TRANSFORMERS 427 



load between any number of transformer banks operating in par- 

 allel on single-phase circuits. 



/ Kv - A - } 



j = \per cent IZ/i 



\per cent 7Z/ i \per cent /Z/2-f-; . . . 



/ Kv.A. \ 



j \per cent 7Z/ 2 , 



12 ~ / Kv.A \ / Kv.A. \ 



\percent/Z/i \ per cent /Z/2-f; . . . 



where /i = load current in transformer bank No. 1; 

 /2 = load current in transformer bank No. 2; 

 I L = line current for any given load ; 



'-== ) = capacity rating of bank No. 1, divided by its 



per cent IZ/ 1 



per cent impedance; 



( ' ) = capacity rating of bank No. 2, divided by its 



\per cent IZ/2 



per cent impedance. 



The above formulae are, however, only correct when the relative 

 ratio between the resistance and reactance of all the transformers 

 are equal. If not, the sum of the individual load currents will be 

 greater than the current in the line, due to a phase difference 

 between the currents in the different transformers. The error 

 introduced by the inequalities in the values of this ratio is gen- 

 erally so small that it can be safely neglected. 



For delta-delta connected transformers the effect of different 

 impedances is also an unequal division of load among the three 

 transformers. The curves of Fig. 268 show the relation of current 

 in the three legs of the delta, assuming two legs always to be alike 

 in percentage impedance and capacity. The abscissa? represent 

 ratio of impedances of like legs to the odd leg. 



