428 ELECTRICAL EQUIPMENT 



. . . , . . , per cent 7Z . 



But since Z is proportional to ,. -r we can write 



Where Zi, Z 2 , and Zs, are the impedances of the different legs. 



, . . , per cent 7Z 



>ortional to T ^ r we can 



Kv.A. 



/per cent 7Z\ /per cent 7Z N 



= \ Kv.A. ) 1 = 



/per cent 7Z\ /per cent 7Z 

 \ Kv.A. / 3 \ Kv.A. 



If I L = line current for any given balanced load, and 7i, 7 2, and IB 

 are the leg currents, with the same load, the ordinates of the curve 



represent the ratio of leg current to line current j^ = j^ an d T^> 



IL IL IL 



respectively. 



If, for example, we have three transformers connected in delta- 

 delta, with capacities and impedances as follows : 



Kv.A.i = 100, per cent 7Zi = 2; 

 Kv.A. 2 = 100, per cent 7Z 2 = 2; 

 Kv.A. 3 = 50, per cent 7Z 3 = 2.3; 

 Line voltage = 1000; 



we find that 



and 

 also 



= 0.40. 

 L 



If 7i = 100 amp., the normal current for that transformer, 



100 

 = 6~68 amp ' 



7 3 would then be equal to 147X0.40 = 59 amp. or 18 per cent 

 overload on leg 3. 



Again, if we assume that 7 3 = 50 so as not to overload leg 3, 

 then I L = 125 and 7i = 85 and legs, 1 and 2 are, therefore, carrying 

 only 85 per cent of their rated capacity. This means that without 

 any overload on any of the three transformers, the system can 



