Bugman1400
Senior Member
- Location
- Charlotte, NC
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Transformer Inrush
- Thread starter bantam_dave21
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
"Capable of" a 150C rise is not a correct statement of the properties of a transformer.That link does not provide anything useful......whats magnet wire got to do with 150degC rise of xfmrs? Its the rise that I have a question about. It makes sense if you were referring to 150deg absolute.
The thermal design of the transformer determines what temperature rise will result from full load current. The maximum temperature that the winding is capable of withstanding, combined with the temperature rise, puts a limit on what the ambient temperature can be and still allow the transformer to operate at full load.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Bantam...
Was your query answered to your satisfaction?
Regards, Phil Corso
Was your query answered to your satisfaction?
Regards, Phil Corso
Do you have a data sheet or a weblink on the xfmrs that are capable of 150degC rise? That sounds high and I would like to read up on that. Thanks!
Start here. Temperature Rise and Insulation System Ratings:
http://www.powermagneticsinc.com/temperature-rise-and-insulation-system-ratings/
Yes, 150degC is really hot but understand that the insulation class of the common DTDT is 220degC.
bantam_dave21
Member
- Location
- Yorkshire, UK
Bantam...
Was your query answered to your satisfaction?
Regards, Phil Corso
Hi,
yes and no is the honest answer. I've read lots of interesting things as a result of these replies. What I was after which it seems now isn't really practical was a general rule of thumb I could apply in this instance and perhaps others and or a way approximating what in rush could be that I could use as a general approach for instances like these that I come across. I thought there was perhaps a guide folks use for various topologies through various size ranges in a similar way most DOL motor starting would generally (or to my experience anyway) be expected x6 FLA @ 0.4pf.
The good thing is I've had a bit of confirmation of what I could expect worst case to be which has assisted me to urge the client to re think their system model. Look at other ways of either omitting the transformers or supporting their loads differently so the back up generator they want can be more realistically sized.
thank you all very much.
Bugman1400
Senior Member
- Location
- Charlotte, NC
I think the rules of thumb posted in post #18 by templdl are valid. You don't have to worry about the pf if you just use the FLA multiplier. Typical inrush for larger xfmrs is mainly first and second harmonics with near 0.1 PFs since you are basically charging the inductive field.
Bugman1400
Senior Member
- Location
- Charlotte, NC
I have a follow up question on this same topic that I thought I would post under this thread. Typically, 8-12X xfmr FLA is used to ensure phase OC settings can ride through inrush. I say phase because I mostly see xfmrs that are delta connected on the highside where ground current is not an issue during xfmr energization. Also, typical time values I see as a rule of thumb are 12X @ 0.1 sec and 24X @ 0.01 sec. My question is about xfmrs that are connected solid g-wye on the highside. Would I also apply this same 8-12X xfmr FLA and time values for the ground OC setting too?
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
160218-1449 EST
Bugman1400:
My intutitive guess is:
Assumptions:
Line-to-line primary voltage remains the same.
Total KVA rating remains the same.
Three individual transformers are used and are scaled to work at the same input and output voltages.
The same core material is used and the design is for the same steady state maximum flux density.
All the primary lines to the transformers are turned on at the same instant, and there is no load on the secondary.
The turn off time was such as to leave one transformer in its maximum residual flux state.
The next turn on time is such that the transformer with the highest residual flux is maximally driven into saturation. This produces the maximum inrush current to the saturating transformer.
When the delta connection is used the transformer with the maximum saturation provides only part of the composite current in the input line. The other transformer current is much closer to a sine wave.
When the primary is connected wye, then the one saturating transformer provides all the line current. This saturation current is larger because the primary is wound for a lower voltage. I believe that the composite line current, which is now only the single transformer, will have a 1/2 cycle RMS current, or peak current, greater than the composite current when the primary was loaded with the delta transformers.
.
Bugman1400:
My intutitive guess is:
Assumptions:
Line-to-line primary voltage remains the same.
Total KVA rating remains the same.
Three individual transformers are used and are scaled to work at the same input and output voltages.
The same core material is used and the design is for the same steady state maximum flux density.
All the primary lines to the transformers are turned on at the same instant, and there is no load on the secondary.
The turn off time was such as to leave one transformer in its maximum residual flux state.
The next turn on time is such that the transformer with the highest residual flux is maximally driven into saturation. This produces the maximum inrush current to the saturating transformer.
When the delta connection is used the transformer with the maximum saturation provides only part of the composite current in the input line. The other transformer current is much closer to a sine wave.
When the primary is connected wye, then the one saturating transformer provides all the line current. This saturation current is larger because the primary is wound for a lower voltage. I believe that the composite line current, which is now only the single transformer, will have a 1/2 cycle RMS current, or peak current, greater than the composite current when the primary was loaded with the delta transformers.
.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I think that you are getting lots of answers because there are lots of possible different details which can give different results.
Regarding connecting a transformer with a solidly grounded wye primary:
In general, when a wye winding is used as the primary of a transformer, the neutral should be allowed to float. This is because the neutral is being 're-derived' by the winding, and huge currents can flow if the neutral is connected.
I am _guessing_ that this came up because there is a desire to take a transformer designed as a 1KV delta:0.4KV wye and using it in 'reverse' to step 0.4KV up to 1KV. If this is the case, then this will cause a huge increase on the inrush current.
The reason is that when transformers are designed, the secondary coils are placed on the inside near the core, and the primary coils are placed on the outside. This increases the 'leakage inductance' of the primary coils, which acts to reduce inrush current. When the transformer is used in 'reverse' you don't have this feature working in your favor.
Finally, with proper switching, my understanding is that transformer inrush can be greatly reduced or eliminated. Transformer inrush is caused by a combination of residual core flux and energizing phase angle. With proper switching control you can drop the transformer straight into steady state operating current.
Unfortunately, beyond telling you that this is possible, I do not have the experience to walk you through the details of selecting appropriate components.
-Jon
Regarding connecting a transformer with a solidly grounded wye primary:
In general, when a wye winding is used as the primary of a transformer, the neutral should be allowed to float. This is because the neutral is being 're-derived' by the winding, and huge currents can flow if the neutral is connected.
I am _guessing_ that this came up because there is a desire to take a transformer designed as a 1KV delta:0.4KV wye and using it in 'reverse' to step 0.4KV up to 1KV. If this is the case, then this will cause a huge increase on the inrush current.
The reason is that when transformers are designed, the secondary coils are placed on the inside near the core, and the primary coils are placed on the outside. This increases the 'leakage inductance' of the primary coils, which acts to reduce inrush current. When the transformer is used in 'reverse' you don't have this feature working in your favor.
Finally, with proper switching, my understanding is that transformer inrush can be greatly reduced or eliminated. Transformer inrush is caused by a combination of residual core flux and energizing phase angle. With proper switching control you can drop the transformer straight into steady state operating current.
Unfortunately, beyond telling you that this is possible, I do not have the experience to walk you through the details of selecting appropriate components.
-Jon
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