176 BELL SYSTEM TECHNICAL JOURNAL 



The use of the higher voltage circuits, each transmitting many 

 thousands of kilowatts, of itself tends to increase the problems of 

 coordination. However, the greater separations obtained by the use 

 of private rights of way for these main transmission circuits in most 

 cases eliminate the need for coordinative measures to control normal 

 induction (manifested as noise in the telephone circuits) and, in case 

 noise presents a specific problem, the greater separations simplify and 

 render less extensive those specific coordinative measures which may be 

 required. Induction due to power system abnormals too is mitigated 

 or rendered easier of control. 



In the case of distribution lines, the adoption of increasingly higher 

 voltages is accompanied by more systematic grades of construction 

 and greater clearances from communication circuits. The result, of 

 course, is that fewer abnormal conditions of operation occur and the 

 number of related disturbances in the communication circuits is cor- 

 respondingly reduced. The possibility of contact between power and 

 communication circuits is also reduced. This trend toward better 

 grades of construction applies also to transmission lines and, as noted 

 previously, to other parts of the power system. 



System Stability. — During recent years considerable attention has 

 been paid to the development of methods for improving system electri- 

 cal stability. One of the most important of these methods is the use of 

 higher speed switching, — at present, faults can be cleared in 15 cycles, 

 or less, of a 60 cycle wave. So far, high speed switching has been ap- 

 plied mainly to transmission circuits. However, as development 

 proceeds and cost of equipment required is reduced, the field of appli- 

 cation of high speed switching may naturally be extended to distribu- 

 tion systems. The result in the case of either transmission or distri- 

 bution will be, of course, to reduce the duration of transients. Akin to 

 high speed switching, the use of high speed excitation of rotating equip- 

 ment has been developed. This may tend to increase the maximum 

 fault current values somewhat which would make coordination more 

 difftcult. However, the reduction in the severity of instability surges, 

 in so far as such surges involve faults-to-ground, affords definite bene- 

 fits from the coordination standpoint. It requires further study and 

 observations to determine what, if any, inherent limitations or advan- 

 tages it may possess with respect to coordination work. 



The way has been paved for the development of high speed switching 

 by steady improvement in relaying practise. Selective operation of 

 protective relays in power systems, during the early stages of relay 

 development, was largely dependent upon an additive sequence of time 

 intervals which might aggregate a considerable period in the case of the 



