CHAPTER XXI 



OIL-COOLED, AND OIL- AND WATER- 

 COOLED TRANSFORMERS 



The order of tests on oil-cooled transformers is the 

 same as that for air blast transformers. The trans- 

 former should, if possible, be filled with oil at least four 

 hours before starting the tests; if not possible, the cold 

 resistance, polarity, ratio, checking of taps and impedance 

 tests may be taken before. Under no condition, however, 

 must an oil-cooled transformer of over 10,000 volts be 

 operated at normal potential without being filled with oil. 



Cold Resistance 



If the transformer has not been filled with oil, a 

 thermometer should be suspended inside the tank to 

 measure as nearly as possible the temperature of the 

 windings. If filled with oil, record the temperature of 

 the oil. Always use a spirit thermometer to obtain 

 the temperatures inside the tank. As the leads are 

 not brought out in the same manner as in the air blast 

 type, the circuits should be carefully checked before 

 starting the tests. 



Heat Run 



The methods and connections used are practically the 

 same as for air blast transformers, except that oil-cooled 

 transformers should be started at an overload, so as to 

 heat them up rapidly and thus shorten the run. Where 

 practicable, they should be run with 50 per cent excess 

 current for two hours, and 25 per cent excess voltage for 

 three hours. In some cases the time of overload run must 

 be shortened, though occasionally a longer time is 

 required. When normal voltage is applied, the alter- 

 nator must be operated at normal voltage. 



During the heat run, a careful search should be made 

 for oil leaks in the tank and oil gauges. If the trans- 



[185] 



