178 Hh.l.L SYSTEM TI-.CIINICAL JOURNAL 



by governmental regulation, dependence is extensively placed on the 

 Petersen coil as a substitute. This device may be regarded as a special 

 type of neutral impedance. The Petersen coil has been applied to but 

 limited extent in this country although its possibilities for moderate 

 voltage systems, especially for situations warranting only single circuit 

 supply, are receiving consideration. 



In this country, the increasing use of neutral impedance as well as 

 the use of other types of current limiting devices is an aid to coordina- 

 tion since it reduces the magnitude of abnormal induction. 



Lightning Control. — The major problem of the transmission art at the 

 present time is the control of lightning in its effects on service. In 

 those sections of the country in which lightning is prevalent, this 

 natural hazard accounts for a large proportion of transmission circuit 

 faults, approaching 100 per cent in the case of the heavier, higher class 

 trunk transmission lines. The seriousness of this problem and the 

 researches which some of the larger power utilities and apparatus 

 manufacturers are conducting for its solution are being fully reported 

 from time to time before the Institute and need not be discussed here. 

 It is suflticient to say there is encouragement that methods for the solu- 

 tion of this problem, as it affects high voltage trunk circuits, will be 

 known in the not too distant future. Where adequate methods are 

 found and applied the results, of course, will be a decrease in the num- 

 ber of system disturbances which induce transients in communication 

 circuits. 



Present measures in power system practise, especially at the higher 

 voltages, directed toward the control of service interruptions caused 

 by lightning include improved application of overhead ground wires, 

 improved grounding connections at the supporting structures, the 

 improved use of wood for lightning insulation, and the use in shunt with 

 line insulators of fused gaps or other valve devices to "spill" the surge 

 without dynamic current follow up. There is also under consideration 

 the application on grounded neutral systems of single-phase switching. 

 All of these measures, with the exception of the last, are helpful from 

 the coordination viewpoint since their effect is to avoid or reduce sys- 

 tem faults or at least to decrease the magnitude of earth fault currents 

 and hence of the accompanying voltages induced in nearby communica- 

 tion circuits. 



Single-phase switching involves the use of individually controlled 

 and operated single-phase circuit breakers. Upon the occurrence of a 

 single-phase fault-to-ground, the breakers on the faulty phase only 

 would open, leaving the other two-phase conductors in circuit to main- 

 tain connection momentarily between source and load. In a short 



