TRANSMISSION LINE 399 



quantities are fixed by other considerations than the reactance 

 and reactance drop. 



257. Capacity. The capacity of a line per mile of conductor 

 between the conductor and neutral is 



00 O 



,10-* farads (Equation 45). 



D-R 



This value applies for each conductor of a single-phase or poly- 

 phase line. If the conductors of a three-phase line are suspended 

 in one plane instead of in the form of an equilateral triangle the 

 capacity of the central conductor is slightly greater than that of 

 the others, but since all lines are transposed the total capacity of 

 each of the three is the same and is given with sufficient accuracy 

 by the formula above if the distance D is taken as the shortest 

 distance between conductors. 

 The capacity reactance per mile of conductor is 



X c = 7 ohms, 



and the charging current per mile of conductor is 



where e is the voltage between conductor and neutral. 



For transmission lines up to 50,000 volts the capacity is very 

 small and its effect on the regulation may be neglected. If, how- 

 ever, any part of the transmission is carried out through under- 

 ground cables, the capacity may be very largely increased and may 

 not be negligible. Above 50,000 volts the capacity of the line 

 must be considered in calculating the regulation. For lines up to 

 100 miles in length and for voltages up to 100,000 volts the^capacity 

 of each conductor may be considered as a condenser connected at 

 the centre of the line between conductor and neutral. If more 

 accurate results are necessary the fact that both the reactance and 

 the capacity of the line are distributed over the whole length must 

 be taken into account. 



Due to the presence of the charging current in a line the current 

 flowing into the receiving circuit may be very much larger than 

 the current entering the line at the generating station. 



