220/11kV Transformer- Minimum Rating? 1

[Marmite]

I'm looking at a small waste to energy plant where the generator output will be around 20MW at 11kV. Unfortunately the only possible utility connection is at 220kV. Is it possible to build a 220/11kV transformer with such a small rating?
Regards
Marmite

 

[bilalk]

My experience with transformer manufacturers is that they can build for you anything you want. However, here are some points I would like to make aware of before issuing your supplier's bid:
- figure out the right tx configuration (Y-Y, D-Y, Yg-Yg, D-Yg, and so on) because this will affect the zero sequence currents, harmonics transfer between generator and utility system;
- what are your local distribution utility requirements at the connection point;
- any protection requirements?;
- applicable tx standard for %Z, fusing, construction design, and so on; and
- the minimum efficiency standard you would need to go by (whether it follows minimum efficiency standards or TOC)

 

[ScottyUK]

Hi Marmite,

Have seen 275/11kV at 24/36MVA ONAN/ONAF, they were our station service transformers at Teesside. They came from the old ABB works at Dundee, but the capability exists to build them elsewhere.

 

[krisys]

Normally for small and remote hydroelectric generating stations, 25/35 MVA, 220/11.5kV transformers are commonly used. 220 kV class transformers are abundantly available at this rating.

Now to dispatch your 20 MW power, you may also need a transformer of 25/35 MVA rating. You should be able to buy from the competitive market.

 

[Marmite]

Thanks for the info guys. I vaguely recollected reading something about minimum practical ratings and was wondering whether we would be pushing it. Your answers have given me reassurance that this is not the case.

Thanks & regards
Marmite

 

[prc]

Marmite, you are right.For every voltage there is a minimum rating below which it is difficult to handle the HV winding due to very small size of the conductor and difficulty in making impusle proof HV winding. But 25 MVA is quite common at 220kV. A couple of years back I was involved in making a 16 MVA 220/33 kV 3 phase unit with OLTC. The minimum size in my experience is 10 MVA 220 kV that was made nearly 40 years back.

 

[prc]

An after thought-as mentioned earlier,technically the minimum feasible rating at 220 kV is 10 MVA and at 132kV 5MVA.Such units are occasionally made as users insist for such ratings.I thought this may be rare today,but find this is common in some emerging small grids and my employer is suplying them often. But it is always wise to go for 20&10 MVA ratings at 220/132 kV levels,even if load is less than the nominal ratings. The price increase for such increased MVA is negligible or nil, but you get a better transformer with better efficiency at actual working. In Indian grid,when I was starting my career some 50 years back, demand was for 20MVA 132/66 kV auto-transformers. For such utilities today we are making 100 or 160 MVA 3 phase 132/66 kV units. At that time GE,Pittsfield,US( regretfully that great factory is no more) supplied 20 MVA single phase 220/110 KV autos to local Indian grid. Today they want 200 MVA 3 phase 220/110 kV units.
Another change coming is increaed rating for subtransmission transformers.Earlier 220/33 kV rating was limited to 60 or 80 MVA due to the limitaion in rupturing capacity of 33kV breakers. Today there are applications of 180MVA 220/33kV units at high power density areas eg. high capacity oil refineries.

 

[FPelec]

I agree with prc, the smallest size for a 230 kV transformer should be around 10 MVA. The smallest HV/MV unit I have ever seen is a 3.9 MVA 150kV/3 kV transformer for railroad application, which is a standard size in Italy.
However, if you need only a few hundreds kVA at 11 kV, another solution could be to use SF6 insulated station power transformers (available from 25 kVA to 1 MVA):

https://library.e.abb.com/public/76941f54e79c84bcc1257b3400367ea2/Br_HV_TIP_en_7a.pdf

Si duri puer ingeni videtur,
preconem facias vel architectum.