Parallel transformer application
Parallel transformer application
(OP)
When transformers are paralleled, what effect if any do circulating currents have on deferential protection and other protective relaying?
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Parallel transformer application
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RE: Parallel transformer application
This generally means that they should be on the same tap.
In that case there should be no circulating current.
Differing impedance voltages will affect the load sharing ratio but should not cause circulating currents.
That said, delta transformers and delta banks may experience circulating currents even when not connected in parallel.
But;
The circulating current should be seen by both CTs in each differential pair and should not cause a problem.
However, circulating currents do add to transformer heating and for proper protection you may consider monitoring the current in each winding as well as the line currents.
I have seen delta connected transformers badly overheated by circulating currents even though the line and load currents were quite low.
In parallel single phase transformers, a voltage unbalance of twice the impedance voltage will cause a circulating current equal to full load rated current.
What is your application?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Parallel transformer application
While it is currently planned to keep the tap changers in the neutral position full time, if ever activated all the units must be lead in sequence with a leader follower scheme?
RE: Parallel transformer application
RE: Parallel transformer application
RE: Parallel transformer application
RE: Parallel transformer application
Master-follower is one way to coordinate load tap-changing. Other methods use measured circulating current or circulating reactive power to bias the controls in a way that keeps them together. See beckwithelectric.com for more information.
RE: Parallel transformer application
RE: Parallel transformer application
RE: Parallel transformer application
RE: Parallel transformer application
Also I found this, it look like it applies to smaller transformers, however it can also be used as a rule of thumb for utility applications?
http://www.facilitiesnet.com/whitepapers/pdfs/schn...
RE: Parallel transformer application
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Parallel transformer application
RE: Parallel transformer application
Also thank you for bring up inrush. While I have not thought about it in depth, should it be a potential problem I will reconsider.
RE: Parallel transformer application
This may not arise in practice very often. The slight difference is more for the purists and those who wish to have a complete understanding of the subject.
In practice, the actual load so seldom matches exactly the transformer rating on a day when the ambient temperature is up to design maximum and there is no wind, that the slight reduction in capacity is not an issue.
If you have occasion to parallel two transformers with greatly different X/R ratios, then you may want to calculate the phase angles.
It may be easier and quite safe to be not aware of the issue.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Parallel transformer application
My biggest concern however are replacement units. I plan on specking a common impedance and X/R ratio (one the has been around for a long time and will continue to be for the given MVA). Does 12% Z sound typical?
RE: Parallel transformer application
RE: Parallel transformer application
RE: Parallel transformer application
RE: Parallel transformer application
For a 35 MVA transformer,you will find %Z specified from 8-20 % depending on the region and utility. Common range is 10-15 % on max rating.
C57.1210- 2010 for transformers recommend 7% (with OLTC) 7.5% with out tap changer for 200 BIL Transformer. It is 9&9.5 % at 550 kV BIL. It is on self cooled MVA base ie at 20MVA base.
IEC/TR 60909-2-2008 (data of equipment for short circuit calculation) gives the range of %Z and X/r ratio at different parts of the world. These are on maximum rating base ie at 50 MVA.
RE: Parallel transformer application
I did find this word doc which actually lists recommended % impedance for the IEC and IEEE:
https://www.google.com/url?sa=t&rct=j&q=&a...
By chance, does anyone know how these numbers are derived? Are they just arbitrary based on industry preference or is there a solid engineering reason behind these recommendations?
In this application BIL is in the 450 to 550 range.
RE: Parallel transformer application
RE: Parallel transformer application
I am debating between 10 and 12% Z (base self cool rating). I did check and the X/R in newer units is significantly lower on resistance. My reason for nit picking these value is because parallel use transformers is expected to go up and I would like to have all units be the same Z.
In terms of fault current, anything I should look out for other then the obvious 3x increase assuming infinite source?