GENERAL CONSIDERATIONS 271 



Transmission Voltage. The transmission voltage should be 

 chosen to insure the most economical ensemble. Many factors 

 affect the problem variously, and their nature makes a mathe- 

 matical expression difficult and, as a rule, unsatisfactory. The 

 distance of the transmission is naturally the factor which governs 

 the choice of the voltage to the greatest extent. The cross-section 

 area and, consequently, the weight to the transmission conduc- 

 tors, varies inversely as the square of the voltage for a given load. 

 The cost of the conductors is, therefore, reduced 75 per cent every 

 time the voltage is doubled, and it would, consequently seem 

 proper to use the highest voltage possible in any given case. 

 Though with increasing voltage, the cost of the conductors de- 

 creases, the cost of other apparatus and appliances increases. 

 This involves transformers, switching equipment, lightning 

 arresters and line structure and insulators, while, of course, the 

 necessary safety requirements become stricter with higher 

 voltages. 



With very high voltages and long lines the capacity current of 

 the lines becomes considerable, especially in sixty-cycle systems, 

 and may reach values higher than the full-load current. Its 

 greatest objection is that it loads the generators with current which 

 represents no power, and where small units are used it may often 

 render it impossible to throw one machine on the line alone. 

 Much more serious, however, is the impairment of the voltage 

 regulation incident to very long lines, i.e., the voltage variation 

 between no load and full load, especially for inductive loads. By 

 providing synchronous condensers, it is, however, possible to 

 compensate for the wattless currents and improve the regulation. 



Another factor which has a limited bearing on high potentials 

 for transmission purposes is corona, as experience has shown that 

 if the voltage on a given line is raised beyond a certain point the 

 air at the surface of the conductors breaks down as an insulating 

 medium and becomes luminous. The most serious objection to 

 corona comes from the losses, which increase at a high rate as the 

 voltage is raised above this luminous or so-called visual critical 

 point. This critical voltage increases with the size of the con- 

 ductors and their spacing, and by properly choosing these values 

 the losses may be materially reduced or entirely eliminated. For 

 high altitudes corona starts at lower voltages and this should be 

 given careful consideration (see section on " Station Wiring." 



