Abstract :
The question of thermal transference in transformer oil was brought forward by Mr. W. Pollard Digby in 1915, ¿ and further elaborated in a report of the I.E.E. Research Committee. ¿ Suggestions were there made for an experimental investigation to verify the conclusions reached, and the research reported herein, besides achieving this, has produced further useful information. The cooling power of three transformer oils has been investigated by means of a special apparatus, representing transformer conditions in a simplified form for experimental work. Attention has been given both to the temperature-rise of the oil and to that of the windings. It has been found that classification of oils according to temperature-rise of the fluid does not necessarily yield the same order of merit as classification according to temperature reached by the windings. The physical properties of the oils have been measured, and an attempt has been made to correlate the cooling data with these. It appears that the group of variables (ac/v) is important in both cases. Here a, c and v are respectively the coefficient of expansion of the coil, its specific heat per unit volume, and its kinematical viscosity. The extreme variations of viscosity with temperature limit the precision with which this information can be applied to predict the relative performance of very similar fluids. The temperature-rise of the windings differs from that of the fluid, in that it is influenced by the thermal conductivity of the oil. Further, the influence of the thermal conductivity in this case is probably greater than the influence of the group of properties represented by ac/v. The work was carried out, at the National Physical Laboratory, by Messrs. W. F. Higgins and A. H. Davis.
