Step-up Transformer Sizing

[mersan55]

What is the sizing criteria or standard for selecting kVA rating of a Gas Turbine and Steam Turbine Generator?
Equal kVA rating of Generator and Transformer creates temperature rise problems in some cases as per my previous experiance.

 

[mersan55]

missing wording. I mean the following;

for selecting kVA rating of a Gas Turbine and Steam Turbine Generator Step-up Transformer?

 

[tlrols]

In a perfect world you will generally perform a simulation (i.e. load flow) with the generator operating at its full output (either turbine based of generator based) and size the transformer to handle the full output (in MVA). Note that you should accomodate possible future upgrades to either the turbine or the generator. All of the above is typically covered by using a transformer rated at 125% of the MW output capacity of the generator. Transformer ratings are slippery numbers to nail down...strap on a fan pack and you get more capacity rather cheaply...

 

[mersan55]

thanks for your quick response.
In my case Steam Turbine rating is 32125 kVA with 0,8 pf.
11/31,5 kV transformer will be used. Alternatives are 35000 kVA ONAN or 25000/35000 kVA, 32000/40000 kVA.
Of course 25000/35000 kVA is the best solution in terms of investment cost. But I coulnt find any standard or criteria giving the calculation method.
No chance to calculate and prove it without computer based load flow calculation?

 

[ScottyUK]

Bear in mind that turbines love cold days and that turbine output can rise more rapidly than the GSU transformer's ability to get rid of heat does as ambient temperature drops. You need to look at the capabilties of the generator and the transformer over the full specified ambient temperature range, not just at standard ISO conditions. The manufacturers should be able to give you spot data at max, min and nominal temperatures; if they can't then it's time to find a new manufacturer...


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If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

Keep in mind that not all power capacity of your generator will be exported. You have to subtract the in-house loads(electric-driven) from your available output. Though newer designs have low station service loads, GSU could be sized less than 100%.

 

[mersan55]

Do you know below standard is applicable for my case?

C57.116-1989

IEEE guide for transformers directly connected to generators

 

[bacon4life]

Yes, the standard would be a good read for you.

In C57.91, the predicted nominal life of a transformer at 110C hot spot temperature ranges from 7.4 years to 20.55 years depending on which end of life criteria is selected.

We have a fairly conservative policy for temperature limits that trips the banks if they get above 110C, thus the our banks have to be a bit over sized to have some operating margin and to account for measurement inaccuracies.

Also be sure to include the higher auxilary and load losses in your calculation for the smaller bank sizes.

If your transformer has taps, be sure that the capacity is not derated on the tap you are planning to use.

 

[dpc]

Bottom line: You do not want the capacity of the entire facility limited by the size of the generator step-up transformer. As Scotty said, the generation system may be capable of considerably greater output than the nameplate suggests and operators will use this capacity when necessary.

No one ever got fired for making this transformer a little too large.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)