Cooling of oil-immersed transformer windings after shut-down

The 1916 A. I. E. E. Rules require that the temperature rise of transformer windings be observed by the resistance method. Since the measurement of resistance usually requires considerable time, there is always a drop in temperature between the instant of shut-down and the time of observing this resistance measurement. There are three general methods of determining the temperature at shut-down. These are: 1. To take a cooling curve and extrapolate back to the instant of shut-down. 2. To use an arbitrary correction. 3. To calculate the rate of cooling. The usual theoretical formula for calculating the cooling of a body is not in a convenient form for practical use. Furthermore, the conditions in a transformer are generally such that it would be difficult to apply. However, it is shown in the paper that the cooling of oil-immersed transformer windings, for a limited time (four or five minutes) after shut-down, is approximately a function of the watts per lb. of copper, and that when it is necessary to make calculations, more accurate results can be obtained by this partially empirical method than by attempting to use the theoretical formula. Under “Conclusions”, the general advantages and disadvantages of each method are given. In the Appendix are developed certain formulas used in the paper.