192 BELL SYSTEM TECHNICAL JOURNAL 



star connections in which case the full triple-harmonic magnetizing 

 current flows in the two systems which the transformer interconnects, 

 the relative magnitudes in each depending on their relative impedances. 

 Where a star-star bank is connected at one terminal of a line, with a 

 star-delta at the other, the neutrals at each end being grounded, prac- 

 tically the entire third harmonic required by the star-star bank may be 

 expected to circulate in the line connecting the two. 



An effective method of control for cases in which star-star connec- 

 tions are required due to phase relations is the provision of a third set 

 of windings or tertiaries in the transformers, the impedance of the 

 tertiaries with respect to the other windings being sufficiently low to 

 furnish an adequate path for the triple-harmonic magnetizing current. 

 An alternate method of control, which also provides like phasing on the 

 two sides of the bank, is the use of zig-zag connected transformers. 



In four-wire multi-grounded neutral distribution systems, it has 

 been found helpful in controlling the residual triple-harmonic currents 

 from the single-phase load transformers to provide star-delta connected 

 banks at various points in the network with neutrals connected to the 

 system neutral. In some cases, these have been three-phase load 

 banks, in others, special banks installed as a method of control. 



The subcommittee is continuing its work on wave shape and balance 

 through a laboratory study of transformer harmonics and transformer 

 connections. These tests are being made on small model transformers, 

 typical of the designs which are used for large sizes on transmission 

 systems. It is planned to develop the theory applicable to harmonics 

 from transformers on three-phase systems from the work on these 

 laboratory models. It is planned to supplement the work by tests 

 on large transformers in the manufacturer's shops and in the field. 



A number of severe noise situations have been created during the 

 past few years when star-connected generators, operating with 

 grounded neutral,''-^ have been connected directly or through star-star 

 transformer banks to transmission or distribution systems. The 

 interference in these cases resulted from triple-harmonic residual com- 

 ponents impressed on the system by the particular generator operating 

 with the grounded neutral. The magnitudes of these currents depend 

 on the triple-harmonic components in the generator phase-to-neutral 

 voltage and the impedance to ground of the system. The methods of 

 control which have been successfully applied in these cases include the 

 following: 



1. Isolating the generator neutral and supplying the system ground 

 through a suitably designed transformer bank. 



2. Grounding the neutral of only those generators designed to be 

 free from triple harmonics in their phase-to-neutral voltage. 



