Connecting delta xfrmr to wye system

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charlie

Senior Member
Location
Indianapolis
Re: Connecting delta xfrmr to wye system

Assuming you are connecting the delta primary to the three phases of the wye input. No problem. :D
 
Re: Connecting delta xfrmr to wye system

Thanks Charlie ............ I thought not, but there is a customer who doesn't want to do this. I don't have many details yet, (ie. size of xfrmr etc.)
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Connecting delta xfrmr to wye system

By connecting a delta primary and wye secondary you are trapping triplen harmonics that are generated in the wye secondary from backing upstream on the dlta primary side. Common PQ practice.
 

templdl

Senior Member
Location
Wisconsin
Re: Connecting delta xfrmr to wye system

Re. the question "I am trying to find the disadvantages of connecting a delta transformer to a wye system."
If I understand this question correctly, as an example you have a 480y/277 supply and would like to reduce it to 208Y/120 with a 480v delta-208y/120v transformer. Is your question with respect to going from a wye system to a delta transformer?
Is the other option that you are considering going from the Wye system to transformer with a wye primary and not a delta.
 

templdl

Senior Member
Location
Wisconsin
Re: Connecting delta xfrmr to wye system

Sorry, the previous post was sent incommplete of some odd reason.
To continue:Wye-wye transformers require a 5 legged core and are $$$$$$$$$$$$$$$.
Step-up transformers are available as 208v delta-480y/277 secondaries.
This is how I understand it:
A 75kav transformer with a 208v pri is rated 208FLA. The allowable OCPD is 208 x 125%= 260a which is not standard where you are allowed to go to the next standard rating or 300a. Since the information that I have been provided by a former transformer design engineer tells me that the inrush for a 150degC 75kva transformer is about 10x the FLA, then the inrush would be in the are of 2080a which is well below the 3000a magnetic trip (300 x 10) of a 300a breaker. Therefore nuisance trips should not be a problem if the ori breaker is not sized to small. You could expect some problems with a 200a breaker and even with a 225a breaker. Please not that transformers with lower temperature rises 115 and 80degC) and K-factors can have much higher inrushes, increasing inrushes as much as 60% greater than a 150degC transformer.
Remember that any think that is done to a transformer to mack it more efficient and run cooler will increase the inrush.
 
Re: Connecting delta xfrmr to wye system

As I said, I don't have a lot of info yet. The wye system I think is 4160 volts and I am assuming they want to connect a 480/277 volt delta, pad transformer. Does this help?
 

templdl

Senior Member
Location
Wisconsin
Re: Connecting delta xfrmr to wye system

No big deal. Simply use a 4160v delta pri-480y/277 secondary which is a very common application, a L-L-L connection, the neutral is not used. Just assure that someone qualified to work with the 4160v side does the installation.
The primary must be protected with fuses.
 
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