Xfmrs 2

[Redskinsdb21]

If a Transformer is listed at 12470Y/7200 Primary and 480Y/270 secondary (Shown AS 12470Y/7200V for primary and 480Y/270 secondary) does this mean the primary must be wired in a Y configuration if to be used at 12470 V and if you use a neutral on the primary then it would be 7200 V? With the Y being beside 12470 does this mean it cannot be used to replace an existing Delta-Wye wired transformer?

I am trying to replace an existing 12,470 pri/480/270 sec Xfmr that I am told is in a delta/Wye configuration but have not been able to go out and check yet.

 

[Kiribanda]

You have not given the kVA rating (size) of the rating. But I am sure that the primary is connected as WYE at the factory
and it is not that easy to change it. Since, each primary winding voltage rating is 12470V/(1.732)=7200V, one cannot
increase that voltage so that voltage per turn is not exceeded. Therefore, you cannot connect primary to 12470V DELTA.

 

[RRaghunath]

The transformer is essentially a YNyn transformer with (L-L) voltage ratio 12470/480V.
This cannot replace a Dyn transformer for two reasons:
1) Each of the primary windings is rated for 7200V (L-N) and not for 12470V that would be the case in case of Delta primary.
2) The YNyn transformer has no phase shift between primary & secondary, unlike Dyn transformer. This may or may not be relevant depending on your power system configuration.

 

[Redskinsdb21]

RRaghunath Wouldnt the 12,470Y mean its Wye connected 12,470 Line to Line and the 7200 mean its Wye connected Line to Neutral?

Kiribanda I am trying to follow you. I see that you cant connect to 12,470 Delta but I believe you could for Wye if wired Line to Line and the 7200 would be if wired Line to Neutral? Or am I thinking of this incorrectly?

 

[Kiribanda]

I am not sure whether I understood your concern correctly. If your supply voltage
is 12470V 3-ph, 3Wire, then you donot have a NEUTRAL to connect to the STAR point
of the primary. In any case to use this transformer you need 12470V/ 3-ph/ 4Wire
power supply otherwise you are going to float the neutral which will create other
problems.

 

[cranky108]

"does this mean it cannot be used to replace an existing Delta-Wye wired transformer?" It depends on other system factors.
Since a Yg-Yg transformer will pass zero-sequence currents from low to high, and a D-Yg transformer won't, the rest of the system needs to be considered.
Most Yg transformer must be grounded, or the reduced insulation near the neutral could breakdown. So without more information, I think that you can't float the neutral. Do you have a system ground on the high side?

 

[VTer]

Typically 12470V systems are grounded and the utility does not necessarily provide a neutral conductor but this is something you need to confirm with the serving utility. Many times these systems are multi-grounded and I prefer to use concentric neutral conductors and ground the concentric neutrals on both ends - at the transformer and the source feeding it. The transformer HO bushing should also be grounded.


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