Transformer Winding Type

[cabfa]

Do any of you have any recommendation or idea of which is a better transformer, one with a circular winding or one with a rectangular/ sheet type winding?
Thanks

 

[dpc]

I'm going to assume you're asking about a power transformer.

(Flame shields up)

My personal opinion is that "circular" core type transformers have been demonstrated to be superior to rectangular core units in withstanding the forces associated with through-faults.

Rectangular core transformers generally have a lower first cost but simply don't hold up as well if you are looking for a 30, 40 or 50 year transformer life.

(shields down)

 

[JBD]

Unless you are a utility or you want to have a specially built transformer it does not matter. Most purchasers chose a manufacturer first and then accept their standard offering.

Do you have a particular reason for asking?

 

[dpc]

JBD,

It matters to a lot of my industrial (non-utility) clients. I'd have to disagree with your approach to buying transformers in this day and age. Unfortunately, ordering a transformer from GE or ABB, etc does not mean that they will actually build it. In many size ranges they will sub it out and just put their name on it.

 

[apowerengr]

Cabfa - Your question has different design criteria mixed. You can build circular or rectangular "sheet" windings or disk or helical windings. As for the threshold where one manufacturing style is justified I believe it depends on voltage - I feel once you get to 35kV or above you can justify the costs associated with the advantages of circular disk or circular helical windings. At low voltages (<25kV) you can probably get what you need with a rectangular design providing you don't have frequent through-faults or severe duty such as arc-furnaces, etc. For me it is kind of like AL vs. CU - different applications depending on what I need to accomplish. There are some papers out there - Waukesha Electric Systems website will discuss the different types and advantages for power transformers.

 

[JBD]

dpc

I believe the primary builder of dry type transformers in the US is ABB, I can't think of any major still building their own.

I still think that for most applications <5000kVA and <35kV whatever is standard is usually fine. Of course special applications (ie.e furnaces) will always need evaluation. This is why it is important for Cabfa to tell us his application

 

[subtech]

I was priviledged lately to be able to speak at length with a transformer design engineer of considerable experience from California. He informed me that where he is employed, the cost break point for circular disk vs. rectangular sheet wound units comes at about 69KV and 20 MVA. Above these levels, the circular disk design has a slight advantage price wise and a significant advantage performance wise. Where I am currently employed, we have both types in service and speaking strictly from the performance aspect, I greatly prefer the disc or circular wound unit. The natural thermo-siphon action of oil and hence heat dissipation in a properly designed power transformer (disc wound) is a great advantage when compared to similar rated sheet wound units. We have several 69/13 KV 30 MVA units (55 deg C rise) that will consistantly operate with loads of 22 MVA with an ambient temp of 37 deg C with the first stage of forced air cooling not yet necessary. In my opinion, these same units are limited in over-loading capability by the protection applied to them, not so much by thermal constraints. If the range of loading on the proposed unit(s) varies greatly and signigicant time may be spent in the upper end of their rating range, you would be wise to consider very carefully the disc wound unit, even if the price is slighly higher than the sheet wound unit.

 

[cabfa]

All,
Thanks for your replies. The transformer I am looking to buy is a 13.8kV to 4kV delta-wye substation transformer. The transformer will be around 10MVA. I have seen that transformers with rectangular winding appears to be less expensive but I have heard a few bad things about them. I want to be able to buy a lasting transformer that won't be giving us trouble in the near future.

 

[rcw retired EE]

One measure of winding integrity is the change in measured impedance after a short circuit test. In theory, if the winding design is adequately braced there will be little winding movement or deformation during a sudden short circuit test. If a winding does move, the leakage reactance is affected and the measured impedance changes. (I beleive ANSI allows a 7.5% impedance change after a short circuit test.)

One supplier used this reasoning to justify its chepaer rectangular windings. Their test results showed no change in impedance before or after the short circuit type test. I don't recall if they were using sheet windings. It was for small to medium (25 MVA) transformers. Square windings allowed a smaller tank and some other savings, according to that supplier.

 

[ScottyUK]

And another test which is useful for detecting winding movement is the FRA test developed by the CEGB labs in the UK. A frequency sweep is carried out and the complex impedance the winding is plotted giving a 'fingerprint' of the winding. If carried out when the transformer is new then any future movement is normally detectable by repeating the test and comparing the results with the original.

http://www.transformerscommittee.org/info/F03/F03-FRA.pdf

gives the general idea. We try to ensure an FRA test is carried out on all new transformers.

----------------------------------

Sometimes I only open my mouth to swap feet...