Offshore power transformer YNd1 vs YNd11

[osizer]

Hello everyone,
I have a question regarding the vector group of a power transformer installed in the configuration shown in the attached diagram. Which option would be more appropriate for offshore power transformer, for such a setup – YNd1 or YNd11?



A brief description of the configuration (in case the diagram link expires):
Onshore, there is a 3-winding transformer 400/220/42 kV with a YNyn0d11 connection. A STATCOM is connected to the 42 kV winding, and the 220 kV winding is connected to the export cable. On the offshore substation, there is a 220/66 kV transformer with either a YNd1 or YNd11 connection. The wind turbine transformers are in a Dyn11 configuration.


Additional question:
Do YNd1 and YNd11 transformers differ significantly in construction, considering that in practice, it is possible to obtain the same phase shift from a YNd1 transformer as from a YNd11 transformer by swapping two phases?

 

[Kiribanda]

Looking into the SLD followings are my findings.
1) I have seen that in NORTH AMERICA the winding configuration is always Dyn1 whereas in the other side of the world following IEC standards the winding is mostly Dyn11. A solid reason is not known yet to me. The change can be made by reversing the polarity. Construction of both YN1 & YN11 is same. It is an internal difference. If you specify correctly at the beginning of the project the vector group you need, then the manufacturer is able to deliver it. But not after witness testing.
2) When you have two WYE earthed systems at either side at 240kV, please make sure that during an earth fault the EF relays at ONSHORE are not desensitized more than 10%.

 

[dpc]

For transformers built to ANSI standards, the standard convention for phase shift (for delta-wye or wye-delta transformers) is that the low voltage side lags the high side by 30 degrees.

As long as there is no need to parallel with an existing transformer, there's no operational difference between the two configurations. As Kiribanda notes, the phase shift can be changed by reconnecting the primary and secondary windings. See here: https://www.eng-tips.com/forums/238/faqs/1154

 

[Nayeem1993]

There is no significant advantage of Dyn1 over Dyn11 or vice versa. You can choose either of them, just make sure that transformers operating in parallel have same vector groups.

 

[FPelec]

I agree, there is no difference between Dyn1 and Dyn11. On the other hand, it seems like a strange setup to me. Why not use 400/230 kV and 230kV/66kV autotransformers instead? It would reduce costs and losses, and provide solid neutral grounding for the 66 kV network — which, in turn, would lower insulation requirements and costs on the offshore grid equipment.

 

[bacon4life]

The 230 kV/66 kV ratio is a bit higher than typical for autotransformers. That being said, it seems weird to have the 66 kV system ungrounded.

Many regions are eliminating tertiary windings on 400/230 kV autotransformers.

 

[osizer]

Thank you all for your answers. You have confirmed my way of thinking.
I am aware of the conditions of transformers parallel operation, but in this example it will not happen, because there is only one offshore transformer.

In the context of autotransformers, from my observations, it is not a very popular solution in my region. Apparently in the US it is a common solution in the offshore industry - maybe someone is able to confirm this?

The layout given is highly simplified. I plan for my own educational purposes to build a model of an offshore wind farm in a simulation program.
In this layout, the 66 kV side will eventually be grounded using a ZNyn11 grounding transformer. I do not yet know whether the grounding will be direct or through a resistor.

 

[prc]

As already mentioned, the US follows a Yd1 generator step-up transformer + Dy1 distribution transformer at the consumer end. IEC world follows Yd11 + Dy11 transformers. This is just a practice with no other issues. The voltage vector at the generator and the consumer will be in the same position. So, in your case,, you can adopt Yyo Auto + Yd1 + Dy1 transformers, assuming you are in the US.

400/220 kV transformers can be auto-connected to reduce costs and losses. But if your tertiary Statcom rating is high, you should stick with the three-winding connection.

Auto-connection will be advantageous only up to a voltage ratio of 1:3 . So, at 220/66 kV, Auto-connection will not give you any advantage. But why not go for a Yyo connection instead of Yd11 for 220/66 KV connection? You can avoid a 66 kV grounding transformer by such a connection.

 

[FPelec]

We normally use autotransformers for 230 kV / 69 kV and 400 kV / 135 kV transformers as well. While the advantage is less significant than in 400 kV / 230 kV units, we still find it beneficial.