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Unit main transformer sizing?
3

Unit main transformer sizing?

Unit main transformer sizing?

(OP)
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 !

RE: Unit main transformer sizing?

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
www.hydropower-consult.com

RE: Unit main transformer sizing?

(OP)
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 ?  

RE: Unit main transformer sizing?

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

RE: Unit main transformer sizing?

(OP)
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.  

RE: Unit main transformer sizing?


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
www.hydropower-consult.com

RE: Unit main transformer sizing?

(OP)
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.

RE: Unit main transformer sizing?


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
www.hydropower-consult.com

RE: Unit main transformer sizing?

(OP)
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.
 

RE: Unit main transformer sizing?

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.

RE: Unit main transformer sizing?

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?

RE: Unit main transformer sizing?

(OP)
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.
 

RE: Unit main transformer sizing?

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
www.hydropower-consult.com  
   

RE: Unit main transformer sizing?

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
 

RE: Unit main transformer sizing?

(OP)
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 ?

RE: Unit main transformer sizing?

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

RE: Unit main transformer sizing?

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. blush One failed breaker caused a disturbing number of problems.
  

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

RE: Unit main transformer sizing?

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

RE: Unit main transformer sizing?

(OP)
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.

 

RE: Unit main transformer sizing?


ELEP:

Thanks for the line diagram. One first note: The UAT next to the generator must have a voltage ratio of 18/6.9 kV.

In general the configuration looks okay. With 320 MVA transformer rating and 330 MVA generator rating you've assumed a minimum base auxiliary load of 10 MVA as a throuput for the 18/6.9 kV UAT. However, it would be sufficient to deliver the power house auxiliary via the 220/6.9 kV UAT allone and to skip the 18/6.9 kV transformer. Then increase the UMT rating to 330 MVA and you've got the most economical plant lay-out. But nobody can argue that the lay-out you presented in your single line diagram is wrong. A power plant lay-out always is the result of a certain design philosophy, coupled with experience and the desire to guarantee a safe operation. But the most important point for the client is the cost aspect, whether we engineers like it or not.

My comment regarding your post dated 11 Jul 08 0:45 is as follows:

If the generator is designed for 280.5 MW/330 MVA the throuput of all transformers directly connected to the generator terminals cannot be higher than 280.5 MW/330 MVA. The apparent power available to the 220 kV GIS therefore is 330 - 40 = 290 MVA in case the auxiliary power demand is at its maximum of 40 MVA. And there is no difference if you skip the 18/6.9 kV UAT and rely entirely on the outside 220/6.9 kV UAT. In this case the 220 kV GIS is receiving 280.5 MW/330 MVA from the generator but the GIS has in turn to deliver 40 MVA to the power house auxiliaries. In such case you have to increase the UMT rating from 320 MVA to 330 MVA.

I hope all this helps and good luck for the discussions with your client.

Wolf
www.hydropower-consult.com

RE: Unit main transformer sizing?

For my pinion you forgot other options.
generator work on full 330MVA, but system must export reactive power from  grid,
2. I again say, GSB need not only for aux bus feeding.
In case of generator fault and 6.9kV fault all energy of aux bus flow to fault generator.
Regards.
Slava
 

RE: Unit main transformer sizing?

ELEP,

Ou plant does not have a well-implemented breaker fail scheme. There is an excellent and very complex scheme protecting the 275kV substation, but not on the 16/11kV UAT. The UAT breaker failed to open when commanded, and a cascading series of faults eventually took out a 900MW generating block, hence my comment to install a robust breaker fail scheme. A BF scheme could have contained the problem before it got out of hand. There are a couple of ways to implement BF and I don't have a strong opinion as to which is better: any scheme would have been better than none. Maybe one of the protection experts can offer some advice?
  

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

RE: Unit main transformer sizing?

(OP)
thanks for explaination! wolf39:

For starting only Aux. Tfr 220/6.9kV will be used, once Genr. is synchronized, the UAT 18/6.9kV will be feeding auxiliaries & Aux. tfr will be  off.

 

RE: Unit main transformer sizing?

(OP)
slavag:
Thanks.
I couln't understand about reactive power export, will you pl. explain bit more.
-
One more fact about GCB I got from various Utility operational people is that they usually keep GCB tripping bypassed, instead they trip UMT HV CB & UAT LV CB. In my openion they might have facing many problems about GCB. Because GCB has to interrupt more current on less voltage(generally 13.8kV, 18kV, 22kV etc.) so there might be many problems about frequent contact wearing, mechanism etc.even GCB manufacturers are very few in the world.
 

RE: Unit main transformer sizing?


slava:

I may have missed a point, but at rated voltage and with an active power of 280.5 MW and an apparent power of 330 MVA the generator in any case is capable to produce about 174 MVar reactive power (overexcited). For underexcited reactive power one has to refer to the generator power diagram of the OEM. In case the generator synchronous reactance is very low, a 174 MVar reactive power (underexcited) would also be possible.

Regards,
Wolf
www.hydropower-consult.com

  

RE: Unit main transformer sizing?

Hi.
1. Reactive power export, that mean, absorb of reactive energy from grid.
I have check why UMT/GSU MVA is usually more than MVA of generator. In case of absorb of reactive power, via UMT flow reactive energy for the aux. bus feeding.
2. BFP system in the power station is very intresting system.
We provide now retrofit of  gen-trafo block protection systems and will change all BFP logic.
Firstly, we separate BFP initiation on two ways, protections with high level of currents. And w/o current detectors,for example: reverse power,voltage protection, turbine trips, stator e/f, buchholtz, pressure relief, etc.
This signals are sended to HV BFP system via CB contacts with no current detector.
Protections with high level of current are sended to HV BFP w/o CB position, but with about 0.8Inom current detector.
In additional, each protection system ( we used fully duplicated protection scheme) include four BFP signals (DC1 and DC2 ).
Best Regards.
Slava

RE: Unit main transformer sizing?

Hi.
ELEP, Im not right, I try check few options and see, that
Wolf comments is right. More, I check configuration of few old power plants with generators and UMT with more or less same size and more or less same configuration--- What Wolf said.
1+1=2, rule work.
Best Regards.
Slava.
Star to Wolf

RE: Unit main transformer sizing?

(OP)
thanks!
slavag;scottyUK;wolf
for your valuable suggestions.

slavag:
Please check attached LBB schemes, if you mean like this, if it is same then you have covered faults  when HV CB open as well as HV CB closed. This is best LBB scheme I think.  

RE: Unit main transformer sizing?

Hi ELEP.
Firstly, for BFP, we use independent relay with independent CT inputs and independent contacts. This is part of BBP ( or stub protection) of HV side.
Yes, I meant more or less to this, for the high current level BFP init.
50BF, I>, for more detaliesied, is gate "OR" from every phase or max. comparatore from three phase. And before out "TRIP" gate "AND".
Next point, we add timer not before "AND" to trip, we add it after, it's serious difference.
Pllease pay attention on this point. Timer BFP depend on two criterions.
Next point, this timer, lets say T1 send trip to block CB .
and start timer T2, after time T2  
via BBP will sended trip to all HV CB's of actually zone.
In parallel to this, added no current BFP init signals and are started same T1, but w/o current detector. CB aux. contacts are disconnect plus of those BFP init. signals and not in logic, externally.
Next point, for avoide unwanted trips we add HV disconnectors conntacts to BFP systems and add some switch
"BAY OUT OF USE".
Best Regards.
Slava

RE: Unit main transformer sizing?

ELEP, is there a mistake in the sketch of your 10th posting?
I believe UAT transformer primary should be 18 kV ,and not 220 Kv.The trf you have shown UAT 220/6.9 KV ,directly coming from 220 KV grid should be named SST ie station start up transformer.There is  also some interesting feature in the vector relation ship of these transformers.GSU will be Yd11,UAT Dy1 and SST Yy0.If GSU is Yd1,then UAT shall be Dy11.

RE: Unit main transformer sizing?

Prc.
You are right.
Standard confuguration GSU/UMT is Yd1, UAT is Dy11 and SST ( we call it RAT, reserve aux, transformer) is Yy0.
Best Regards.
Slava

RE: Unit main transformer sizing?

(OP)
slavag:
We are also in favour to use dedicate CT & relay for BFP as it is the most important protection but due to economic problems many client ask us to use same CT(diff. or OC/E/F. slavag, I represent this scheme  just for understanding, actually inside relay & wiring it is different. This schemes can be designed in many different ways, also it depends upon protection design engr, but purpose is same as shown in the diagram. One thing I couldn't understand how keeping timer after AND gate will work BFP. If it is like this then only one prot. device will initiate the same. Because AND gate is used to sum initiations from all protecting devices, will you please explain more?.

prc:
the diagram I shown is just for shake of understanding. You are right it should be SST & UAT should be 18/6.9kV, I have attached updated dia. with tfr, vector group,pl. find, your comments are most welcome.

-thanks both of you for sharing views.
ELEP
 

RE: Unit main transformer sizing?

Hi ELEP.
Forgot add important point.
Please pay attention:
Not all CT's you can use for the current detector of BFP.
In case of fault on the 18kV bus, or on the generator current will be high after opened CB's and exsitation up to few seconds.
Best Regards.
Slava

RE: Unit main transformer sizing?

(OP)
Good explaination slavag:
right, One pict is better than 1000 words!

yes, I do agree, all CT's can't be used for BFP. I think for some old installations it can as there are lots of difficulties to install new CTs.

thanks!

ELEP;
 

RE: Unit main transformer sizing?

I just want to share what I know about GSU transformer sizing. One case I dealt with, the sizing was based on the generator rating , auxiliary loads, and estimated losses. Power flows scenarios are analyzed with different PFs (e.g. 0.8, -0.8,1).
GSU size is selected based on worst scenario.

Hope it helps.

Yigo

RE: Unit main transformer sizing?

(OP)
thanks  YIGO:
will you please ref. SLD in my posting dt.14 Jul, 08 23:18. For aux. load we have UAT(Unit Aux. Tfr.) and it will be always feeding plant auxiliries. Station SST(Station Start-up Tfr.)will be used for starting purpose only, once Gen. is synchronised it will be off. So we need to size UMT(Unit Main Tfr.) based upon net generated power going through UMT. I was actually searching any standard for sizing the same but I come to know it is general practice  in many countries & many consultants that UMT size is equal to Gen. MVA rating. Yes, we need to consider estimated no load/full load losses as well.

If you have any ref. or calculation pl. try to post.

thanks & best regards,

ELEP:

  

RE: Unit main transformer sizing?

Elep,

I came across your post late, but felt that you may look at IEEE C57.116-1989 since your original question was about standards/references. It is called IEEE guide for transformers directly connected to generators. The guide describes the selection of Unit Transformers and Unit Auxiliaries Transformers.

Regards,

KH2

RE: Unit main transformer sizing?

(OP)
thanks!
:KH2

yes! IEEE C57.116-1989 is very helpfull.

RE: Unit main transformer sizing?

You could also look in IEEE C57.91 Transformer loading guide to check that the transformer will have a long enough life if this is a baseloaded unit.  Running a transformer at nameplate conditions could lead to a lifetime as short as 65000 hours.   

RE: Unit main transformer sizing?

(OP)
thanks:bacon4life

Transformer loading is nicely explained in IEEE C57-91, I think it can be easily applicable for distribution transformers. Grid connected and generating transformers are always running on full load I believe, and  they are desigend like that I think.

 

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