THE DESIGN OF STATIC TRANSFORMERS 



where a bank of air-blast transformers was located in a base- 

 ment right below the switchboard. In installing the switch- 

 board, various undesirable materials fell through into the 

 transformers, which had been carelessly left with their top 

 dampers open. Whether due to this initial cause or not, con- 

 siderable trouble was subsequently experienced with burn-outs 

 of these transformers. One of them actually overheated to the 

 extent of igniting its insulating materials, and the combustion 

 was intensified by the air-blast. Yet to shut down the air blast 

 under such circumstances would also intensify the heating. 

 My own opinion is that when the air-blast type is used, it is 

 necessary to take the very greatest care. Thus even the con- 

 tingency of rats destroying the insulation or causing other 

 damage is by no means remote, and has constituted a serious 

 danger in certain important installations. The manufacturers 

 of air-blast transformers direct that all accessible parts of such 

 transformers shall be given a systematic cleaning at intervals 

 of once a month or thereabouts. This cleaning should include 

 getting at the inside of the transformer and wiping away dirt 

 and dust from the coils and from the spaces between them, 

 Even if the transformer is protected from the entrance of any 

 foreign matter through carelessness, nevertheless an undesirable 

 amount of dirt and fine dust will always gain access to the 

 interior as it will accompany the circulating air. Compressed- 

 air plant, enabling a considerable pressure to be directed 

 against remote parts, will be of much assistance in cleaning 

 air-blast transformers. 



Forced-Water-Cooled Oil-Transformers. In transformers for 

 outputs of 500 to 5000 kva or more, the most-widely-employed 

 design is that in which the active material is deeply immersed 

 in oil and has, arranged above it, a coil of cooling pipes through 

 which water is circulated. Such transformers are usually 

 enclosed in a heavy boiler-plate casing. The general external 

 appearance is well illustrated by Fig. 95 which relates to a 

 Westinghouse 10 000-kva three-phase 66 000- volt transformer 



