I would like to know the advantages DY connected distribution transformers have over YY connections.

 

The Delta - Wye transformer has a distinct advantage of ISOLATION and the primary filters odd harmonics divisible by three "tripl'n", the Wye - Wye transformer will not filter harmonics and generally has no isolation, can be the cause of phone and other communication problems and they are not allowed in some juristictions. They can not be connected in parallel due to the phase shift primary to secondary.

See the enclose site for additional reference

http://www.riedel-trafobau.de/english/pdf_eng/b/gen_e.pdf

 

Responding to Solomon's offline post asking for comparison of the Y-Y and D-Y transformer connections:

I must apologize for misunderstanding your question. My initial response was aimed at distribution systems related to industrial or bulk-power consumers.

The use of Y-Y transformers for what used to be called rural-distribution here in the USA, is quite common. A principal advantage is they are cheaper for the reason discussed earlier... insulation grading. Also, single-phase units provide flexibility benefits:

a) Simpler pole mounting.

b) In case of failure the system can operate at reduced capacity with just two xfmrs.

c) In a 3-phase bank of single-phase xfmrs, different sizes can be used.

Economy and flexibility aside, there is also strong support for their use in distribution systems, especially here in Florida. You may not be aware, but, Florida is considered the lightning capital (isokeraunic-ly speaking) of the world. Discussion still goes on about neutral-earthing.
The consensus is that the Y-Y connection with both sending and receiving-end neutrals solidly-earthed has the lowest thru-surge transmission ratio.

Regarding References. In the 50s and 60s General Electric and Westinghouse provided a great amount of technical application information. Today, GE is principally concerned with their Credit Division. And, Westinghouse is virtually defunct, now that it is "owned" by ABB! but, I happen to have a Westinghouse "bible" titled:

"Electrical Transmission and Distribution Reference Book" ca 1950

It contains a table listing characteristics of some 45 power distribution systems in the USA covering many voltage levels! Virtually all were solid-earthed at both the sending and receiving ends. If you are unable to obtain a copy, I can send the table as a pdf file.

My comments regarding the D-Y connection are still valid.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

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