Large single phase power transformer installation 2

[acog]

I am working on a project which requires either 2 x 300MVA, 500kV/115kV three phase transformers or alternatively 2 x 3 x (~100MVA?) single phase 500kV/sqrt(3) / 115kV/sqrt(3) transformers.

Single phase transformers are being explored as an option due to complications in transporting such a large three phase transformer.

I am interested in the problems presented by single phase transformers as opposed to three phase power transformers. Specifically any protection and switching issues.

Also,

If I use the single phase transformer option, then to derive an auxiliary supply: I assume the best option is to simply put another tap at medium voltage on each single phase transformer (for example at 22kV/sqrt(3)), then run a cable from the medium voltage bushing on each 500/sqrt(3) / 115/sqrt(3) / 22kV/sqrt(3) transformer to a step down auxiliary transformer (e.g. Dyn1, 22/0.4kV, 315kVA)?

Any comments will be greatly appreciated.

 

[Power0020]

It is not so clear for me about tapping.

Usually singe phase units cost more, more copper, more steel, bigger foundations, more cables, more maintenance time and cost, and more space.

Extra care should be taken during the design to clearly do the vector group connections.

The virtue is that in the case of failure of one phase, you can use a replacement spare for that phase (some utilities usually put a forth spare transformer next to the live bank).

Protection and switching issues? I don't recall any, you just get more fault probabilities by exposing live connections outside.

As I mentioned earlier, you will need surge arrestors at both sides if you connect the bank in delta.

You still need another transformer to drop from 115 kV to 22 kV...I don't know if units with tertiary connections exist on single phase basis but this will save another transformer!

 

[davidbeach]

At 500/115kV you're probably looking at two full windings rather than an auto transformer. You can always throw in a third winding rated for your 22kV and connect those in delta.

As I first read it I was thinking that 300MVA isn't that big, but realized that I'm used to autos and the 300MVA being what the Planners would call it based on top rating, not base rating. Single phase may be what you have to go with if you aren't close to a major rail line or can't take delivery from a barge.

With single phase, a seventh unit (700MVA total) gets you to N-1 whereas with 3-phase you need a third unit (900MVA total) if you want that redundancy.

 

[acog]

Thanks guys,

Really appreciate your input.

"At 500/115kV you're probably looking at two full windings rather than an auto transformer."

I have a separate line of enquiry with a transformer manufacturer, but can you please elaborate on this? I obviously assumed an auto-transformer would be the optimal solution in this instance.

I assume the insulation is not pushing us to a two winding design because the auto TX would have graded insulation.

So I am assuming the limitation you're referring to is the current carrying capacity of the single winding? At ~500A it is large, but not impossible.

The project is in a developing country and is located far away from ports or significant transport infrastructure so I assume the large three phase transformer solution is not economically viable once we factor in all the road upgrades.

Its going to be an interesting yard design trying to put a delta connection in at 115kV.

 

[davidbeach]

The lower the ratio between voltages the practical it is to go with the auto. At over 4:1 you're getting up there.

 

[bacon4life]

There have been several threads about the decline in use of tertiary windings. If you have an alternate source of station service power, you might consider skipping the delta winding.

 

[Mbrooke]

Isnt 3:1 the general rule of thumb between auto and isolation transformer?

 

[Mbrooke]

The project is in a developing country and is located far away from ports or significant transport infrastructure so I assume the large three phase transformer solution is not economically viable once we factor in all the road upgrades.

Its going to be an interesting yard design trying to put a delta connection in at 115kV.

I could be wrong, but I found a video which appears to use single phase auto-transformers with a delta tertiary:

https://www.youtube.com/watch?v=T0m0tnCLSHY

 

[FPelec]

Yes, 500 kV/115 kV is not a very favorable ratio for an autotransformer, but you can still save 25% of sizing MVA if compared to an equivalent 2-winding transformer.
If the neutral is solidly grounded on both sides, autotransformer could still be a cost-saving solution.
As regards single phase construction, for a 300 MVA rating I would choose use 3-phase, 3-limbs units without delta winding; total weight would be about 300t, transport weight about 230 t, so that site transport should not be a problem unless you have very challenging route to the substation.
As regards delta winding, it would only increase unit cost, weight, losses and mechanical stress during external short circuits.
Even if you need some hundreds of kVA for auxiliary power, station service transformers (115 kV / 0.4 kV) would be a more economical and safe solution.


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[dpc]

Banks of three single-phase transformers (with a fourth as a spare) used to be common for large installations where reliability was important. Installation will be more complicated and costly, but there are no significant issues that haven't been worked out decades ago. Protection is somewhat more expensive because there are three separate tanks, so temperature, level and pressure instrumentation is all times three. For 500 kV to 115 kV, I'm assuming that both windings will want to be wye (star) for ground sourcing, so you may need at least a buried tertiary.

I would initiate discussions with likely transformer suppliers and get their general thoughts and recommendations on all of these issues. Agree with David that this seems a bit outside the range for an autotransformer.

 

[crshears]

Our utility has a pair of autotransformers, 3-phase single tank, 500 kV/115 kV, 28 kV delta tertiary, rated at 225 MVA full-cooled, manufactured in 1967 by Canadian General Electric; admittedly not a recent installation...

A wind farm connects their 115 kV collector system to our 500 kV system with their own pair of autotransformers, same rating, same windings, manufacturer unknown; built within the last five years.

Not a large count of them to be sure, but they're out there.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

At 500/115kV voltage ratio, auto-transformers will be costlier than two winding transformers esp if OLTCs are used. With single phase transformers, a stabilizing tertiary winding is a must. So it can be used for auxiliary power supply by putting a distribution transformer,close to tertiary terminals.Base MVA of an auto-transformer will be (500-115)/500 x Line MVA.

 

[MatthewDB]

If you go the route of single phase units, why use six? You could do three 200MVA units instead.

Generally when I've seen single phase units, there is a fourth transformer installed. The primary, secondary and tertiary are connected to a bus that runs along the entire bank. If a unit fails, the jumpers from the faulty transformer can be reconnected to the bus for the fourth transformer, bringing it back online in hours if needed.

What is the latitude of the substation? At high latitudes solar flares can saturate transformers from geomagnetic currents. Single phase units are the most sensitive to geomagnetic induced currents. Three phase three limb cores with non-magnetic supports are the most resistant to saturation.