Trafo vector group 6

[slavag]

Hello All.
Could you please explain: why trafo mnf's build in US ( and not only in US) trafos YD1 and YD11 and many Eroupean mnf's YD5 and YD7.
What is a reason?
Thanks in advance.
Best Regards.
Slava

 

[davidbeach]

ANSI standard for YD or DY transformers is that the high side leads the low side by 30 degrees. That leads to 1's and 11's. Don't know why it would be seemingly more random elsewhere. ;-)

 

[prc]

As David said US they normally use YD1 connection and UK,India and Australia follow YD11 connection for Generator Transformers.I never knew Europe use YD5 or YD7 for GSUs.Or is it for rectifier transformers?In that area many special transformer connections were and being used to meet the specific rectifier connection or to suppress the harmonics.

 

[ScottyUK]

prc,

UK practice tends to have dYN1 for generator step-ups, Dyn11 for distribution transformers, plus less common ones like ynYN0, and various oddities with other vector groups.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
I asked not only about GSU, common practic as you saied is Ynd1,distribution trafos are Dyn11 or Ynd11, as I understand because inductive load.
but I see excitation trafo for the generator used with vector group 7 ( or 5 ) and same for rectefire bridge.
What is a phisical reason.- OK secondry voltage is lag primary on 210 ( 150) deg. For what?
Best Regards.
Slava

 

[sibeen]

Slavar, I’m not sure what you mean by “as I understand because inductive load”. The transformer connection doesn’t care whether the load is inductive, capacitive, resistive or non-linear. The phase shift across the primary/secondary only has to do with the way that the windings of the transformer are connected.

There is nothing ‘special’ about any of the configurations. A Dy1 transformer can (depending upon construction) be fairly easily converted into a Dy11 transformer by changing the wiring at the primary of the transformer.

As to why anyone would construct a Yd5/Dy5 or Yd7/Dy7 I have no real idea. I have never seen any of these in the field in Australia, New Zealand or South Pacific. The only ‘hand waving’ explanation I can think of is that some manufacturers found it easier and less expensive to wire their transformers in this way. It makes no practical difference for the loads at the secondary as the voltages are still in a positive sequence.

As for their use in rectifiers, I cannot see this. For a twelve pulse rectifier a 30 degree phase shift is needed between one six pulse bridge and the other. This is provided satisfactorily by a Dy1 or Dy11. OEMs quite often design a twelve pulse rectifier using a transformer with a dual secondary and a 30 degree phase shift between the secondaries, and it is quite common practice for rectifiers where the DC has to be isolated from ground. The ‘normal’ practice is to use a Dd0y1 or Dd0y11 but there is nothing stopping the design being a Dd6y5 or Dd6y7; but I can think of no practical reason why this would be done.

 

[davidbeach]

You can take a Dy1 transformer (per the name plate) and connect it in such a manner as to have it function as Dy1, Dy3, Dy5, Dy7, Dy9, or Dy11. Sometimes it is really handy to get a different phase shift, particularly when different paths to the same load have different numbers of Dy transformers.

At one of our plants the standby source (one Dy from the 500kV reference) was 30[°] ahead of the normal plant aux load bus (two Dy from the 500kV reference). Doing a motor bus transfer from normal to standby bus meant waiting for the motor bus to slow down enough to slide back 330[°] relative to the standby source before before the transfer relay would close into the standby source. That took about 45 power system cycles and we'd loose a couple of critical motors. Rewired the standby transformer so that it was 30[°] behind the normal source and now the transfer takes less than 15 power system cycles and the motor bus drifts back about 40[°] but the 10[°] out of phase with the new source isn't a problem. Now we don't loose any motors going from normal to standby.

Back to normal is now more problematic but we get to schedule when it happens while the normal to standby transfer always happens at a bad time. Better to have the unwanted transfer happen flawlessly and the scheduled transfer need to have a motor shutdown and restarted than loosing motors at the same time we've lost the 500kV system.

 

[prc]

Scotty ,Thank you for correcting me.In India we always have YNd11 for GTs except when US consultants are involved,then they call for YNd1 connection.All our practices are based on UK and hence I was believing it is UK practice too.But since you are UK,I will stand corrected.I think as per IEC norm, we have to write YNd1, and not as dYN1 as mentioned by you.

 

[ScottyUK]

Hi prc,

Maybe I have fallen into bad habits - by the way I was taught, for a GSU transformer the delta is the primary hence it appears first. Anyone know what the 'official' position is on this?


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If we learn from our mistakes I'm getting a great education!

 

[vima]

As per my understanding excitation trafos and interposing trafos have Yd5 and Yd7 connections.Yd5 and Yd7 are 180 out of phase with Yd11 and Yd1

 

[slavag]

Sorry Folks.
I would like ask next Q on this issue.
OK, 5 and 7 group is some exotic.
Why for GSU used YD1 and for distribution YD11?
Must be some reason!!!!
Best Regards.
Slava

 

[slavag]

I found some explanation of this:
"In case of distribution transformer the load current remains lag due to inductive load so the phase voltage to coinside with laging current the D to Yn 30 (lagging in voltage) used. the same case is for generator transformer".
What do you think about it?

 

[dpc]

slava,

For ANSI delta-wye transformers, the low voltage winding lags the high voltage winding by 30 degrees regardless of delta-wye, wye-delta, or anything else. This can result in YD1 or YD11 depending on the application.

 

[slavag]

Dear All.
Thanks a lot for your help.
I think, I finished with this issue.
Best Regards.
Slava

 

[prc]

slavag,I have some explanation.To my knowledge, in US ( as per IEEE also) GTs are with Yd1 and distribution trfs are either with Yd1 or Dy1 connection.It means the generating voltage vector and final load voltage vector will be in same sense.I dont think it has any significance,but only a practice.In India, it is Yd11 and Dy11 or Dy5.

As my knowledge goes delta is used in GTs to avoid zeroesquence currents get in to generator stators in the event of a LG fault on transformer HT side.Generators cannot stand zerosequence currents.In DTs HV delta helps in reducing fault currents without increasing cost from uniform insulation.The quote you mentioned is from some book? Can you give some reference as I could not understand the logic or advantage in it.

 

[slavag]

Hi PRC.
Thanks a lot for your time and explanation.
The quote aren't from book. it's from:

http://answers.yahoo.com/question/index?qid=20071109004105AATosRU

My Q. come from my freinds from Russia: " What is standard groups in the word, why used YD1 and YD5/7"
Russia used only YD11 group.
Best Regards.
Slava