Step-up transformer vector group

[LDKGR]

Hi everyone,
I am trying to finalize the specification of a step-up transformer but I can't find a lot of information about vector group.
The existing step-down transformers of the installation are Dyn11 type. After a discussion with a transformer manufacturer his proposal was YNd11.
A rough description of the installation is [gen-set 0.4kV] - [step-up transformer 0.4/20kV] - [20kV line] - [step-down transformer 20/0.4kV] - [loads].

Thanks

 

[jghrist]

There is not technical reason for using one vector group over another. US standards would have the high side leading the low side by 30°, which is YNd11 or Dyn11.

 

[LDKGR]

Thanks jghrist for your answer.
Perhaps the question was not so clear. So, is there any regulation (or other tech restriction) which determine to use an exact vector group for the step-up trafo if the vector group of the step-down trafo (Dyn11) is known ?

 

[electricnewbie123]

A few things to consider would be:

- Are there any changeover supplies at the other end? If so, the phase shift degrees will need to be carefully coordinated to avoid mismatch of phasing between the two supplies at the receiving end
- Ground fault protection philosophy will also dictate the type of vector grouping. In the arrangement you have mentioned (having a YNd11), I presume the intention was to use the d11 for the 20kV side. If this is the case, grounding of the entire 20kV network is going to be challenging. You may need to consider corner point delta winding earthing or having earthing transformers installed. In my opinion, I would go with the 20kV side to be solidly or resistively earthed via an NER or NEX in which case a star winding is what I would prefer for the 20kV side

 

[prc]

My understanding so far is delta connection is required on LV side of GSU to prevent the zero sequence current entering in to generator(as rotating machines have very little with stand capacity for zero sequence currents) on any LG fault on HV side. So in US YNd1 is usual connection for GSU and outside Ynd11.I have seen Ynyn GSU for small ratings, but never came across Dyn.

 

[LDKGR]

Hi electrichie123, there is no changeover supplies. Step-up transformer is fed by a standby gen-set. YNd configuration means STAR connection in medium voltage 20kV side (I'll use solidly grounded neutral) and DELTA connection in low voltage 0.4kV side.

 

[ScottyUK]

For large machines the LV delta is primarily there to allow high resistance earthing of the generator neutral point to ensure that any stator earth fault does not result in significant damage to a very high value asset. Phase-phase faults on the generator bus are rare because the output busbars are usually of the isolated phase type, and the GSU transformer itself is often a bank of single phase types.

The generator itself doesn't know or care about the GSU vector group, but the GSU vector group may be of significance if the station and unit auxiliary supplies are required to operate in parallel for any reason.

 

[LDKGR]

Hi ScottyUK,
thanks for your answer. Consequently which is your proposal for my simple case ?

 

[ScottyUK]

As I'm UK-based I guess I will go for YNd1 transformer, and earth the neutral point through a resistance to limit the earth fault to a few amps. I am assuming that you do not have any line-neutral loads on your generator, and I'm looking at this from the generator owner's perspective. The utility you connect to may well have the final say on the transformer - they may accept a zero sequence source from the star winding, or they may not. Without knowing how the 20kV system is designed to operate it is hard to be more specific.

 

[prc]

LDKGR, the convention of transformer vector group representation is high voltage connection first(capital letter) followed by low voltage connection, clock number correspond to position with HV at 12 o'clock.
With delta on LV side, any LG fault on HV will be seen as a LL fault on LV. Scotty, my understanding is, UK you follow YNd11 for GSUs always. My understanding wrong? In India, we follow UK practice except where US consultants are involved where we use YNd1.

 

[ScottyUK]

Hi prc,

There are few if any British generating plant manufacturers left, with the market now dominated by foreign companies - YNd1 isn't uncommon among the younger stations, especially those with North American links. I think the UK practice you refer to is that of the old CEGB, but now ownership of generation is fragmented there's no common standard. Dyn11 is the default vector group on the distribution system other than where 'unusual' types are required - typically star-star or star-interstar (zigzag) - for specific purposes.