Sizing of a GSU 1

[Coca1995]

How do you size a GSU? Lets say i have a generation plant DER with 15 MW capacity, the GSU size should be 15 MVA? Or is it always sized a bit higher than normal plant capacity?

 

[dpc]

You need to look at the generator capability curves and nameplate ratings. Size for at least the highest MVA rating, plus a little extra. You don't want the output capacity of an entire power plant restricted by your generator transformer. I doubt anyone ever got fired for slightly oversizing the GSU. Transformers are MVA machines. What's a DER?

 

[edison123]

MW ≠ MVA

Muthu

http://www.edison.co.in

 

[waross]

Many 15 MW generators are 18.75 MVA. A 15 MVA transformer may be a little undersized.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[prc]

Normally the GSU rating taken in MVA is Generator MW x1.2 ie if Generator is 50 MW, GSU will be 60 MVA or 63 MVA, the nearest standard rating. The GSU need to handle only Generator power less the power taken by auxiliaries.

 

[davidbeach]

Nah, I wouldn't go that tight. Two rewinds later, the insulation systems have improved to the point that the 15MW generator is now a 17.5 MW generator (maybe the prime mover can't keep up with that, but that's different). Do you (or the future you) want to find out that you have to replace the GSU to gain all of the benefits of the latest rewind? Maybe not an issue with a recip engine as the prime mover, but hydro runners seem to have a lot more oomph than the originally installed generator, and the replacement of a 1960ish runner with a 2020ish runner along with a 2000+ rewind might call a GSU size into question. Squeezing out the last dollar (chucky, euro, loonie, etc.) today might have consequences tomorrow.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[thermionic1]

What kind of generation is the DER? What capacity factor does one expect from the plant?

 

[Coca1995]

Thank you guys. Your feedback is the only thing helping me out here. Auxiliaries on the plant itself are taking most of the juice out of the rated capacity of the plant. Its a natural gas DER. @dpc distributed energy resources, its a word that utilities here uses i believe lol

 

[cranky108]

Can the auxiliaries be fed from a start up source? Seen that happen.

 

[dpc]

I agree with the other comments - probably not safe to assume the auxiliaries will always come off ahead of the GSU. Turbines and generators of this size are often loaded based on temperature and if they can run above nameplate, they will, since the marginal costs for the added energy are pretty low. The transformer can be fitted with multiple stages of cooling also. You probably know this, but don't neglect any derating necessary due to high ambient temperatures.

 

[bacon4life]

For distribution transformers, the vast majority of the cost is fixed for things like the tank, bushings, accessories, and shipping, so upsizing from 15 MVA to 20 MVA may only cost a few percent rather than 33% more.

For evaluating transformer bids, I include factors for the expected lifetime cost of losses. For base loaded units, it can make sense to somewhat oversize the transformer. The oversized transformer will have lower I*I*R losses and will not have the fans/pumps run as much.

Lots of folks use the term DER, but the term DER doesn't really tell us anything about the operating characteristics of the plant. For example, saying solar PV provides obvious assumptions about daily load factor whereas with a wind plant there is an obvious correlation between wind induced natural cooling of the transformer and peak production from the plant. As an example of the variety of DER definitions, under the Washington State law (RCW 19.405.020) definition of DER, a natural gas generator is not a DER.

 

[wcaseyharman]

You might also check your grid codes or rate schedules as they may require a power factor less than 1 (this doesn’t seem as common in distribution as transmission systems in my experience) If so of course your transformer MVA should be at a minimum MW/PF if the VAR support is on the generator bus.