Verifying Transformer Rating

[nhcf]

I have a power transformer marked
KVA = 2501/2800/3250/3500
Class ONAF
55/65 deg

Is it correct that the 2501/2800 are ratings at 55/65, no fans, and the 3250/3500 are 55/65 with fans?

 

[nhcf]

To expand - an existing transformer rated as shown above is the GSU for a 1.6MW/2.0 MVA genset. We wish to add a second identical genset, and I have stated a larger GSU is required. Others have challenged stating because generators are operated at close to unity pf (0.95-.0.99 pf leading) per interconnect agreement, total generation with two generators will remain below the 3500 KVA FA rating of transformer. Shouldn't the transformer be rated for full range of generator capability curve?

 

[dpc]

Yes, you have the ratings correct.

As to sizing - it depends. As long as the transformer is adequately protected and it is felt that the rating will not be exceeded, I be OK with one xfmr. Obviously a second transformer would allow for more flexible loading, greater reliability, and probably extend the life of both transformers, it would represent a major expense. I'd design the system so it can be added later.

 

[nhcf]

Thanks dpc.

To confirm - you are indicating you'd consider tying both the existing and new generators (total output 3.2 MVA/0.8 pf) through the single, existing 3500 KVA transformer, provided protection was configured to ensure transformer ratings were not exceeded? This would end up decreasing the overall KVA/reactive capability of the facility, although in normal operation this should not be an issue. As the existing GSU and gear is already installed, designing for a future connection would also involve some expense. I am guessing the mgmt. will elect to leave the existing xfmr and interconnecting switchgear line up as is. I had envisioned a second or new, larger transformer.

 

[dpc]

Adding a second transformer is a major cost. Replacing the existing transformer with a larger one could work, but is still a major expense. Just allow space and possibility for the second, if possible. I don't see how you can justify the cost a new transformer at this time, based on my understanding of the project. Also, transformer ratings can be exceeded in some situations.

 

[nhcf]

thanks again. The only justification would have been a need to match the MVA / pf capabilities of the generator, however if not required by the utility and adequate transformer protection provided, then I see the logic in using the existing unit.

 

[dpc]

Money is always a factor. Just don't paint yourself into a corner.

 

[johannes60]

Hi all,

when talking about genset ratings keep in mind that acc. to ISO 8528-1 'Application, ratings and performance' only 'Continuous Power (COP)'-rated genset are capable of delivering 100% of nameplate power on a 24/7 basis. Others like 'Prime Power (PRP)' usually are either limited to 70% on average over a 24-h-period or have limited running hours per year (500hrs at 100% for a 'Limited-Time running Power (LTP)'-rated genset). So except for COP-Operation the transformer may be downsized w/o compromising its life expectancy.
Load factors may also be different between manufacturers and deviate from the ISO standard.

Johannes

 

[nhcf]

Which KVA value does the %impedance apply to? I assume in this case the 2500 KVA value?

 

[dpc]

If this is an ANSI rated tranformer, the impedance is based on the lowest kVA rating.