404 ELECTRICAL EQUIPMENT 



thus causes an excessive stress on the windings. No triple fre- 

 quency harmonic appears, however, in the line voltage, which 

 remains normal, because the third harmonics across the three 

 transformers are in phase with each other. 



The triple frequency component does not exceed 75 per cent 

 of the fundamental and with densities commonly used has an 

 average value of 50 per cent of the fundamental. An exception 

 to this, however, is the case when the transformers are operated 

 with grounded neutral and connected to a transmission line pos- 

 sessing electrostatic capacity. In such a case the induced triple 

 harmonics may be intensified to values as high as two or three 

 times normal. 



To obviate the above increase in voltage, it is necessary to 

 make neutral connections in such a manner that the triple har- 

 monic exciting currents necessary for sine wave excitation can 

 flow, thereby eliminating the triple harmonic voltage. This is 

 accomplished first, when the transformer neutral is grounded, 

 and a Y-delta bank of transformers with grounded neutral of 

 sufficient Kv.A. capacity is connected to the line, second, when 

 the primary neutral is connected to the neutral of the generator, 

 this case only being possible for step-up transformers. It should 

 be noted that by grounding the high voltage neutrals of both step- 

 up and step-down transformers the danger from triple voltage 

 intensification is not eliminated. 



It should be kept in mind, however, that when such ground 

 connections are relied upon for eliminating triple third harmonic 

 voltages, such voltages are restored by disconnecting any ground 

 connection, and also that the third harmonic ground currents are 

 liable to subject parallel telephone or telegraph systems to serious 

 interference. 



The above does not refer to three-phase core-type transformers, 

 which, owing to their construction, are not subject to these addi- 

 tional strains. 



No stable neutral can be maintained on a bank of transformers 

 with both high- and low-voltage windings Y-connected when un- 

 grounded, since it may shift to any position. 



Open-delta. When single-phase or three-phase shell-type trans- 

 formers are used, it is possible to maintain operation if one phase 

 is damaged. Such a combination is shown in Fig. 248, and is 

 termed the open-delta or V connection. In three-phase core type 



