398 ELECTRICAL EQUIPMENT 



When speaking of the voltage and current or line voltage and 

 line current of a three-phase system, without further qualifica- 

 tions, the delta-voltage and the Y-current are understood. 



Delta-connected transformers must be wound for the full-line 

 voltage but for only 58 per cent line current. The windings must, 

 therefore, have a greater number of turns than for star connec- 

 tion, while they can be of a smaller size. 



The maximum insulation stress in case a permanent ground 

 occurs does not exceed the normal voltage stress, provided the 

 ground is at the transformer terminals. When, however, the 

 ground occurs on the transmission line at some distance from the 

 transformer terminal the reactance drop due to the charging current 

 adds to this stress. On this account with long distance high-volt- 

 age transmission lines operating on the delta-delta system, a dead 

 ground of one wire may cause the potential of the other two wires 

 to rise above ground considerably above normal potential, thereby 

 increasing the insulation stress. This increased stress may exist 

 both at the generating and receiving ends of the transmission line. 



With a delta-connected 220-volt distributing system the ground 

 connection should be made to the mid-point of the winding of one 

 transformer. This gives 110 volts to ground from the phase wires 

 next to the ground connection and about 200 volts from the other 

 phase to ground. 



Where 2200-220 volt transformers are connected delta-delta 

 for three-phase power service, one of the units is occasionally made 

 larger than the other two, and a tap from the middle point of the 



low-voltage winding brought out so that a 7T v ft single-phase 



three-wire service may be obtained for lighting purposes. 



If one transformer or one phase of the three-phase transformers 

 is disabled, the other two may then be used in open-delta. 



The capacity of a group of delta-connected transformers is 

 equal to VsxEXl Kv.A., where E represents the transformer or 

 line voltage and / the line current. The current in the trans- 



former windings is equal to =. 



V3 



Delta-Y. Delta- Y connection or vice versa, as shown in Fig. 

 243, is used to a great extent, and it is especially convenient and 

 economical in distributing systems, in that a fourth wire may be 

 led from the neutral point of the low-voltage windings. 



