388 ELECTRICAL EQUIPMENT 



can be connected to the 10 per cent tap, thereby compensating 

 for the line drop. As this tap is used when the load is greatest 

 it follows that, theoretically, the taps should be of full capacity; 

 i.e., the current carrying capacity of the high-voltage winding 

 should correspond to the lower voltage value. Often, however, 

 reduced capacity taps are specified, and reliance is placed on the 

 ability of the transformer to carry the increased current safely. 



o 



5% Tap 



2^ Ta 



FIG. 226. 



On account of the low-voltage winding the capacity of the trans- 

 former is, however, based on the full rated voltage. 



Sometimes large power transformers have their high-voltage 

 windings so arranged that the two halves can be connected either 

 in parallel or series. The former connection corresponds to 

 only half the voltage of the latter and is for use during the first 

 period of operation of a system when the load is light and when the 

 lower operating voltage is sufficient. When the load has increased 

 so as to necessitate a higher voltage, the two windings are con- 

 nected in series, thereby doubling the transmission voltage. 



Transformers are sometimes arranged for supplying simul- 

 taneously two loads, one at full voltage and the other at half 

 voltage. The question then often arises as to how much each 

 side may be loaded without causing overheating of the trans- 

 formers. This can readily be ascertained from the curves in Fig. 

 227. For example, with a full voltage load of 75 per cent it is pos- 

 sible to load the half voltage circuit for 40 per cent current, which 

 is equal to 20 per cent the capacity. 



Where taps are not essential for the satisfactory operation of a 

 system, they should be avoided as much as possible, especially in 

 very high-voltage transformers, and standard practice does not 

 contemplate any taps for voltages below 6600, nor above 66,000. 

 It is evident that taps are difficult to insulate and bring out to the 

 connection board and that they, therefore, introduce additional 



