Review of the Subject.-Part I of the paper takes up first altitude. For transformers where a part of the loss is by radiation the division of losses by radiation and convection from tall vertical (unaffected by air density) the effect is reduced by the ratio of the planes. It is shown that the loss by convection from tall planes convection loss to the total loss. Also the effect on the winding rise can be expressed by the formula over the ambient is further reduced by the fact that the winding rise WC= 0.0014 A 0 5/4 over the oil is not affected by altitude. The effect of the copper loss, in which W, is the watts, A the square inches of surface and 0 the however, is to increase the effect of altitude because the resistance is temperature rise in degrees centigrade.increased by temperature.Based upon the above facts it is shown that the temperature rise '
May 1925COONEY: SELF-COOLED OIL-IMMERSED TRANSFORMER TEMPERATURES 611 the value of an insulating material. Of all the electrical tests even though the workmanship is perfect. All of the joints tested that we made on the various types of joint, we found that the were made by the same man so that the quality of workmanship dielectric-loss test was the most sensitive, and that it gave the was undoubtedly the same in all cases. In none of the joints most consistent and conclusive results. Experience has shown that we opened and examined was it possible to say that that a joint designed for normal dielectric strength and low the cause of the failure was due to poor workmanship. dielectric loss will stand up better under severe operating con-Unfortunately, when the earlier work was done on the joints, ditions than one with high dielectric strength and high dielectric we had only single-phase voltage available. Some of this work loss.has been repeated with three-phase voltage with substantially All the joints tested were opened and carefully examined and the same results. Dielectric strength tests at high and low photographic records made of the paths of carbonization. A few temperatures, show very little variation, especially if the results of the joints were tested with radio apparatus installed in the are comparative. The joints that show the h;gher dielectric ground circuit, and the tests stopped at the first signs of internal strength at low temperatures, invariably have the higher dielecdischarge. It was from a careful study of the paths of carboni-tric strength at high temperatures.zation that the weak spots in the cable joints were found and the We feel that the proper design of a joint to be put into a cable design changed so as to eliminate them as far as possible.installation is well within the province of a cable manufacturer, We have not discussed the question of workmanship in this but that the question of workmanship is a matter with whichl paper. A poorly designed joint will not give satisfactory results, the operators should deal.Synopsis.-The temperature rises of dynamo-electric machines, temperature rise above top oil separately, since each follows the in changing from one steady thermal state to another, follow an above exponential law quite closely, then adding them together to get exponential law of the form 1 --t, where 1/O is the thermal time the winding temperature rise above room at any time before condiconstant, or the time required to attain 63. 2 per cent of the final tions are constant. A procedure is explained in detail for calculachange in temperature rise. It is shown here that while the winding ting fi,for either the top oil rise or the winding rise above top oil from temperature rise over room of self-cooled oil-immersed transformers the weights of materials the iron and copper losses and the winding follows this law only after a certain time has elapsed, quite ac-and top oil constant temperature rises at any given load. Comparicurate results may be obtained by calculating the time-tem...
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