PETERSEN SYSTEM OF GROUNDING 57 



system and the system with neutral ^rouncled through the reactor, 

 recourse may be had to the indirect method of reference to the solidly 

 grounded neutral system, as in the discussion of residual currents of 

 the non-triple series on page 52. Such a procedure, of course, in- 

 volves a reference to general experience also. It has been shown in 

 the earlier discussion that for a triple series harmonic of order m 

 the neutral current wMth the reactor is approximately 1/ {m^ — 1) 

 as large as with the dead-grounded neutral if a low impedance path 

 for triple frequency magnetizing currents is provided, as by a delta 

 winding. The establishment of this system of neutral currents, even 

 though they are small, when a previously isolated system is grounded 

 through a Petersen reactor, constitutes an addition to the residuals 

 which produce induction in neighboring circuits. However, it is not 

 to be expected that the added inductive effects would be important. 

 Where no low impedance path for the triple series magnetizing cur- 

 rents exists, the reactor is relatively less effective in suppressing 

 residual currents of this series. The triple harmonic neutral currents 

 of a power system connected in this manner and grounded through a 

 Petersen coil might in some cases lead to inductive effects of some 

 significance. 



In general, for harmonics of orders not divisible by three, grounding 

 through a moderate resistance (large, however, compared to other 

 impedances involved in a short circuit to ground) will be more ad- 

 vantageous as regards residual voltages, and less advantageous as 

 regards residual currents, than grounding through the reactor. 

 Grounding through zero impedance would, of course, generally lead 

 to the smallest residual voltages and the largest residual currents of 

 these frequencies. For frequencies belonging to the tr-iple series, 

 grounding through the reactor will be considerably more advantageous 

 than grounding through a moderate resistance as regards both residual 

 voltages and residual currents. It may be expected that with moder- 

 ate neutral resistance, residual currents and voltages of the triple 

 series will both be nearer in magnitude to those obtaining with zero 

 neutral impedance than to those obtaining with the Petersen coil. 

 The moderate neutral resistance is relatively more effective at the 

 higher frequencies in reducing residual currents of all harmonics 

 and residual voltages of the triple series; for harmonic residual 

 voltages not belonging to the triple series, it is relatively more effective 

 at the lower frequencies. 



I wish to express my gratitude for helpful suggestions and criticism 

 received in the preparation of this paper from Messrs. L. P. Ferris 

 and R. G. McCurdy, and also from Mr. R. K. Honaman. 



