304 ELECTRICAL EQUIPMENT 



but the fundamental e.m.f. wave is, of course, larger than in each 

 of the phases. 



If the neutral is grounded the potential difference from line to 

 ground may not be the line voltage divided by V3; but, super- 

 imposed on this voltage, there may be the triple-frequency e.m.f. 



A B 



A A f\ A A /\ A. /-\ A A 



vy vy v/ v/ w 



r\ r\ /"\ ,0 h ^"\ /\ /\ /^\ /^v 



^ x> > ^ O 



V\XX/^ c . x , , 



O x2/ TT T 



FIG. 180. FIG. 181. 



and the maximum value of the wave may be greatly increased, 

 thus increasing the insulation strain. 



In a balanced three-phase system, third harmonics can, 

 therefore, only exist in the voltage from line to neutral or Y-voltage ; 

 in the current from line to line, or generator delta current; and 

 in the line current only if the generator neutrals are grounded or a 

 return circuit provided. 



Grounding of Generator Neutral. With two generators oper- 

 ating in parallel, a difference of potential will exist between their 

 neutrals equal to the vector difference between their phase e.m.f J s. 

 With the neutrals interconnected a local current would flow, lim- 



