YnDyn0 vs Dzno, advantages and disadvantages 2

[slavag]

Dear all.
We have two options for replace a old transformers YnDyn0.
by same new or by Dzn0.
Dzn0 is cheaper, but we do not have any experience with such transformer.
Please share your experience in this case.
Thanks in advance.

 

[Mbrooke]

What does this transformer do? What voltages are involved? What capacity?

 

[slavag]

6.6kV to 3.3kV, 7.5-10MVA.
6.6kV factory generation bus- co-generation with grid, 3.3kV factory small distribution bus with additional generation.

 

[prc]

Slavag,unfortunately in the industrial world, price may not be reflecting the cost always!So let us consider only cost.First let us understand the existing transformer. What that D for? I believe it may be a stabilising tertiary winding.Please confirm.Only HV will be indicated with a capital letter. So such a connection will be indicated as YNyn0d11. There will be three windings of approximate copper quantity M-M-0.33M = 2.33 M.In the second case also, there will be three windings of copper M-0.585M-0.585 M =2.15M . But will not D connection create problem with existing solidly grounded system? To me a better solution is to go for YNyn0 with 3 limbed core type transformer that will be less costly than both the alternatives before you.

 

[slavag]

Yep, d is stabilizing tertiary winding in the existing transformer (Yn/yn0,d).

 

[inpran]

prc, a three winding transformer is being replaced by a 2 winding one. YNyn0d (the vector group for delta winding is not mentioned by slavag)is being replaced by a DZn0. Correct me if I am wrong here please, zig-zag transformers are not used for loading purpose. At least I have never come across one.
Further, in the calculations above, the amount of copper used for YNyn0d11 ic clear. But the amount of copper used in a DZn0 is not Can you please explain why there are 2 0.585M in a 2 winding transformer?

 

[odlanor]

I guess less expensive will be Ynd11 with zig-zag grounding transformer.

 

[slavag]

I must 0 vector group Ynd with zig zag is not suitable.

 

[stevenal]

No thoughts of using an autotransformer here?

 

[prc]

In zig-zag connection, there are two windings each of E/3 where E is the line voltage.One winding will be from one phase while the other will be from another phase.When these are connected together, it will be E/root3 when voltages are added vectorially.In normal star winding, it will be E/root3.I think you are confusing about grounding transformer and Zn connection for other purposes. It is true in grounding transformers there will be only one Zn winding which will be used only for getting the virtual neutral for the system.But is also normal in grounding transformers to add a y secondary to feed to auxiliary loads (ZNyn).

It seems in early days of electric power transmission,to use Ynzn was usual to save on the stabilising tertiary winding ie use Ynzn instead of YNynd11.Advantage is that zn will serve the purpose of a delta in reducing the zero sequence impedance,but with a small saving in copper. Any LG fault in secondary of such transformers will be seen as L_L fault on primary so that maltripings on primary will be avoided, Even today in such regions YNzn transformers are being procured (quite large 100-125 MVA)eg.Mumbai (India), Bangladesh and certain South American countries.

 

[slavag]

Stevenal, thanks a lot. I think is good idea. ( usually I work with 500MVA autotransformer ).
maybe with 2:1 voltage levels, it will be cost effective solution.

 

[odlanor]

subtransmission utilities in Brazil use 69kV supplied by source in delta grounded by zigzag grounding transformer
step down transformer are 230/69kV or 138/69kV YNd