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]

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

http://www.hydropower-consult.com

 

[slavag]

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

 

[ScottyUK]

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!

 

[ELEP]

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.

 

[ELEP]

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.

 

[wolf39]

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

http://www.hydropower-consult.com

 

[slavag]

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

 

[slavag]

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

 

[ELEP]

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.

 

[slavag]

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

 

[prc]

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.

 

[slavag]

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

 

[ELEP]

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

 

[slavag]

Hi ELEP.
Please see attached. One pict is better than 1000 words.


New configuration of trafo's is OK.
Good Luck.
Slava

 

[slavag]

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

 

[ELEP]

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;

 

[YIGO]

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

 

[ELEP]

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:

 

[kh2]

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

 

[ELEP]

thanks!
:KH2

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