; Petroleum; Power transformers; Temperature; Testing; Thermal loading; Transformer cores;
Journal_Title :
Power Delivery, IEEE Transactions on
DOI :
10.1109/TPWRD.2003.817516
