TRANSFORMERS 457 



should also be provided for draining off the oil in case of emer- 

 gency. The movement of the oil may be accomplished either by 

 applying compressed air to the tanks or by means of the motor- 

 driven pump of the filter press or other separate pumps. 



Occasionally an intermediate oil tank is provided and installed 

 on the main floor or gallery at an elevation that the oil can be 

 drawn into any of the transformers by gravity. The oil is then 

 pumped from the storage tanks in the basement after being fil- 

 tered. A motor-driven air compressor and vacuum pump may 

 also be required, being operated as a vacuum pump for exhausting 

 the air from the transformer cases so that the oil may be drawn 

 into the same, or as a compressor for pumping in air in the inter- 

 mediate storage tank to assist gravity in emptying the same. 



Cooling Water System. The design of the cooling water 

 system depends on the nature of the development, i.e., whether 

 low-head or high-head, and also on whether a sufficient continuous 

 water supply can be obtained. This is not the case in many sub- 

 stations and under such conditions it becomes necessary to pro- 

 vide cooling ponds and reservoirs. The water from the pond is 

 pumped to the transformers and after passing through the cooling 

 coils it is returned to the pond, where it is cooled. This may be 

 effected either by a spray or by providing a basin of such dimen- 

 sions that a sufficient cooling is obtained by a radiation of the heat 

 from the water to the air. The latter method is much superior 

 to the former in which air is liable to be carried along with the 

 water, causing a rapid oxidation of the iron cooling coils. 



For the generating station transformers it is customary to 

 take the cooling water from the forebay or from the penstocks. 

 In the former case it may be necessary to provide pumps for con- 

 veying the same through the cooling coils. For high-head develop- 

 ments where the pressure may be too high for the cooling coils, 

 a reducing valve must be installed, but this is, as a rule, not neces- 

 sary in low-head plants or with iron cooling coils which can with- 

 stand a much higher pressure than copper coils. 



The water should be taken from at least two separate intakes, 

 and it is needless to say that it must be free from silt and sus- 

 pended particles. For this reason strainers should be provided 

 before it enters the distributing headers, and these strainers should 

 be so arranged that they can be readily removed and cleaned. 



