134 THE DESIGN OF STATIC TRANSFORMERS 



oil-cooled transformer to the average external temperature of 

 the case ma}' be very high, especially in some designs employ- 

 ing large amounts of oil for cooling purposes. The ratio 

 varies in commercial transformers from 1,4 when the circulation 

 is good, up to 2,0 on transformers with had circulation. The 

 ideal figure for this ratio would obviously be unity. A 

 difference of temperature will be observed between readings of 

 thermometers placed on the outside of the case and of ther- 

 mometers placed in a corresponding position inside the case. 



The temperature rise is generally specified as the average 

 from the primary and secondary windings when taken by the 

 resistance method, and as the maximum thermometrically- 

 deternrined temperature of the oil. This is generally found to 

 be at a point just above the top of the core. 



Contrary to the generally-accepted view that the results 

 obtained by the resistance method of measurement of the 

 temperature of the windings are higher than those obtained 

 by thermometric measurements, the reverse may be the case 

 in transformers employing large amounts of oil. There is 

 usually a very considerable difference in temperature between 

 the oil at the top and at the bottom of the tank in oil- 

 immersed transformers of very large capacities, and this 

 difference depends upon the size and shape of the actual 

 transformer in relation to the size and shape of the case 

 within which it is placed. When the temperature-rise of the 

 coils is measured by thermometers, the measurements relate to 

 the hottest part of the windings, since that part of the 

 windings which is nearest the surface of the oil is usually the 

 part in which the thermometers are placed. It may conse- 

 quently be safely assumed that if the temperature of this part 

 of the windings is within the guaranteed limits, the tempera- 

 tures of all other parts will also be within these limits. In 

 large oil-cooled transformers, more especially when they are 

 placed in very deep cases, it is often found that the tempera- 

 ture-rise of the windings, when calculated by the observed 



