Unit main transformer sizing? 3

[ELEP]

If Generator is 330MVA(208.5MW),18kV. How to calculate Unit main transformer size, is it correct to use factor 1.1 or 1.2 times of Generator rating ?.

can anyone help me to know standard/reference.

Thanks in advance !

 

[wolf39]

Usually the unit power transformer has the same MVA rating as the generator itself. However, in case the generator has an overload capacity specified, the transformer throuput has to follow.

Wolf

http://www.hydropower-consult.com

 

[ELEP]

thanks! wolf39
i am looking for any standard or supporting document because my client is asking for the same. Actually, we suggest 320MVA(ONAF2), 18/220kV UM transformer and 40MVA, 18/6.9kV UA transformer. Is this right choice ?

 

[prc]

There seems some thing wrong-You meant 285 MW generator and not 208.5 MW? You are right,generally 1.15 to 1.25 times Generator MW is taken for sizing GSU.In India, usually it is 125 MVA(110MW),250 MVA(200MW),600MVA(500MW),780 MVA(660MW)etc

 

[ELEP]

prc:
Yes you are right, Generator rating is 330MVA(280.5MW),(typing mistake^).

But, is it practice or some reference to use these factors.
1.15 to 1.25.

 

[wolf39]

ELEP:

As said before, if the MVA rating of a generator is known, the transformer MVA rating usually is identical with the generator figure. However, when you start with the MW output of a generator and if the power factor is known to you, just divide the MW by the power factor and you get the proper MVA figure

From the data supplied the power factor of your generator is 280.5/330 = 0.85.

The factor range prc is mentioning does cover a power factor range of between 0.8 and 0.9 as you can see from the table below (factor = reciprocal value of power factor):

p.f. = 0.80 factor 1,25
p.f. = 0.85 factor 1,176
p.f. = 0.90 factor 1,11

It is not necessary to estimate such factors as the generator power factor is one of the fundamental generator data specified by the utility in their order specification and should be easily available.

In case the MW turbine output is known, you arrive at the generator MVA figure by multiplying the turbine MW with the generator efficiency and by dividing the result with the generator power factor. Modern generators have an efficiency of between 0.97 and 0.99, depending on the generator output and generator type.

Wolf

http://www.hydropower-consult.com

 

[ELEP]

Recently, I had discussion with experts, they explained me as follow;
-Though Generator normal rating is 330MVA but it can be overloaded up to 10%, so to sustain Generator overload capacity, size UM transformer 1.10 times of Generator normal rating, even they told there is no any relation of those factors with Genr. power factor. It is coincident that those factors are 1/0.85=1.25.

But I wonder, transformer has also 10% overload capacity then why to oversize UM tranfr ?.

I think it is approprate to choose UM transfr rating equal to Generator rating as both have 10% overload capacity.

 

[wolf39]

ELEP:

Sorry, but your latest comment puzzles me a bit. I thought that my reply was self explanatory. Please note that overload factors have nothing to do with power factors.

By the way: 1/0.85 = 1.176

Wolf

http://www.hydropower-consult.com

 

[ELEP]

wolf39:
I don't doubt your reply, it is self explanatory,please don't get me wrong; but I am unable to convince my client, still they want any reference. Actually, we choose UM transformer rating by referring previous project data. I found those factors in previous projects; even in some project I found UM transformer has the same rating of Gener. Those projects are running without any problem. Both assumptions seems valid. So I am confused what explaination I have to represent, as I am not getting any reference.

Thanks for your kind reply.

 

[edison123]

ELEP

Matching the trafo MVA with Generator MVA is common sense. Like matching the motor kw to the load kw. I don't see standards playing any role here.

 

[burnt2x]

What about the power needed for the auxiliaries of your unit? Do you mean to say 208.5 MW can be delivered through your generator output trafo when you have to deduct the auxiliary power requirement?

 

[ELEP]

burnt2x:
Yes, including auxiliary(sorry it is 280.5MW, typing mistake in earlier post).
Generator size 330MVA, Main Unit T/F 320MVA & Unit Aux. T/F 40MVA. There is no Generator CB.

 

[wolf39]

I don't know of any standard here in Europe (and overseas) dealing with the selection of an MVA unit main transformer output in comparison with the MVA output of a generator. It therefore depends entirely on the utility itself or the consultant whether an overload factor is introduced to the main unit transformer or not. It may be a matter of (bad) experience your client had in the past. However, for extreme climatic conditions it can make sense to be on the safe side.

I would suggest you to contact various utilities in your country how they deal with this matter. For cases a transformer test report is available, one can check the temperature rise of the transformer windings. After comparing these data with the temperature rise figures specified it is possible to determine the permissible overload capacity. Alas, this does help only if a standard size transformer is being selected for your project. For steam plants this may be even the case and the OEM can be asked for permissible overload data.

For hydro application the static excitation system in the majority of cases is energized by tapping the generator terminals, i.e. in front of the transformer LV terminals. However, this doesn't relieve the transformer as we are dealing here with about 0.5 percent at the very most.

The auxiliary power of a hydro power plant in some cases is also available from the bus connecting the generator terminals of a multiple unit power house. For a single unit power station the auxiliary power better is coming from outside via an auxiliary transformer installed in the high voltage switchyard.

One more thing:
Is the auxiliary transformer you mentioned (40 MVA, 18/6.9 kV) really connected to the generator terminals? If so, a certain power flow doesn't pass the main unit transformer, as burnt2x pointed out. For a single unit power house this would be quite an unusual electrical lay-out. If the generator fails you experience a power house black-out, unless you have an extra power supply coming from outside. In case your power house has one generating unit and an auxiliary BASE LOAD of, lets say, 20 MVA, the maximum throuput of our main unit transformer is 310 MVA only. This gives you a 20 MVA overload margin.

The questions I have are as follows:

Steam or hydro?

How many generators are installed?

Does every generator have its own auxiliary transformer or is only one installed?

A power plant single line diagram would be helpful.

Wolf

http://www.hydropower-consult.com

 

[ELEP]

Thankx wolf:
pl. refer oneline.
It is coal fired power plant for South America.

 

[slavag]

Hi.
From my point of view.
Maybe better use 375/400MVA UMT instead 320 MVA and instead one UAT 40MVA, two of 20MVA ( 22/25MVA) and separate aux bus,aux load for the coal PS is about 10-15% of generator power.
It will be more "standard" configuration.
BTW, GCB is always good think, but very expensive for this size of generator.
Just my 0.2$.
Best Regards.
Slava

 

[ELEP]

thankx slavag:

Not shown in the oneline, but we fixed two winding UAT 40MVA, 220/6.9/6.9kV(20MVA each) and Aux. Start-up Trfr. 40MVA 220/6.9/6.9(20MVA each).
As there is a separate Aux. Start-up Trfr. we don't feel need to use GCB.
Regarding UMT size, it is 320MVA, so that the total transformer capacity connected to Gen. terminal including UAT(40MVA) is 360MVA, which is almost 10% more of 330MVA Genr. capacity.
Is this right configuration ?

 

[slavag]

Hi ELEP.
I cann't say right ot not. Isn't my level in such projects.
I don't know your aux configuration, your steam system configuration..
1. GCB is importand for reduce stress on the generator in the fault time.
2. Yes, you are right 1+1=2, but, if you will work with aux bus.., only on the SAT/RAT and will provide all gen. power via UMT? in case of UAT 6.9 CB fault, for example.
I don't know, maybe it some rule of tumb, in lot of such application I see UMT--MW===GEN--MVA.
Best Regards.
Slava

 

[ScottyUK]

Looks quite similar to our station configuration. Our GSU is 400MVA for a 365MVA generator. UAT is 48MVA, and the station transformer is 36MVA.

Based on recent experience at site, if you don't put in gen CBs on the 18kV bus for the GSU and UAT then make sure you have a very secure breaker fail scheme. One failed breaker caused a disturbing number of problems.


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

 

[slavag]

Just idea,
ELEP don't forgot about losses on the trafos.
It's few tens MVar.
You need manged reactive energy too.
And of course, Im with Scotty, GCB is important, but ELEP you can say in older applications GCB isn't use, is true, but you buils new one.
Regards.
Slava

 

[ELEP]

ScottyUK:
thankx,
-For any fault there will be UMT 220kV HV & UAT 6.9kV CB tripping alongwith turbine. We have designed few power plant with Gen. CB when there is no separate aux. source. In this case Aux. supply is taken from existing 220kV S/s so we ommited Gen. CB.

Any how ScottyUk, I would like to know more about breaker fail scheme used in your plant ?

In our earlier projects, We applied BF protection scheme which use a trip signal, a timer(62BF(instead of 50BF)) and an auxiliary contact of the C.B. for the detection of a C.B. failure. We are planning to use same for this project.