Predetermination of self-cooled oil-immersed: Transformer temperatures before conditions are constant

The temperature rises of dynamo electric machines, in changing from one steady thermal state to another, follow an exponential law of the form 1 − ∊−βt, where 1/β is the thermal time constant, or the time required to attain 63. 2 per cent of the final change in temperature rise. It is shown here that while the winding temperature rise over room of self-cooled oil-immersed transformers follows this law only after a certain time has elapsed, quite accurate results may be obtained by calculating the time-temperature curves of the top oil temperature rise above room, and the winding temperature rise above top oil separately, since each follows the above exponential law quite closely, then adding them together to get the winding temperature rise above room at any time before conditions are constant. A procedure is explained in detail for calculating β, for either the lop oil rise or the winding rise above top oil from the weights of materials the iron and copper losses and the winding and top oil constant temperature rises at any given load. Comparisons of temperatures by test and calculation are presented.