Transformer in electric generation plant

[hisham.i]

Hello,

I was looking into some drawings for electric generation plant, and i figured out a high voltage transformer 13.6kv/380KV 600MVA that connects the plant to distribution line.

My question is why they use only one transformer 600MVA? Why they don't use 3 transformers 200MVA each so in case one has some fault the other can supply electric power also.

Regards,
Hisham

 

[jim hardy]

Some plants do use multiple transformers.

It's a tradeoff. The complexity of switching and relay protection schemes grows exponentially. Complex things are more prone to mis-operate than are simple ones.

And - if it's just one generator, why not consider the generator - transformer pair as a unit?

 

[nsaspook]

Hello,

I was looking into some drawings for electric generation plant, and i figured out a high voltage transformer 13.6kv/380KV 600MVA that connects the plant to distribution line.

My question is why they use only one transformer 600MVA? Why they don't use 3 transformers 200MVA each so in case one has some fault the other can supply electric power also.

Regards,
Hisham

Sending two of the necessary three phases on the power line can work in a limited fashion (open-delta high-leg) but at those power levels I don't think the increase in phase current in the other feeders would be very safe at full load so a disconnect on all would happen automatically even if you ignored the phase loss.

http://www.allaboutcircuits.com/vol_2/chpt_10/5.html

Combining all three into one looks like good engineering to me.
http://www05.abb.com/global/scot/scot252.nsf/veritydisplay/a88fb159b6a1a33dc125798a00296d0d/$file/1LAB000086_SPT_brochure_EN.pdf

 

[hisham.i]

Some plants do use multiple transformers.

It's a tradeoff. The complexity of switching and relay protection schemes grows exponentially. Complex things are more prone to mis-operate than are simple ones.

And - if it's just one generator, why not consider the generator - transformer pair as a unit?

Reasonable point of view jim.

But engineering wise, which arrangment is better?

One more question, cost wise, what will cost more 1 transformer 600mva or 3 transformers 200mva (without counting the protection system for each one) just the transformers.

Sending two of the necessary three phases on the power line can work in a limited fashion (open-delta high-leg) but at those power levels I don't think the increase in phase current in the other feeders would be very safe at full load so a disconnect on all would happen automatically even if you ignored the phase loss.

NSASPOOK

I think that you didn't get my point correctly, i don't mean that i want to send each phase on separate transformer, i mean that i want to make 3, 3 phase transformers instead of 1, 3 phase transformer.

 

[jim hardy]

But engineering wise, which arrangment is better?

Are you speaking of three 3 phase transformers each sized 1/3 capacity, or three single phase transformers wired for 3 phase?


three 3phase:
My employer owned two fairly large nuke plants. Mine used a single transformer the other used two with each sized 60%.
I always preferred the simplicity of the single transformer at my plant, though with two you can quickly resume operation should one fail.
They are an expensive part with long lead time and to have a plant down for a year waiting on one would be mighty expensive. To that end we bought a spare.
Good engineering includes lifetime cost and ease of operation. I preferred the single transformer but some other engineers preferred twin. It'll be interesting to see which they choose for next plant.

three single phase:
Sometimes out of necessity one connects three 1phase transformers and wires them for 3phase. You'll see that occasionally on power poles feeding a small factory.

The same 3phase advantage in copper cost for transmission lines applies to the iron in transformers.

Note the flux in center leg is zero.

So 3phase transformers are made this way instead:

observe there's only three legs of iron opposed to the six legs in three 1phase transformers.

There's advantages in harmonic distortion with a 3phase over single phase cores also.

So I believe the single large 3phase transformer is better design. I think most engineers would agree.

Darn - a picture is worth a thousand words and even with four pictures, this STILL came out too wordy.
Thanks for reading.

old jim

 

[nsaspook]

Like Jim said, more connections -> more cost. Each connection must have a disconnect, circuit protection, require extra space and will usually reduce reliability and increase maintenance costs. I see little or no advantage to using 3 smaller units if a reasonable single unit is available.

 

[hisham.i]

Thanks Jim and nsaspook :)

 

[Windadct]

Good questions - MANY reasons:

This dual transformer thinking would appear to add to reliability - or at least reduce MTTR ( mean time to repair), but for the generator transformers the Generator (MV BUS) side is often gas insulated - it is not easy to just disconnect and reconnect these transformers, or each transformer needs it's own output breaker each transformer would probably have to have independent EHV Breakers - more cost, complexity.

Then classic MTBF calculations - expectations say that 2 transformers are 2 x as likely to fail as 1 - so even with better MTTR the MTBF is worse.

Also - no two transformers are identical - the impedance miss-match causes circulating currents - and increases losses, at this power level this is a significant amount of current - which heats the transformers adding to losses and limiting it's actual power throughput. It also makes the already challenging generator protection ( relaying) very complex.

Generation station transformers are designed and maintained from a super critical perspective ( over designed and really taken care of - monitored maintained - etc). Since if the transformer fails, there is no way to stop the energy of the generator from feeding the fault, there is no circuit breaker between the generator and the transformer. Failures of these transformers is very rare. Also there are many generators feed into the grid, so anyone unit can fail and typically not greatly affect the total grid.

So for loads this can apply - but the better solution is still something like a double(+) ended substation, where there is significant load between the transformers, to prevent and minimize the circulating currents - etc. I worked with some banks that had 3 separate feeds as well as Full Building UPS AND N+2 Standby Generation ( Corporate banking has the best power systems - and IMO Hospitals some of the worst--- what does that say about priorities!)

 

[gerbi]

Quick answer: one 600 MVA transformer is cheaper than 3 x 200 MVA, it also requires less space (maintenance is also simpler with one unit). Banks of 3 transformers are used when building/transporting/installing single (very large) transformer is a problem (limits in mass or dimensins of transformer).