Power Transformer Rating for Generation
Power Transformer Rating for Generation
(OP)
Hello,
I am working on the design of a 50 MW generating facility on the USA connecting at 138 kV and need to specify the transformer rating. The facility can control p.f. to 0.95 so I decided to size the transformer up to 55 MVA. Now the issue comes with the ONAN and ONAF steps i define.
I am used to see in IEC that the ONAN is around 70% of the ONAF rating, so in this case a 40/55 ONAN/ONAF. But I see that the ANSI/IEEE practice usually has two ONAF steps and being 33% and 66% higher than the base ONAN rating, in this case 35/45/55 MVA ONAN/ONAF/ONAF. In this case the two transformers are relatively similar but the differrence is bigger for larger generating facilities.
My question is, how would you specify the transformer rating in this case?
thanks,
I am working on the design of a 50 MW generating facility on the USA connecting at 138 kV and need to specify the transformer rating. The facility can control p.f. to 0.95 so I decided to size the transformer up to 55 MVA. Now the issue comes with the ONAN and ONAF steps i define.
I am used to see in IEC that the ONAN is around 70% of the ONAF rating, so in this case a 40/55 ONAN/ONAF. But I see that the ANSI/IEEE practice usually has two ONAF steps and being 33% and 66% higher than the base ONAN rating, in this case 35/45/55 MVA ONAN/ONAF/ONAF. In this case the two transformers are relatively similar but the differrence is bigger for larger generating facilities.
My question is, how would you specify the transformer rating in this case?
thanks,






RE: Power Transformer Rating for Generation
RE: Power Transformer Rating for Generation
Obviously the cooling system becomes critical to the station maintaining output so don't be tempted to cut costs. You should also consider the variations in both voltage and frequency on the 138kV system when specifying the transformer. You would want to be able to maintain output with the grid voltage high, or the frequency low.
Regards
Marmite
RE: Power Transformer Rating for Generation
However, you may want to consider more than 55 MVA. Remember that often generating facilities are called upon to generate Vars to control system voltage. This is also critical during power outages such as the one we had in the north east U.S. not too long ago.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Power Transformer Rating for Generation
The latest incarnation of the LM6000s is 77 MVA, 0.85 pf.
The GSUs we selected are 60/80/100 MVA ONAN/ONAF/ONAF.
At this transformer size, you can pretty much get whatever you want. Call a manufacturer, they would be happy to discuss your needs and their capabilities.
RE: Power Transformer Rating for Generation
If you specify a self-cooled rating, that sets the physical size and design of the transformer to a large degree.
I'm not really clear on what you are asking. One rule to keep in mind is that you do not, under any circumstances, want the output of the power plant to be limited by the size you picked for the transformer. Generators and turbines are often run beyond their nameplate ratings, especially after the warranty period. Sometimes nameplate ratings are based on other factors besides the actual capacity of the equipment. I would talk with the mechanical engineers who would know about the potential over-nameplate capacity of the prime mover, figure out the generator MVA out based on that and then make sure the transformer can handle that on the hottest imaginable day.
David Castor
www.cvoes.com
RE: Power Transformer Rating for Generation
RE: Power Transformer Rating for Generation
RE: Power Transformer Rating for Generation
Then add curves for 0.90 power factor MVA and curves showing the transformer capability output with one and both stages of cooling in operation.
If you load it on a spreadsheet, you can adjust the base ONAN rating and see how much margin the transformer has at the various operating points.
My mentor always sized the transformer so the maximum net MW output at 0.95 power factor was in the middle of the second stage of cooling. If one stage of cooling is lost, the plant can still run full out by running closer to 1.0 pf.
One major client wanted to get even more precise and had us subtract the transformer kW and KVAR losses from the generator net output since the transformer's rating is based on the throughput not the input MVA. (IEC is based on input MVA). That minimizes the transformer's first time costs.
Size the cables, bus and breakers to handle at least 25% more so they do not limit the output either.
Conservative clients specify the rating with worse case scenarios: low voltage, maximum MVAR (lowest pf), one stage of cooling out of service, highest ambient, no auxiliary load and 20% of radiators out of service. Those transformers seldom operate the second fan stage and last a long time, if you can afford it.
RE: Power Transformer Rating for Generation