[ 71 ] 



No. 9. 



THE ELECTRICAL DESIGN OF A.C. HIGH TENSION TRANSMISSION 



LINES. 



By H. H. JEFECOTT. 



[Read June 27. Printed Auar.sx 4, 1922.] 



1. The recent Reports on the Water-Powev Resoui'ces of the British Islands 

 have directed attention to the very important economic possibilities of hydro- 

 electric developments in these countries. 



Compared with the total estimated power available, very little has yet been 

 done in the direction of harnessing the rivers. It is hoped that, as a result of the 

 recent investigations, active steps will be taken to bring about a great extension 

 in the development of this valuable national asset. 



It appears, therefore, to be opportune to direct attention to some of the varied 

 problems that are met with in hydro-electric engineering. 



The present paper gives an account of a method for calculating the per- 

 formance of high tension transmission lines, based on direct evaluation by 

 complex quantities. In it the phase angles and magnitudes of the various vector 

 quantities have not to be separately considered, and the process is almost entirely 

 arithmetical. The method is straightforward, and can largely be systematized 

 into an arithmetical routine, which greatly simplifies the procedure. 



The method is illustrated in its application to Ihree-phase overhead trans- 

 mission lines. It may also be applied to single-phase working. 



2. The problem of the electrical design of a transmission line presents itself 

 usually in this way. At one site — the sending end — A.C. electric power is 

 generated, transformed to high tension, and put into the line. 



At a second site — the receiving end — the power is to be employed, and usually 

 step-down transformers are located. The number of phases, the frequency, the 

 voltage, and the power to be delivered are known at the receiving end. The 

 power factor of the load is also given, or is approximately estimated. The hours 

 during which power is to be used and the fluctuations in the demand throughout 

 the year are also presumed to be known approximately. 



The distance between the two sites is measured, and allowance is made for 

 increased length of the wires due to sag, and also to irregularities in direction of 

 the line both horizontally and vertically. 



It is then required to find the best size of conductor to use in the line, and, 

 the size having been decided on, to determine the efficiency of transmission and 

 the variation of voltage at the receiving end under varying load. 



3. The choice of size of conductor is governed largely by the consideration that 

 the total annual cost involved in its use shall be the least possible. It must 

 always be provided, however, that the size of the conductor falls within the 

 limits imposed by electrical consideratioiis, such as ohmic heating, voltage 



SCIENT. PROC. K.D.S., VOL. XVII, NO. 9. O 



