Large single phase power transformer installation
Large single phase power transformer installation
(OP)
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.
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.






RE: Large single phase power transformer installation
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!
RE: Large single phase power transformer installation
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.
RE: Large single phase power transformer installation
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.
RE: Large single phase power transformer installation
RE: Large single phase power transformer installation
RE: Large single phase power transformer installation
RE: Large single phase power transformer installation
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
RE: Large single phase power transformer installation
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.
RE: Large single phase power transformer installation
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.
RE: Large single phase power transformer 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]
RE: Large single phase power transformer installation
RE: Large single phase power transformer installation
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.