Transformer Winding Costruction 5

[gordonl]

A transformer designer has suggested to me that a disc winding is mechanically superior to a sheet (foil) winding. Any comments thoughts?

In reference, we experienced a 13MVA rectifier transformer failure, apparently due to failure of the low voltage due to mechanical stress between layers. We are looking at buying a spare set of coils for the six other transformers should another failure occur. Our choice is now to stick with the sheet design with nomex for more insulation strength, or switch over to a redisgned disc winding. (The load is approx 150% for 3 minutes every five minutes)

 

[prc]

gordonl,
I have to confess that I dont have experience with sheet windings.But from all the discussions ,I conclude a failure mode as below.
The LV winding is inner most and as the load fluctuate violently and with short durations the winding experiences a pulsating buckling force towards the core.This must be causing the damage to the interlaminar insulation. Under the situation I would suggest to go for the disc winding if your designer is confident.

 

[gmazumder]

it seems that i will have to contradict you all in the above mentioned posts. The sheet winding is known to be more solid in formation and as the fault current gets distributed over the sheet per unit force is lower compared to the stranded conductor winding construction. Generally the failures observed in the sheet/ foil winding transformers have been in the edges and insulation failure within turns are due to non uniformity of the papr production process and the epoxy coating process.
Nomex is good in usage as the temperature withstand properties of the same are better than cellulose paper. But thermally aged cellulose papers are also available and asre definitely cheaper than nomex, an option can be excrcised.

 

[prc]

In the discussions every body agrees that sheet windings are superior from short circuit with stand point of view.But it is not due to distribution of fault current ,but due to uniform ampere turn symmetry,and higher Z for mechanical forces.But if it is not designed or manufactured properly failure is the result and hence my suggestion for disc winding for which designer is confident. gordonl confirms that failure is not due to thermal rundown and hence the question on the use of nomex.Thermally aged paper cannot be recommended for trf,probably what you meant was thermally upgraded paper(a chemical process by which N2 content in paper is increased due to which thermal with stand capability is increased by 10 degree centigrade}Normal cellulose is suitable for 55degree winding temp rise,thermally upgraded 65 degreetemperature rise and nomex 85 degree rise(centigrade}in oil .

 

[SphincterBoy]

"The proof is in the pudding". Examine the
short circuit tests!

Waukesha Electric performed no less than
three (3) short circuit tests on their disk
wound units. Change in impedance consistantly
less than 0.3% after fault.

Every sheet wound unit short circuit test I examined had an impedance change of 1.5% or greater.

Sheet wound is better than disk? Show me a certified short
circuit test and I will believe it!

Even the rebuilders (Sunbelt) will tell you this too!

 

[prc]

S.Boy,
The pudding can taste bad not because of ingredients but also due to cooking!
I reiterate sheet windings should be stronger from short circuit strength angle but it can be disastrous if not properly made or processed.Test results; short circuit results are tricky ,the type of results can come if the windings relatively move axially evenif windings are Ok. So I think from the results reported one cannot conclude one winding or other is superior.

 

[electricuwe]

To prc:

If a transformer is expected to work reliable the windings shouldn't move much during a short circuit.

 

[SphincterBoy]

Electricuwe, are you in Germany?

If so, I would like to get a copy of Siemen's
transformer specs (10 MVA and greater).

Years ago, I evaluated a Siemens (German) unit,
and it was a fine piece of equipment.

The Germans usually have some of the best equipment
money can buy.

 

[prc]

electricuwe,
I fully agree with your statement that the windings should not move.But the point I want to make was even if the windings are strong and ok windings can shift due to poor design /manufacturing.I was only emphasizing the statement made in your earlier posting of 7th Feb first para.

 

[electricuwe]

To Sphincterboy:

Yes, I'm in Germany, but I'm not with Siemens or buy or sell transformers from/to them. So, unfortunately, I do not have access to their transformer specs.

Thank you very much for recognizing the merits of Germans in Engineering.

 

[SphincterBoy]

Electricuwe, I am a big fan of the Germans.

They usually don't miss a beat on anything,
and are quite ingenious, particularly in SF6
insulated substations.

 

Hello
I would like to know about circulating currents in helical type winding. I am also curious to know that if u place regulating winding outside high voltage winding with a single layer what problems may arise. Hope somebody has the answer !
Electrans

 

[electricuwe]

Electrans,

you will not have a circulating current if you have one single conductor. If you need larger cross sections you have to use transposed conductors or provide suitable transposing by other means.

With the regulating winding outside you will incrase losses due to radial components of magnetic field.

See: Kerenyi, Karsai, Kiss: "Large Power Transformers" for detailed analysis on that topic

 

[prc]

There will be circulating currents in helical windings due to unequal flux linkage between parallel strands. To mitigate it is the major concern of all trf designers .There are several unique transposition methods to achieve it .
Single layer regulating winding outside the HV winding is the best method of winding construction from cost and performance angle.Since being away from the main leakage field, stray losses and short circuit forces in reg winding will be minimum in such a construction.

 

[electricuwe]

PRC,

you are right with the losses in the regulating winding, but additional losses are induced in the main winding.

 

[prc]

Electricuwe,
Can you pl explain a little more.I dont understand how additional losses occur in main wdg.If you are refering to Karsai,pl mention the page no too.

 

[electricuwe]

prc,

with a regulating winding,at least for some tap positions, regardless where it is placed in a nearly every possible winding design you will cause a magnetic flux density directed in an other direction as the main stray flux. This will cause additional losses since the conductor shape is usually choosen according to the direction of the main stray flux direction.

I usually try to avoid regulating windings when specifying converter transformers, which is usually possible because the converter can be designed to do the regulation.

My main job is the design of converters. I only have to dig in the transformer details from time to time if we have failures of converter transfomers. Since the Karsai book is quite expensive (>250$) I do not have one at home but I borrow it from the next univerity library.

 

[prc]

Electricuwe,
I am sorry to differ with you. I asked for Karsai page thinking that there is any reference in it .Of course losses will be more than the case with out tappings. But when tappings are provided ,Outermost tappings give least stray losses in windings.If you can explain ,I am ready to correct myself.

 

[electricuwe]

prc,

you may be correct as long as you restrict your design to the standard design of winding one conductor as a helical coil.

One example to manufacture a regulating winding causing lower additional stray losses than the standard arrangement regardless where it is placed:

Wind several strands of wire in parallel and connect them in series. Flux density will be more uniform resulting in lower losses

 

[rterickson]

SphincterBoy,

I would appreciate a copy of your transformer spec and bid sheets when you get a chance, my email is:
rterickson@tid.org

Thanks ~ Randy