32 CONSTANT-VOLTAGE TRANSMISSION 



per, since doing so would not make more than a small 

 improvement in the regulation. The reason for this is 

 that the regulation, P R + Q X , depends mainly on the 

 reactance X, which is several times larger than the 

 resistance R, for the larger transmission lines, and the 

 reactance is practically the same for different sizes of 

 conductors. On the other hand, with constant-voltage 

 operation the maximum load is determined generally 

 by the efficiency, and so it depends more on the resist- 

 ance R. Greater loads can, therefore, be carried by 

 constant-voltage lines made up of larger conductors. It 

 will be economical where large blocks of power are to be 

 transmitted, to make use of conductors as large as 

 350,000 or 400,000 circular mils, copper, instead of 

 building several separate tower lines with small conduc- 

 tors, as is necessary with the usual varying-voltage 

 method of control. It may be noted that still larger 

 sizes of conductors than those mentioned would not gen- 

 erally be economical, owing to the theoretical limit of 

 the load of a line, which is only encountered with ex- 

 tremely large conductors, and which is described in 

 Chapter XII. When this limit is approached, the num- 

 ber of phase modifiers required becomes excessively 

 great. It may thus be stated that one advantage of the 

 constant- voltage system is that it allows the use of large 

 conductors, which increases considerably the power 

 capacity of a single line, and so produces additional 

 savings in the cost of towers and land. 



Since high reactance is not a very great disadvantage 

 in designing a constant- voltage line, and is in some ways 

 an advantage, since it reduces short-circuit currents, a 

 very wide spacing between conductors may be used. 



