Two or three winding transformer cheaper

[Saut]

I have a question for costs on power transformers. Would it be cheaper to get two 150MVA Two-winding transformer, or a single 300MVA Three-winding transformer?

 

[nvcsastry]

THREE WINDING TRANSFORMER OF 300MVA IS OBVIOUSLY CHEAPER
THAN TWO TRANSFORMERS OF 150MVA. BUT IS NOT WISE TO GO ON COST CONSIDERATION, AS THREE WINDING TRANSFORMER IS MORE
PRONE FOR SHORT CIRCUIT FAILURE.

 

[GreyGoose]

Reliability is an issue, not so much because it is three winding versus two winding, but because you have one transformer versus two. If that single three winding transformer has a failure all your downstream loads are out of commission until you can repair/replace that single transformer. However, if one of your two winding transformers fails, at least you can cross-connect the systems downstream (I assume) at run your system at reduced load. Although all large transformers are basically custom made, it would be easier (and therefore cheaper) AND FASTER to replace one two winding transformer with a stock design than go manufacture a new three winding transformer (if the failure is that severe).

But yes, a single three winding transformer would be cheaper to manufacture & install as opposed to two-two winding transformers

 

[Saut]

Thanks for that insight, and that's what I've thought, but I've got quotations from transformer makers that say that the three winding is more expensive than a two winding, due to the manufacturing process and materials, is this true? and they say it also depends on whether is it a high power transformer (> 250MVA) or a small rating.

 

[peebee]

Two-windings are commonly used in or near just about every building in the world. Three-windings are somewhat rare. I'd suspect that the specialty nature of the three's would make them more expensive even if the actual time and materials costs were lower.

 

[dpc]

A three-winder will be more expensive than one two-winder, but I would think, at this size, it would be less expensive than TWO two-winders.

But you also have to consider resulting cost savings in primary protection, breakers, disconnects, relays, substation space requirements, etc.

 

[peebee]

Just curious, are you looking for two secondary voltages? I can't think of any other reason that you'd be considering this rather than one 300kVA two-winding TX.

 

[dpc]

This is sometimes done to limit short-circuit current and secondary feeder size.

 

[busbar]

Voltages are not mentioned, but here’s a long shot: in some areas usual practice would be three [100MVA] single-phase transformers with a fourth identical spare. Overhead rigid bus and shoe-fly spans could allow the spare to be used without relocation, for any failed 1ø unit.

 

[dpc]

Actually, the use of single-phase units in this size range is becoming pretty rare, at least in my experience. It might be done if the configuration was extremely unusual, but I think most people would opt for a three-phase unit due to cost.

Maybe this is a matter of local custom.

 

[busbar]

Understood, dpc. It is indeed likely a "regional variation," but they are in service. I just wanted to cover all workable bases.

 

[Shortstub]

A three-winding transformer is more econiomic than the equivalent capacity in two, two-winding transformers. Approximately to 2/3 to 3/4 the cost.

A three winding transformer, for example, 15-kV primary, 480-Vsecondary and 4.16-kV tertiary, to substitute for two secondary load centers (480-V and 4.16-kV) having the same primary voltage source, has the following benefits:

One source breaker and feeder cable as opposed to two.

One transformer foundation as opposed to two.

Secondary switchgear breakers, bus duct, as well as MVA short-circuit duty can be materially reduced.

One spare transformer as opposed to two.

This configuration was used to appreciably lower the cost of many HPI plants.

 

[Shortstub]

Oops, I meant the "electrical" costs!

 

[GusD]

I like the 2- transformer idea ,rather then a single 300 with 2 secondaries.I am not so sure about the comparative costs between both options,however;If you decide to buy one spare for your 300,the cost of one 150 for a spare would be much cheaper.Another important aspect,as someone mentioned previously,it is the fact that with 2-150 you still have 1 operational transf if one fails.On the other hand if the 300 fails without a spare,it could get nasty to replace or repair on short notice.Moreover,you are "dead in the water" until a replacement is available.
Thanks GusD