TRANSFORMERS 189 



radiated from the outer surface of the tank. When the 

 load is taken off the transformers for resistance measure- 

 ments, always shut off the water. Complete the tests 

 as on other transformers, making careful inspection for 

 oil leaks. As the leads of many of these transformers are 

 brought out through the cover, care must be taken when 

 making connections to avoid dropping tools on the 

 porcelain bushings. 



These transformers are usually made for very high 

 voltages, and are often filled with oil that has been spe- 

 cially refined. The tank is exhausted of air while the 

 transformer is hot, and the hot oil allowed to slowly 

 flow in at the bottom of the tank. To heat up the winding 

 of these transformers, put one-half the full load current 

 through the primary winding, carefully measuring the 

 cold resistance. Take readings every half hour and cal- 

 culate the rise of temperature by resistance. When a 

 rise of 50 deg. C. is reached, decrease the current to 

 maintain the temperature while the tank is exhausted of 

 air. 



In water-cooled transformers with secondary coils 

 made of flattened copper tubing through which the 

 water flows, the amount of water flowing through each 

 section should be measured if all the sections are fed 

 from the same water head. If each section has a regula- 

 ting valve, these valves should be fully opened. Put on 

 a low reading pressure gauge, hold the pressure constant 

 by means of the valve in the main pipe and carefully 

 measure the quantity of water from each section for a 

 given time. Record the pressure and quantity per minute 

 through each section. Never apply a pressure of over 10 

 pounds per square inch to a transformer of this type, as 

 there is danger of opening the soldered joints. In taking 

 overload heat runs, always use the same amount of water 

 as for the normal load heat run. 



Oil-cooled and oil and water-cooled transformers 

 built for voltages above 75,000 have special high tension 

 leads which are filled with insulating material. These 

 leads must be carefully filled before potential is applied 

 to the transformer, and they must be kept filled. They 

 should be carefully watched for leaks. 



