choosing the transformer vector group

[144x]

Hi,
all the transformers in a subtransmission network are Dyn11, in another part of the network a transformer with the vector group of YNd1 is going to be used for an industrial complex .there is no chance of paralleling of transformers but the primary of them are all connected in a ring of 132 KV.is there any hidden problem in choosing the YNd1 vector group? or should all the transformers be selected to be Dyn11?thanx

 

[prc]

I dont think any hidden problem involved in your application.

 

[144x]

you mean all the problems are evident? just kidding .but network owner is reluctant to accept this vector group even though
the industrial complex is radial

 

[nkn5]

I hope you understand why you have to choose either of Y/D or y/n for distribution transformer. Then If I were planning the system, I would select similar vector groups at a voltage level. I mean if 132kv Ring is reference, then 33/11 KV is 30 degree leading, and 415V is 30 degree lagging the reference.

I do not think there are any technical issues if you are not parllelling. Share your comments

 

[jghrist]

The secondary of the Ynd1 will be ungrounded unless a separate grounding transformer is used.

If the primary of the Ynd1 transformer is grounded, it will be a ground source and could affect ground protection.

 

[144x]

let's assume that we can protect each transformer considering all the known rules of protection including zero sequence elimination on the side that is the source of it. unless you mean causing trouble for the part of network that is existing I don't think that protection of the new transformer would be any problem.can you please be more specific?

 

[jghrist]

If the primary Y side is grounded and there is a fault on the primary system, zero-sequence will flow from the Ynd1 transformer into the fault. This may blow fuses or trip overcurrent relays on the transformer, burn up the transformer if the fault lasts long enough, and change the current flowing in ground fault protection on the primary system source.

Zero-sequence elimination will prevent transformer differential operation for the external fault, but you probably also have overcurrent relays. I'm not saying that you can't use the connection, just that the ground source has to be considered.

 

[mgtrp]

I work for a utility, and am very reluctant to allow Ynd connected transformers for supply of industrial customers for exactly the reason given by jghrist - it adds an extra source of ground fault current. Is there a good reason why you want to use this connection?

 

[waross]

I spent some time consulting for a small utility. A very very small utility. I was the de-facto system engineer on an on call basis. Inherited several wye/delta transformer banks. I don't have time here to list all the serious issues with a wye/delta transformer or bank when the primary neutral is connected back to the source. Every time we lost a phase, one or two customers lost their refrigerator or freezer compressors.
I agree with jghrist and mgtrp.
Think of the delta connection as two parts. A phase and B phase forming an open delta and C phas as a single phase transformer.
If the voltage is high or low on C phase, there will be a voltage error when it is connected across the open delta. Expect high circulating currents.
If you lose C phase the single transformer will be back fed and try to supply the loads on the missing phase. If C phase is grounded on the primary, the C phase transformer will still back feed and contribute to the fault current.
There are other effects on the primary side also.

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

 

[144x]

thanx for the responses .can you give any references for study? surely a drawing would help to understand the problem

 

[davidbeach]

I can tell you that on our system that YNd1 transformer would essentially need an interconnection application, and would probably become economically infeasible after all the system mitigations were identified. We want an interconnection information for sources in any sequence, not just positive sequence sources.