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.
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
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
"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
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Parallelling power transformers
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
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
RE: Parallelling power transformers
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Parallelling power transformers
RE: Parallelling power transformers
h
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
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
RE: Parallelling power transformers
"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
"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
RE: Parallelling power transformers
"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
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
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
RE: Parallelling power transformers
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
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