46 BELL SYSTEM TECHNICAL JOURNAL 



For comparison purposes, the voltage to ground of the sound phase 

 with the reactor omitted {i.e., with the system isolated) has been cal- 

 culated, using the same constants as before. The result is 



v' = - ll.GOOe"^^-^' cos 7310/ + 37,350 cos S77t . 



This is practically identical with the earlier result; that is, the voltage 

 to ground of the sound phase is practically the same with the reactor 

 as with the system isolated. 



3. Effects with Respect to Induction in Neighboring Communication 

 Circuits 



An estimate of the value of the Petersen coil must involve a com- 

 parison with other methods of grounding the neutral (including 

 grounding through an infinite impedance, i.e., the isolated system) or 

 of otherwise limiting the effects of abnormal occurrences in a power 

 system. As regards the induction of fundamental frequency voltages 

 in exposed communication circuits, the methods of chief importance 

 in such a comparison, at least so far as American practice is con- 

 cerned, are that in which the neutral is grounded either directly or 

 through a low resistance, and that in which the neutral is isolated. 



When accidental grounds occur on a power system with neutral 

 grounded through zero or a low impedance, the resulting heavy short 

 circuit currents to ground may produce severe disturbances in ex- 

 posed communication circuits. Owing to the fact that these dis- 

 turbances are produced by electromagnetic induction in a circuit 

 consisting of the communication conductor as one side and the earth 

 as the other, they cannot be avoided by enclosing the communication 

 conductors in lead-sheathed cable, even when this is placed under- 

 ground. 



With the Petersen coil, according to the explanation in the first part 

 of this section, the neutral current of fundamental frequency due to a 

 fault to ground is made equal to the charging current of the system 

 to ground with one phase grounded, and this is generally a small 

 fraction^ — a few per cent, or less — of the neutral current in an identi- 

 cal system with neutral directly grounded. 



The Petersen coil will thus in many cases largely prevent the electro- 

 magnetic inductive effects at fundamental frequency which appear 

 when a fault to ground occurs on a system grounded solidly or through 



» Exceptions to this statement exist in the case of extensive high voltage networks 

 where, with a ground on one phase, the charging current to ground with isolated 

 neutral may be of the same order of magnitude as the short circuit current with dead- 

 erounded neutral if the fault is remote from a point of main power supply. 



