Step up and Step down transformer

[ToonBlue]

Why is Delta star transformer used for step down transformer and star delta for step up transformer?

According to this book,
Electrical Power Systems
By C L Wadhwa

 

[westward10]

Power is generally distributed in delta to save the expense of a fourth conductor which is only required for final distribution hence star to enable the neutral.

 

[Pete999]

Google, Transmission of Electricity via the Grid this may give you a better understanding of how Electricity is transmitted

Comparison between Star and Delta Connections

Comparison between Star and Delta connections. Star Connection and Delta Connection are commonly used in three phase power systems.

www.electronicshub.org

 

[Lucien Nunes]

Power is generally distributed in delta

I would rather say it is transmitted in delta but distributed to consumers in star, hence the need for a DY transformer at the final substation.

Another property of DY transformers that is of importance here is that 3rd harmonic currents from the load on the Y side are trapped by the transformer and merely circulate around the winding on the primary D side. They are not transmitted along the lines and therefore do not create line losses.