424 ELECTRICAL EQUIPMENT 



when the low-voltage delta is closed and the maximum voltage 

 difference will be double line voltage. 



For delta-Y connection, such a reversal w.ll not produce a 

 short circuit when the Y is closed, but the voltages and phase 

 relations will be unequal. The maximum potential difference 

 will equal the line voltage. 



A reversal of one or two high-voltage windings with a Y- 

 delta connection will immediately produce a short circuit when 

 the delta is closed, and the maximum potential difference will 

 be double line voltage. 



With Y-Y connection the result of reversing a high-voltage 

 coil will be the same as for the delta-Y connection. 



Effect of Ratio on Parallel Operation. For successful parallel 

 operation, correct ratios between the high- and low-voltage wind- 

 ings of the different banks is, as previously mentioned, also essen- 

 tial, otherwise a cross-current will be established, even if the ratios 

 are only slightly different. This current is then due to the dif- 

 ference of the two voltages divided by the sum of the impedances 

 of the two transformers, and its effect is to balance the voltages of 

 the two transformers with a resultant equilibrium of the two 

 transformers. 



To determine this current, assume that e\ and z\ are the volt- 

 age and impedance in low-voltage terms of one transformer and 

 62 and Z2 are corresponding terms of the second transformer, con- 

 nected in parallel with the other. The circulating current would 

 then be 



Z1+Z2' 



where z\ and 22 are expressed in ohms. Or expressed in percentage 

 of normal current by the following formula: 



Per cent f = cent voltage difference Q 

 Sum of per cent impedance 



For example, suppose that the voltage ratios of two trans- 

 formers are such as to cause a voltage difference of 2 per cent. If 

 each transformer furthermore has a 2 per cent impedance, the 

 circulating current is equal to 



Per cent 7 = ^^X100 = 50 per cent, 



~ 



