Thermal Overload Tests on a 400 MVA Power Transformer with a Special 2.5 P.U. Short-Time Loading Capability

Comprehensie load/ thermal testing was performed on a 400/400/125 MVA transformer representing a new design generation for large power transformers with a short time emergency loadability of 2.5 p.u. This particular overloadability factor required special design features to control the very high leakage flux resulting from such a loading condition and hence ensure that the transformer satisfies existing standards limits on hot spot. This paper presents these design features along with a description of a full size transformer that was built for the sake of developing these flux control measures. Hot-spot temperature measurements in the windings and in the outer core packets and structural parts under a number of overloading conditions are presented and discussed. The results confirm the soundness of the design to handle the 2.5 p.u. overloadability while satisfying the hot-spot requirement according to the existing loading guides for 1.5 p.u. overloading. The measurements also show that the hot spots will rise much faster than what an exponential function based on the time constant of top oil would predict.