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Power Transformer Rating for Generation

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bow72

Electrical
Dec 24, 2009
1
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,

 
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I'm used to seeing generator step up transformers with just a single rating, often an ONAF rating with full cooling running all the time.
 
I agree with Davidbeach.
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
 
I also agree with David. I have only ever seen one rating in a GSU transformer. The fans will kick on autmatically when the temperature requires it. Pumps will run continuously.

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.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.
 
I am currently working on a power plant expansion in California.

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.
 
There are ANSI standards for ratios forced-cooling ratings versus self-cooled as a function of the transformer rating.

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
 
I'd also check the coldest day too, generator output will be far higher on the coldest day than on the hotest day.
 
Also be aware that the design life suggested by IEEE C57.91 is only between 7 and 20 years for a tranformer running continuously at it's rating. Over sizing the tranformer a bit to run cooler may result in a much longer life if this is for a baseload unit.
 
If it is a gas turbine plant, the MW output increases as temperature decrease, as David said. The transformer cooling capability also increases in the lower ambient, providing some compensation. Do a chart of turbine MW output versus ambient temperature and use the ANSI transformer temperature derating curve to chart the transformer capability on the same chart. (0.5% per degree C sticks in my mind but I don't have my reference with me.) Subtract the power plant auxiliary load from the generator output because the transformer only sees the net output.

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.




 
I suggest to use the rated power of 44/55 MVA, ONAN/ONAF. For ONAN the rated power is up to 80%.
 
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