TRANSFORMERS 



429 



carry only 125 amp. line current or 87 per cent of the rated capacity 

 of the three transformers. 



At the point where r = 0, we have the current in legs 1 and 2 

 equal to the line current, giving the condition of open delta. By 



T Ratio of Impedances 



FIG. 268. 



decreasing the capacity of leg 3 to zero, which is the same as 

 increasing its impedance to infinity, we have but two legs on which 

 to carry the three-phase load. 



With delta- Y-connected transformer banks a small difference 

 in the per cent impedance has, as for the voltage ratios, a negligible 

 effect. For example, if two transformers having impedances of 

 6 per cent are connected in delta-Y with another transformer 

 having an impedance of 3 per cent, the potential of the neutral 

 point will be shifted at full load by an amount approximately equal 

 to one-third of (6%-3%) or 1 per cent of the normal voltage of the 

 transformer. 



Mechanical Design. For self-cooled power transformers of 

 moderate capacity the tanks are generally made of corrugated 

 sheet steel (Fig. 269), the bottom of the top edges of which are 

 permanently cast into the base and the top rim simultaneously 

 with the pouring of the castings, thus forming a perfectly cast- 

 welded joint. For larger sizes tubular tanks are usually supplied. 

 These are of the plain steel-plate construction with a* number of 

 wrought-iron tubes, so arranged with connections at top and bot- 

 tom as to allow a natural circulation of the oil between the tank 



