366 



ALTERNATING CURRENTS 



If the transformer primaries be connected in Y, with no neutral 

 connection, no third harmonic currents or multiples thereof can 

 flow into the Y, as these currents are all in phase with one another. 

 In order that currents may flow to a common point, there must be 

 phase difference, as the currents flowing toward the point must 

 be equal to the currents leaving the point at any instant, or 

 electricity will accumulate at the point. If no third harmonic 

 currents can flow in the transformer primaries, none can flow in 

 their secondaries, hence there will be no circulatory harmonic 

 currents between the transformer secondaries and the converter 

 armature if the primaries are connected in Y without a neutral 

 connection to the main generator. However, if the neutral of 



FIG. 334. Double-delta connection of transformers to 6-phasc synchronous 



converter. 



the transformer primaries be carried back to the main gener- 

 ator, the third harmonic currents and multiples thereof can 

 return to the generator through the neutral. Therefore, the 

 secondaries cannot be connected either diametrical, six-phase 

 star, or double-Y with inter-connected neutrals, if the primaries 

 are connected in Y with a neutral return to the generator. 



The harmonic currents other than the third and multiples 

 thereof are not eliminated by three-phase connections, but they 

 are reduced to small values by the use of series reactances or by 

 using transformers having high leakage reactance. 



Figure 322, page 351, shows the connections for a 500 kw. 

 converter and transformers taking power from 6,600-volt, three- 

 phase, 60-cycle mains and delivering 550 volts direct current. 



