STAR VERSUS MESH CONNECTION in 



180 C.), and not give off any appreciable quantity of inflammable 

 vapour. 



For very high voltages anything exceeding 30,000 volts the 

 oil-insulated type of transformer is the only one practicable. 



57. Star and Mesh Connections for Transformers. 

 Comparison of Single-phase and Polyphase 

 Transformers for Polyphase Circuits 



The coils of transformers intended for three-phase work may be 

 connected either mesh- or star-fashion. The mesh connection is 

 usually preferred, for if one of the coils should burn out, causing its 

 fuses to blow, the supply of current to the corresponding phase would 

 still be maintained, the remaining two coils taking up the load. But 

 with a star connection the failure of one of the coils would evidently 

 cut off the supply to two phases one of the line wires becoming 

 entirely disconnected. Thus a breakdown in the latter case would 

 have far more serious consequences than in the former. On the other 

 hand, when dealing with very high voltages, the dielectric stress on 

 each coil is reduced by adopting the star connection, since the phase 



p.d. then becomes only = 0'577 of the line p.d. ( 16). 



The relative advantages and disadvantages of a single three- 

 phase transformer as compared with three independent single-phase 

 transformers have frequently been discussed. For smaller sizes (up 

 to about 200 k.w.) a single three-phase transformer is both lighter and 

 cheaper, for a given output, efficiency, temperature rise, and regula- 

 tion,* than three single-phase transformers. For very large trans- 

 formers, on the other hand, the cost is lower in the case of three 

 single- phage transformers (the weight is, as before, greater than that 

 of a three-phase transformer). A three-phase transformer has the 

 advantage over three single-phase ones of maintaining better balance 

 of the p.d. on the three phases, owing to the interlinking of tho 

 magnetic circuits.! 



58. Auto-transformer or Compensator 



In addition to the ordinary transformers already considered, which 

 have distinct primary and secondary windings, there is another type, 

 known as the auto-transformer. In this the primary and secondary 



* Sec Chapter X. 



t Eborall, " Howard Lectures on Polyphase Electric Working," p. 40 (1902). 



