A fundamental approach to transformer thermal modeling - Part I: Theory and equivalent circuit

A simple equivalent circuit to represent the thermal heat flow equations fo r power transformers is presented. Key features are the use of a current so urce analogy to represent heat input due to losses, and a nonlinear resisto r analogy to represent the effect of air or oil cooling convection currents . An interesting historical note concerns the fact that the foregoing effec t was first quantified in 1817, It is shown that the idea of "exponential r esponse" is not the best way to think of the dynamics of the situation. It is also shown that one can consider ambient temperature to be a variable in put to the system, and that it is properly represented as an ideal voltage source, Physical experimental verification, on an in-service 250 MVA transf ormer, is described in a companion paper.