448 ELECTRICAL EQUIPMENT 



In cases where it is impractical to apply the hot-air method for 

 drying, the " internal " or short-circuit method may be used. 

 The transformer should then preferably be taken out of the tank 

 or, otherwise, the manhole cover should be removed and the valve 

 in the base opened to give as great a circulation of air as possible 

 under the conditions. 



This method requires one winding to be short-circuited and a 

 voltage applied to the other so that sufficient current will flow in 

 the windings to raise the temperature to approximately 70 C. 

 The amount of current necessary to effect this temperature ranges 

 between one-fifth and one-third of the full-load current, depend- 

 ing upon the room temperature and the design of the transformer. 

 The impedance volts necessary to give the specified range in cur- 

 rent varies from 0.4 to 1.5 per cent of the rated voltage of the 

 winding to which the impedance voltage is applied. 



The temperature of the winding can be determined by the 

 increase in resistance, which is calculated as follows: 



Let R c = resistance at room temperature, or cold resistance; 



k = room or coil temperature for cold resistance; 

 Rh = hot resistance ; 

 tn = temperature of windings hot; 



then 



_fi>(238-K)-238fl c 



R c ' 



and rise 



= t h -t c . 



A simple method for determining the temperature of the wind- 

 ing is to assume that for each per cent increase in resistance the 

 temperature rise is approximately 2J C. 



The duration of the drying run depends upon the voltage and 

 size of the transformer and also upon its condition as to moisture 

 at the time it is dried. For transformers under 20,000 volts the 

 drying should be continued not less than twenty-four hours; 

 20,000 to 30,000 volts, 48 hours; between 30.000 and 40,000 volts, 

 seventy-two hours. Higher voltages may require longer. It is 

 obvious that some consideration must be given to the capacity of 

 the transformer. Transformers of less than 100 Kv.A. may only 

 require twenty-four hours. For transformers between 200 Kv.A. 

 and 500 Kv.A. the process may be limited to thirty-six hours; 



