Transformer Nameplate Rating

B

bwat

EE
Location
NC
Occupation
EE
Am I correct that the amount of (apparent) power you can push through a transformer is actually less than the transformer's nameplate rating because you have to consider the losses of the transformer?



That's something I hadn't really considered before. I always mentally assumed you could put 10MVA through a 10MVA transformer. And so if you wanted 10MVA on the secondary side, maybe you'd be up >10MVA on the source side, but you'd still be under the damage curve by design. But thinking through this, I'm wondering what is true. Are you actually limited to strict nameplate size on the source side such that max MVA delivery is actually nameplate minus transformer losses (including var consumption)?
 
In reality, no. The KVA rating can be exceeded since the short term heat can be cooled at night or with fans. The only time I have seen heat show up that was slightly less than the VA rating was a large amount of harmonics. The business ran 24/7. That did degrade the transformer at a faster rate than expected.

I also wouldn't worry too much about the possible ideal vs non-ideal transformer with relation to the rated VA. It is more of a consideration for protection studies and money calcs when you want to consider the load and no load losses.

It is not going to effect the load you can put on a transformer.
 
How about this for a fun answer: it depends upon where you live (or where you buy your transformers).

Per IEC standards, a transformer kVA rating (and the 'base' for % impedance, % losses, etc) is the _input_ value.

Per ANSI/IEEE standards, the transformer kVA rating is the _output_ value.

The relevant standards are behind paywalls, so looking them up is an exercise left to the student. Here is a LinkedIn description of the topic with relevant standards numbers:
www.linkedin.com

Rated Power of Transformers

During the initial years of transformer development, transformer power rating was specified as output, in kW (sometimes in HP) as the case with other electrical machines like motors and alternators. Typical transformer ratings in 1893 were 1.
www.linkedin.com www.linkedin.com
 
winnie said:
How about this for a fun answer: it depends upon where you live (or where you buy your transformers).

Per IEC standards, a transformer kVA rating (and the 'base' for % impedance, % losses, etc) is the _input_ value.

Per ANSI/IEEE standards, the transformer kVA rating is the _output_ value.

The relevant standards are behind paywalls, so looking them up is an exercise left to the student. Here is a LinkedIn description of the topic with relevant standards numbers:
www.linkedin.com

Rated Power of Transformers

During the initial years of transformer development, transformer power rating was specified as output, in kW (sometimes in HP) as the case with other electrical machines like motors and alternators. Typical transformer ratings in 1893 were 1.
www.linkedin.com www.linkedin.com
Click to expand...

That's fantastic, thank you!

So since I'm typically working with ANSI/IEEE C57 transformers, I was (accidentally) correct in assuming these were the output ratings.
 
bwat said:
Am I correct that the amount of (apparent) power you can push through a transformer is actually less than the transformer's nameplate rating because you have to consider the losses of the transformer?



That's something I hadn't really considered before. I always mentally assumed you could put 10MVA through a 10MVA transformer. And so if you wanted 10MVA on the secondary side, maybe you'd be up >10MVA on the source side, but you'd still be under the damage curve by design. But thinking through this, I'm wondering what is true. Are you actually limited to strict nameplate size on the source side such that max MVA delivery is actually nameplate minus transformer losses (including var consumption)?
Click to expand...
Just for the record, apparent power is KVA. True power it KW
 
winnie said:
How about this for a fun answer: it depends upon where you live (or where you buy your transformers).

Per IEC standards, a transformer kVA rating (and the 'base' for % impedance, % losses, etc) is the _input_ value.

Per ANSI/IEEE standards, the transformer kVA rating is the _output_ value.

The relevant standards are behind paywalls, so looking them up is an exercise left to the student. Here is a LinkedIn description of the topic with relevant standards numbers:
www.linkedin.com

Rated Power of Transformers

During the initial years of transformer development, transformer power rating was specified as output, in kW (sometimes in HP) as the case with other electrical machines like motors and alternators. Typical transformer ratings in 1893 were 1.
www.linkedin.com www.linkedin.com
Click to expand...
Learned something new today...
 
bwat said:
Agreed. Did I seem to say something otherwise? In my first sentence I specified "apparent" and then only used MVA everywhere.
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The nameplate rating would be the same as the apparent power. Ture power is the variable, that is why I made the statement.
 
Strathead said:
The nameplate rating would be the same as the apparent power. Ture power is the variable, that is why I made the statement.
Click to expand...
Sure, but I think you might be misunderstanding the topic on this thread, or I'm not understanding your point for stating things about the relationship between apparent and real power?

(Not trying to be a jerk. I'm only replying/clarifying in case somebody reads this thread in future years and wonders if there was something here related to kW vs kVA, but there isn't. This is related to input kVA, output kVA, and the nameplate kVA. Winnie's post #3 is gold and I'm not too sure what you were getting at)
 
bwat said:
Sure, but I think you might be misunderstanding the topic on this thread, or I'm not understanding your point for stating things about the relationship between apparent and real power?

(Not trying to be a jerk. I'm only replying/clarifying in case somebody reads this thread in future years and wonders if there was something here related to kW vs kVA, but there isn't. This is related to input kVA, output kVA, and the nameplate kVA. Winnie's post #3 is gold and I'm not too sure what you were getting at)
Click to expand...
There is a difference between VA and watts though. Maybe isn't normally all that significant when dealing with transformers, but can be significant when dealing with AC motors and generators.

A transformer generally passes low power factor from a supplied load through and onto the primary circuit, but since transformers are normally rated in VA and not watts, this is somewhat ignored. But correcting the PF of the motor before the transformer will lower the VA that does pass through the transformer and if significant enough amount of correction is applied you could use a smaller transformer to supply the same load than if you didn't have any PF correction because you have corrected the apparent power to be closer to the actual watts.
 
kwired said:
There is a difference between VA and watts though. Maybe isn't normally all that significant when dealing with transformers, but can be significant when dealing with AC motors and generators.

A transformer generally passes low power factor from a supplied load through and onto the primary circuit, but since transformers are normally rated in VA and not watts, this is somewhat ignored. But correcting the PF of the motor before the transformer will lower the VA that does pass through the transformer and if significant enough amount of correction is applied you could use a smaller transformer to supply the same load than if you didn't have any PF correction because you have corrected the apparent power to be closer to the actual watts.
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I don't know who is arguing this and why we started talking about watts or PF at all. I think I need to just let this drop... This thread was super helpful, but now we're just onto stating somewhat irrelevant facts that nobody is asking about.
 
bwat said:
I don't know who is arguing this and why we started talking about watts or PF at all. I think I need to just let this drop... This thread was super helpful, but now we're just onto stating somewhat irrelevant facts that nobody is asking about.
Click to expand...
You used the term "apparent power" in the OP. Apparent power is generally considered to be VA which is only the same thing as true power (watts) if you have unity power factor.

Now that you cleared up that is not what meant when you said that, it probably can be dropped for the sake of this thread.
 
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