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Parallel transformer application

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?

RE: Parallel transformer application

Parallel transformers should have the same terminal voltage.
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

(OP)
These are three delta wye grounded 20/25/35MVA 3 phase units being run in parallel. My understanding (if correct) being transformers of different makes or ages will never have truly identical primary to secondary ratios which leads to slight secondary voltage difference. The plan is to keep all units the same impedance when anyone is replaced to prevent overloading from asymmetrical load division across them.



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

No problem with any protection

RE: Parallel transformer application

(OP)
When circulating currents are present, are these seen as reactive (lagging) power flow to CTs?

RE: Parallel transformer application

Yes. We use it for check stability of differential protection.

RE: Parallel transformer application

Lagging on one, leading on the other; reversed at the other winding.

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

Yes.

RE: Parallel transformer application

There is also the case of equal percent impedances but unequal X/R ratios. Current sharing will be good but the KVA with rated current through both transformers will be less than the sum of the individual KVAs.

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

RE: Parallel transformer application

Can't you have more issues with sympathetic inrush with parallel transformers?

RE: Parallel transformer application

(OP)
What equation(s) can be used to determine the max loading of the bank in question when units have unequal X/R ratios?

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

When the X/R ratios are different the phase angles of the current will be slightly different through each transformer. Even though each transformer may be loaded to its rated KVA, the current vectors add, well vectorily, not arithmetically. The result is sum of the transformer KVAs will be slightly more than the load KVAs.
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

(OP)
It will probably not even be an issue if truly slight. The application calls for 3 units in parallel, however the normal load will only be 2/3 of the total banked rating 4-5 hours a day during peak load. The 1/3 remaining capacity is when (or if) one unit is taken out of service the remaining two will be loaded to their full OFAF rating.


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

Preferred impedances are listed by BIL in Table 3 of IEEE C57.12.10. 12% is off the chart, but users are free to specify other values.

RE: Parallel transformer application

(OP)
Do you have a copy of the table?

RE: Parallel transformer application

Yes, but it is copyrighted. Give me you high side BIL, and I'll do a lookup.

RE: Parallel transformer application

Better to go by the impedance of existing transformers. Don't look at X/R.As already discussed, its impact is negligible. But if you specify it, based on old transformers, you will end up with very poor efficiency transformers. R % (depends on load losses only) is coming down all these years, as transformer losses are coming down.

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

(OP)
Thanks! I looked at the IEC technical report. What do they mean by "rated short circuit voltage"? Is this the given fault current for the voltage shown in the table expressed in a % value? The use of voltage is throwing me off in this case.

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...


Quote:



Table1 of IEC 76-5

Rated KVA %

Up to 630 4.0
631-1250 5.0
1251-3150 6.25
3151-6300 7.15
6301-12500 8.35
12501-25000 10.0
25001-200000 12.5


Quote:


B) Oil Filled
Table 10, C57.12.10
Without LTC With LTC
HV LV LV 2400 LV 2400and
BIL 420V and above above

60-110 5.75 5.5 ---
150 6.75 6.5 7.0
200 7.25 7.0 7.5
250 7.75 7.5 8.0
350 ----- 8.0 8.5
450 ----- 8.5 9.0
550 ----- 9.0 9.5
650 ----- 9.5 10.0
750 ----- 10.0 10.5



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

The BIL for your highest voltage transformer winding should not be a range, it should be one of the standard values; 450 or 550.

RE: Parallel transformer application

(OP)
My mistake. The actual BIL for "common" transformers is either 450 or 550 out in the system for various reasons, but the new units will be 550 BIL.

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?

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