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Parallelling power transformers

Parallelling power transformers

Parallelling power transformers

(OP)
Attached is a sketch I made of a potential problem with a proposed design. There are two transformers that can be paralleled. They aren't under normal conditions.

The ratings are shown on the sketch. The system operates at 4160V.

What if the transformers were paralleled under the load shown (almost 3000 Amps)?
 
My questions are these:

1. Would the load be divided between the transformers and there would be no issue....except that the main bus is too small 2000Amps and should be increased to 4000 Amps or so?

2. The system would only be safe if it were interlocked so that the two feeds can only operate independently - a dead bus transfer only.

3. It's impossible to know which transformer is feeding which loads under the parallel conditions, therefore the load must be less than the smallest of the two transformers.

4. This is a totally wrong design and should be rejected altogether.


 

RE: Parallelling power transformers

Presumably your load is increasing or your wouldn't be paralleling the two transformers.  I guess that' why you mentioned going with a 4000 A bus.

Some things to think about:
 - circulating current. Your transformers have different impedances and this will lead to circulating current.  The voltages on the incoming feeders could be slightly different and this too leads to circulating current.  Probably need to account for both effects since this could cause one of the two low side breakers to trip under load conditions.

 - fault current.  You're essentially doubling the available short circuit current on the low side bus.

You can do some calculations and see if the risk is acceptable or not.

RE: Parallelling power transformers

the transformers will not share load equally because of difference in %Z.  

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
 

RE: Parallelling power transformers

When in parallel, T2 will be delivering 9383 KVA when T1 is delivering 10,000 KVA.

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

RE: Parallelling power transformers

(OP)
Thanks,

I realize there will be more short circuit current and will account for that. The load is actually varying.

I knew about the circulating currents...but how close do the transformers have to be in order for this to not be an issue.
This is not built. I'm just looking to raise the alarm flags.
Is there a way to calculate the circulating currents?

RE: Parallelling power transformers

When in parallel, T2 will be delivering 9383 KVA when T1 is delivering 10,000 KVA.
The transformers may be expected to share the load in the ratio of T1> 51.6%, T2> 48.4%

If the load exceeds 19,383 KVA, T1 will be pushed past it's 10,000KVA rating.
This assumes that the X:R ratios are similar.  

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

RE: Parallelling power transformers

ohms law

RE: Parallelling power transformers

The different impedances won't cause circulating currents. Different supply voltages or different turns ratios will cause circulating currents.
 

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

RE: Parallelling power transformers

Bill, Uk% of paralleling trafos must be same.
 

RE: Parallelling power transformers

Beckwith has several good white papers on paralleling transformers that have LTC's.  They do however show how to calculate circulating currents.

http://www.beckwithelectric.com/docs/tech-papers/advxpara.pdf

Each portion of your bus needs to be rated for the current it is carrying. From your diagram I don't see why anything would need to be above 2000A even though the total load would be 2945 amps. As long as they are fed from the same 15 kV source, you can calculated how the current will split on each portion of your system.

To prepare for either transformer failing, you would need procedures to automatically shed load in excess of the remaining transformer emergency rating.

After paralleling, also note that both the 1000A load and the 1945A load will see voltage sags for faults on either set of feeders. This is a tradeoff to your previous system that had no extra voltage dips, but required manual reconfiguration in the case of a transformer failure.

RE: Parallelling power transformers

Slava: If the 10,000/14,000 KVA transformer is re-rated to 9383 KVA, the PU impedance becomes 7%. The transformer may be connected in parallel if we consider it to be a 9383 KVA rather than a 10,000/14,000 KVA transformer. It will share the load based on the ratio of 9383:10,000 KVA.
If the X:R ratios are not the same, the sum of the individual transformer currents may be more than the actual current.

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

RE: Parallelling power transformers

bdn2004 - you may also check the LTC of each transformer to determine how you want operate taps if they are not similar.

RE: Parallelling power transformers

I am just wondering if in this particular case, would anyone consider moving the fans and controls from transformer with higher %Z (T2) to one with lower %Z (T1) to allow both units to share at least 20MVA of load without issues. When paralleled, the one with the fan rating and higher %Z is useless?  

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
 

RE: Parallelling power transformers

I meant to say that the additional fan capacity on T2 is useless.  

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
 

RE: Parallelling power transformers

The fans on T2 would likely be running at 10,000 kVA, allowing it to operate somewhat cooler than the 65C OA rise. A cooler transformer usually lasts longer, so I definitely would not remove the fans.   

RE: Parallelling power transformers

I was suggesting moving the fans from T2 to T1 assuming it's practical to do so, not removing them completely.  

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
 

RE: Parallelling power transformers

BTW the 65C rise rating should be around 11200kVA assuming you get additional 12% with additional 10degC rise. The 10MVA rating is at 55degC. I believe the actual rating of the T2 transformer is:
10/12.5MVA ONAN/ONAF at 55degC rise
11.2/14MVA ONAN/ONAF at 65 degC Rise

Not clear from attachment if T1 is rated for 55degC or 65degC rise tho

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
 

RE: Parallelling power transformers

(OP)
Thanks for the great answers. I have another question.

T1 provides an alternate feed for 4 switchgears, none of which would are "on" through T1 at the same time. There is differential relaying proposed around T1 primary, and all 4 of the downstream switchgears alternate feed mains.

I can see a scenario where someone may try to parallel the two transformers T1 and T2 and close in the main on one of these alternate feeds on one of the switchgears. It seems the currents would divide through the transformers as has been pointed out - so the differential relaying would never sum to zero and trip. Am I looking at this right?

RE: Parallelling power transformers

The differential would have to include CTs in the 2000 A breaker connecting to the 2000 A bus.  Otherwise, if current flows through that breaker, either feeding T2 load from T1 or feeding part of the T1 load from T2, the differential will trip.

RE: Parallelling power transformers

(OP)
The proposed design is actually more like this attached drawing. T1 is to be a backup for T2, and T2 to serve as a partial backup for T1. My question is, since both transformers would be sharing the load, 49/51% would there be harm in closing in all the breakers shown for a potential total load if we reduced the load so that neither transformer would overload?

For example:
What if we reduced the load to 20MVA total.(2775A)

T2 at 51% = 11MVA on T1 = 9.8MVA

Now it appears that the 2000A main bus would be undersized but not the 2000A bus to the transformers....

Is this ever done like this?

 

RE: Parallelling power transformers

You have your loadings interchanged. T1, having the lowest impedance will take the greater portion of the load.
The load sharing may be:
T1- 10 MVA
T2- 9.383 MVA
You may want to check the X:R ratios of the transformers. My old text book claims that although transformers with different X:R ratios will still share the load in the ratio of the KVA ratings when the KVA ratings are normalized to the same PU impedance  base the total delivered KVA will be slightly less than the sum of the individual transformer KVAs.

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

RE: Parallelling power transformers

Looks like the second drawing didn't get attached.

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