6 TRANSMISSION LINE FORMULAS 



Fig. 2. It is evident that the voltages E and ,, and the 

 power factors cos 6 and cos </>, at the two ends of the line, 

 are not the same in value. 



A long transmission line acts as a condenser and this 

 fact also must be taken into account. A condenser con- 

 sists of two electrical conductors placed close together but 

 insulated from each other so that a direct current cannot 

 pass between them. However, if an alternating voltage 

 be applied between them, a charge of electricity propor- 

 tional to the electrostatic capacity of the condenser will 

 flow into and out of the conductors. The result is that an 

 alternating current will appear to flow between them, pro- 

 portional to the capacity susceptance of the condenser. 

 This current, called the charging current, will be 90 

 out of phase with the voltage, and, unlike most currents 

 in ordinary practice, it will lead the voltage in phase, in- 

 stead of lagging behind it. The amount of the charging 

 current may be determined by means of the tables of 

 capacity susceptance of transmission lines, in Part III. 



A current in phase with the voltage will flow between 

 the conductors, but it is only noticeable at very high 

 voltages. Part of it is a leakage current flowing over the 

 insulators, and part is a discharge through the air, and 

 produces the glow called corona, on high-voltage con- 

 ductors. 



The elements of a transmission line accounting for the 

 leakage current and charging current are shown in Fig. 3, 

 in which resistances and condensers are shunted across the 

 line all along its length. 



Considering for the present that the voltage of the line 

 is the same at all parts and is equal to E, the current in 

 phase with E flowing across from one conductor to the 



