Vector group selection

[safy2004]

Hi.. every body!
we want to install new power transformer, but we have problem.
My question about vector group. Is there any relation between the vector group and short circuit current. one engineer told me that if I want to use the power transformer in area near the generation, I sould select YNd1, because the short circuit current will be high, while in other areas too far from generation plants, YNd11 can be used because of the low level of short circuit current!
Is that correct?

 

[davidbeach]

Nope. Vector group and fault current have no relationship with each other.

 

[ScottyUK]

Typically GSU transformers are YNd1 and load transformers are Dyn11, at least in this country, and maybe that's where your associate's misconception comes from. David is absolutely correct, there's no direct relationship between vector group and fault current.


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If we learn from our mistakes I'm getting a great education!

 

[safy2004]

Thanks, david and scotty!
But if there is no relationship, why sometimes YNd1 is used and other times YNd11. Is it just standard or there is any other reason for using different vector groups in different transformers.

 

[waross]

In theory, you may have three cores, three primary windings and three secondary windings in a given transformer. The order in which the winding leads are connected to the bushings and the physical arrangement of the bushings determines the vector group.
In theory, any transformer may be connected for any vector group.
In practice there are some limitations. For example if a vector group is selected that requires a set of windings be changed from a delta configuration to wye or vice versa, the rated voltage will change.
Another practical issue may be grading of the insulation from end to end of some of the windings.
The selection of a vector group is often made to match existing transformers or system standards to facilitate parallel operation.

A question for those who are more familiar with vector groups:
Does the vector group of some transformers change if the transformer power flow is reversed? That is whether the transformer is used for step-up or step down?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kiribanda]

safy2004,
ANSI C57.12 and IEEE C57.12.00 specify that the angular displacement shall be 30 deg where LV phase vector lags the HV phase vector (Clock 1)whereas Clock 11 is specified by IEC 60076-1.Therefore you will see any trafo made in North America(NA)per ANSI is always clock 1 and any UK based transformers are Clock 11. But it does not prevent any NA ANSI trafo manufacturer to make a 12 clock transformer and any IEC trafo manufacturer (e.g. HYUNDAI)to make a Clock 1.There are so many HV substation installations in NA where the design has come from UK/Japan. In those places you will find Dyn11 or YNd11 as the case may be.

 

[prc]

kiribanda,To my knowledge,there are no such stipulations in ANSI or IEC.If so please tell us the relevant clauses.I agree with all your other conclusions.In India we are making GSUs with normally Yd11( our legacy with UK) but switch over to Yd1 if the consult is from US!But then the station auxiliary trf will be Dy11 if GSU is Yd1 and Dy1 if GSU is Yd11 so that the vector will be in line at the secondary of station start up trf. That is the reason ( I believe ),in UK-Europe when GSU is Yd11, down trfs(distribution) are Dy1.

 

[dpc]

The standard ANSI phase relationship in delta-wye or wye-delta transformers is that the low side lags the high side by 30 degrees. Other phase shifts may be specified.



David Castor

http://www.cvoes.com

 

[Kiribanda]

prc,
Please refer
1)section 5.6.2 of ANSI C57.12
2)section 5.7.2 of IEEE C57.12
3)section 6 of IEC 60076-1
Also please see what dpc has posted.

 

[ScottyUK]

Kiri,

My copy of IEC 60076-1 doesn't make any stipulation in section 6, just gives some examples. I'll have to check if this is the most recent update. I don't have the IEEE standards available right now so I'll take your word for it.


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If we learn from our mistakes I'm getting a great education!

 

[prc]

I am travelling and hence have no access to these standards.As Scotty said, to my memory there are no stipulations in standards as regards preferred vector groups as it will vary depending on application and utility practice.My company is making GSUs as per IEC with Yd1 and Yd11 depending on the customer preferences.It has nothing to do with standards.

I will check the clauses mentioned and come back.