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Topic Title: Transformers Vector Group
Topic Summary: Yyn0
Created On: 14 June 2013 10:53 AM
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 14 June 2013 10:53 AM
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MHIJAZI84

Posts: 56
Joined: 25 July 2008

Hello


I would like to know where and why would we use a Yyn0 Vector Group Transfomer, if any litreture could be pointed for further reading it will be appreciated, thanks for your Help.
 14 June 2013 01:12 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 227
Joined: 25 July 2008

Yy transformers have a major disadvantage in that they propagate triplen (3rd) harmonics whereas Yd or Dy tend to absorb the harmonic currents in the delta winding (and heat it up instead). However a Yy transformer has zero-phase shift between the windings (vector group 0) which may be necessary depending on the situation with other power infeeds/network design. Generally the low-voltage side may require a wye winding in order to derive a neutral for earthing/single phase circuits. Lastly, depending on the voltage rating the high-voltage side if wound as a wYe can be significantly cheaper to construct because the insulation can be graded towards the neutral. I'm not an expert on transformers but that's my understanding and I'm not sure there's a definite easy answer.

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.
 14 June 2013 01:25 PM
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Zuiko

Posts: 517
Joined: 14 September 2010

The need for a star-star system with no phase shift is one benefit (as Timothy says)
another benefit is the ability to connect phase voltage loads
However, if the star point is used for earthing, then the total load should be limited to 10% of the rated current which can be a serious limiting factor.
 14 June 2013 02:12 PM
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MHIJAZI84

Posts: 56
Joined: 25 July 2008

Hello

Thanks for the information, I should have been more precise I understand the Yy0 technicalities my main concern is the actual application. in other words when will you use a Yy0 to explain more

132 kV is reference at 12 Oclock
then a Yd1 Transformer is used 132/33 kV shifting the phasor to 1 i.e. 30 degree phase shift
then we introduce a Dy11 to reshift the vector to 12 so the reason we introduce Dy11 and Yd1 is to correct/obtain the right phase shift between reference and destination

now with this in mind why, when where will we use Yy0 what are the application. thanks for your help.
 14 June 2013 02:55 PM
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Zuiko

Posts: 517
Joined: 14 September 2010

one applicaton is if you want to install a peterson coil to the star point to improve power factor in fault conditions.
 15 June 2013 03:33 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

Originally posted by: Zuiko

one applicaton is if you want to install a peterson coil to the star point to improve power factor in fault conditions.



Zuiko,

Petersen coils are used in ungrounded 3-phase systems or IT system and the aim is not to improve the power factor in ground fault condition, but its to prevent the restrikes at the fault locations known as arcing grounds.
 15 June 2013 03:53 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

YY transformer is hardly used , but its economical and the price more cheaper , usually used in application of high voltage with low capacity..like what was explained by timothyboler , the triplen harmonic because the voltages in any phase of a Y-Y transformer are 120 degree apart from the voltages in any other phase, the third-harmonic components of each phase will be in phase with each other.

But its required few turns of winding and less insulation level...
 15 June 2013 09:46 AM
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Zuiko

Posts: 517
Joined: 14 September 2010

Jack,
You are correct in your thinking that it is to prevent restrikes. It does this by improving power factor. Electric arcs are non-ohmic.

By moving the power factor closer to unity, the current and voltage will begin to peak at the same time making arc suppression easier.


They are found in many primary substations at the transformer star point for this reason.

Edited: 15 June 2013 at 10:42 AM by Zuiko
 19 June 2013 02:14 PM
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MHIJAZI84

Posts: 56
Joined: 25 July 2008

Thanks all for the help.

are the reasons described here the only application for a Yyn0 Transformer.
 19 June 2013 04:55 PM
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JohnRRussell

Posts: 22
Joined: 02 December 2003

When you say Yyn0 do you include YNyn0? We've recently commissioned a YNyn0 transformer connecting a hydropower station to a private HV network which is connected in turn to the grid. The need was to provide a neutral earth on the LV (station) side for station auxiliaries, and a neutral earth (switched) on the HV side at times when the private network was disconnected from the grid. Other solutions would have been possible but a YNyn0 transformer was the solution chosen.
 27 June 2013 09:33 AM
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Deslandes

Posts: 2
Joined: 01 October 2007

From what I'm aware all over the UK, 66 kV and 33 kV are both at 11o'clock, therefore to transform from 66 to 33 kV, one requires Yy0. I have been involved in the installation of such a transformer in the North East of England. This was a Yyn0, with an impedance earthed LV neutral to limit earth fault current on the 33 kV network. No HV earth was supplied as it's normal to only earth at the source end on networks below 132 kV.

As someone else has stated, as 132 and 11 kV are typically both at 12 o'clock in the UK network (referenced to the 400/132kV Grid system), these also require a Yy0 transformer. At this voltage I'm guessing that this would be a YNyn0 typically in the UK, as it is standard practice to earth 132 kV at source and line end, and it is typical to earth the 11 kV system at the main supply transformer point. 132 kV is typically solidly earthed to ensure no damaging voltages develop at the HV winding neutral point during an earth fault. At 132 kV and above, HV windings tend to have graded insulation for reasons of economy, so elevating the neutral voltage too high above earth may overly stress the lowly rated insulation close to the star point of the windings.

I've never done a 132 / 11 kV sub, but I would guess that earthing is quite interesting due to having to get a low enough impedance on the site to limit any potential rise between HV and LV earths that would allow dangerous step potentials to develop. Most DNO's have very strict earth impedance requirements before allowing connection of HV and LV earth systems for this reason.

Useful references on the topic include the J&P transformer book, but most DNOs publish freely available information on what phase relationship and vector groups to consider when connecting at different voltage levels on a network. See a link to Eon's below: Relevant info is on page 89

Eon Primary network design manual

This of course differs from country to country. I'm currently working in a location where the entire grid runs at 12 o'clock with solidly earthed neutrals throughout which is quite strange.
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