GSU base rating 3

[Mbrooke]

Someone on Reddit posted a picture of a 500kv GSU nameplate which peaked my curiosity:

https://i.redd.it/xh51so2hiop41.jpg

With one set of coolers the rating is only 164MVA, but with all 6 or 7 its 750 MVA. My question- why is the increase so great? Does this transformer really have a 164MVA core- or is the design such its difficult to conduct heat away from the core without a forced system?

 

[prc]

It is a ODAF cooled unit without any ONAN rating. It has 6 coolers + one standby cooler to dissipate losses. Normally all the six will be running even when load is less. In the unlikely event of any faulty coolers (more than one) load shall be reduced to avoid overheating. With one cooler only running means it can handle only 16 % of total losses. Assume total losses =(1+6)X=7X (where x is no-load losses) At 20 % load (164 MVA) total losses will be (1+ 6x0.04)=1.24X ie approx 16 % of 7X.

It has a core for 750 MVA.

 

[Mbrooke]

What "suffocates" the core to such a degree? Medium size units only dip to about 60% for the loss of both fan stages.

 

[FPelec]

Mbrooke,
I think that the 60% derating refers to typical, core-type, ONAN/ONAF unit.
The attached nameplate refers to a shell-type ODAF transformer.
Without coolers (fans and pumps!) IMHO that unit couldn't even cope with its no-load losses, as prc also said.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[cuky2000]

For information, enclosed is an example of ODAF transformer derating capacity with a 50% inoperative cooling system.

I am not sure if it will be beneficial to operate an ODAF transformer with 1/7≈ 14% of cooling capacity.

>>>>>

 

[davidbeach]

We have several transformers greater than 200MVA rating classified as “FOA” the have a self cooled rating of ... wait for it ... zero MVA. That’s right, don’t turn them on if the pumps and fans aren’t running. Many years ago one of the was tripped when the cooling system failed. Top oil temp continued to rise for another 8 hours. They just can’t move the heat from the core to the exterior without a bit of help.

 

[Mbrooke]

Why shell instead of core? Are there any units over 200MVA with an ONAN rating?

 

[prc]

FOA was the cooling designation as per earlier IEE standard. Now they have lined up with IEC norms and as per latest C57.12.00-2015, it is ODAF.Oil is directed to flow through windings(not core) to remove I2R heat from windings. When unit coolers (oil to air heat exchangers)are used, as in this case, there is no ONAN rating. Pumps are switched on before energizing the transformer. Otherwise heat generated in winding ducts cannot come out to coolers through stagnant oil. These coolers are like car radiators. The story is in 1930s Westinghouse/GE developed these coolers by modifying the room heaters used in US.

But in IEC world (UK, India etc) sometimes customers demand ONAN rating for ODAF cooled units esp for large interconnecting auto-transformers. We then provide radiators ( instead of heat exchangers) with attached fans + axial flow oil pumps with open impellers, so that oil can flow even when pump is not working. Current thinking is to avoid pumps altogether and provide ONAN/ONAF-1/ONAF-2 cooling for even large units eg.1500 MVA 765/400 kV auto-banks in India.

 

[Mbrooke]

Current thinking is to avoid pumps

I like this idea. These pumps fail and can put metal shavings into the oil. I never liked the concept.

Any idea what the ONAN rating is for ONAN/ONAF-1/ONAF-2 units? Are ONAN only units available?

 

[prc]

Normally ONAN:ONAF-1:ONAF-2 ratings are 60:80:100 %.If some body wants 100 % ONAN rating (people ask such ratings only up to about 100 MVA)it is also possible.

 

[Mbrooke]

What does a 100%, 250MVA ONAN GSU unit look like? Just curious...

 

[FPelec]

Mbrooke,
this is a step up transformer for a large synchronous condenser in Sardinia. The ONAN/OFAF rating should be 250 MVA /330 MVA

https://impiantoelettrico.co/prodot...migliorare-la-qualita-dellenergia-in-sardegna

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[Mbrooke]

Thats what I was looking for!