492 ELECTRICAL EQUIPMENT 



The reason for grouping the transformers with the lines is to 

 avoid switching on the high-tension side of the transformers. 

 Paralleling of the high-tension side should also be avoided and this 

 applies especially to delta-connected transformers where the surges 

 set up by arcing grounds on one line may be transmitted to the 

 other line. With transformers having the high-tension windings 

 Y-connected and the neutral grounded, paralleling on the high- 

 tension side may not be so serious and may in certain cases be 

 advisable for the sake of flexibility. 



Means should, however, always be provided for transfer 

 on the high-tension side, and this may be done in various ways 

 as indicated in Figs. 303 and 304, in the former case simply by a 

 tie between the two lines and in the latter by a transfer bus. 

 Such means for transfer should also be arranged at intervals 

 along the transmission lines, perferably at substations (Fig. 305), 

 or places where branch lines are tapped to the main line. 



It is customary to make the generator circuit breakers non- 

 automatic, but for very large and important units it may be 

 desirable to protect them against internal short circuits, which is 

 readily accomplished by means of differential relays as described 

 under " Relays." 



The switch and relay protection of the transmission lines, tie 

 lines, etc., is very complicated and no general rules can be given 

 except to state that the protective features should be of such a 

 selective nature that when trouble occurs, the section involved 

 should be immediately disconnected without the dropping of 

 unnecessary load or power. The protective devices to accomplish 

 this depend entirely on the conditions involved and are best 

 explained by considering a few typical examples. 



Example I: This refers to a system as illustrated by the 

 disgram in Fig. 303 and consists of one generating station feeding 

 a single substation over two parallel transmission lines. 



All the high-tension line circuit breakers are non-automatic 

 and are only intended for sectionalizing purposes, as are the high- 

 tension tie breakers, which should be open under normal operation 

 so that the system would only be operated in parallel on the low- 

 tension side of the step-up as well as the step-down transformers. 

 The low-tension transformer circuit breakers in the generating 

 and substation respectively should be of the automatic type, the 

 former being provided with time-limit Telays and the latter with 



