Power transformer designing modifications

[AGTB]

Hi everyone,

I have an issue with my regular transformer supplier. We need 4 single-phase power transformers (3+1, 400 / 16.7 kV, 60/80/100 MVA; 1F) for a Static VAr Compensator, the problem is that after the initial harmonic measurements, the harmonic content in the grid turned out to be 72A as opposed to the 11A we initially calculated. With an oscilatory basic current of 433A the total load on the transformer jumps from 433.1 to 438.9A, hence the supplier wants a huge increase in the budget, his argument is that the total volume of steel and copper is going to increase a lot but I don't see why the huge increase with such a little percentage of incrased current. Does somebody have a clue? The only semi-logical explanation I can think of is some issue with the allowed losses for the system but even then it doesn`tseem like much. Any help would be greatly appreciated.

 

[Plantbldr]

The supplier may be taking this opportunity to update the order with current comodity pricing for steel, iron, and copper. Raw materials are becoming a large portion of transformer costs.

 

[ScottyUK]

You can't expect the same iron losses per amp of fundamental current as you will get per amp of harmonic current. The relationship is much more complex because core losses are largely due to the reversal of flux each cycle. Harmonics cause flux reversal to occur more often e.g. 3rd harmonic has 3 reversals for each at fundamental frequency, 5th has five reversals, etc.

There are a few ways around this: thinner laminations; better steel; lower flux density. All add cost. If he is already using decent steel and the laminations are as thin as the manufacturing process can cope with, then he only has one option left.

Copper losses in the winding increase due to skin effect an effect which become more problematic as the harmonic order increases, and affects large diameter conductors to a greater extent than small ones. The solution here is either use a much larger conductor or different design of conductor has to be used. Increasing core dimensions to reduce flux density will increase winding turn length too, adding more cost.

The adjustment in price based on commodities value on the stock market is entirely probable to. Most contracts include an adjustment for commodities rate changes between submission of tender and the placement of the order.


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Your body might be a temple. Mine is an amusement park...

 

[edison123]

For harmonic loads, is there not what is called as "K factor" which basically derates the trafo?

* Homer Wisdom - "Operator, Give me the number for 911" *

 

[ScottyUK]

Yes there certainly are K-rated transformers, although I've never heard the term used in conjunction with supergrid transformers at 400kV. Most transformers of that size are custom designs for specific applications rather than off-the-shelf types like distribution transformers where one design covers a multitude of different loads. I couldn't find any reference to K-factor applied to very large transformers, but Googling [blue]transformer k factor[/blue] brings up 725,000 hits so forgive me for not checking them all!



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Your body might be a temple. Mine is an amusement park...

 

[alehman]

K factor only applies to small dry-type transformers.

 

[ScottyUK]

Thanks alehman. Explains why I couldn't find anything.


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Your body might be a temple. Mine is an amusement park...

 

[electricuwe]

I agree to ScottyUK, harmonic currents have a large influence on the design of power transformers.

For small transformers there is an approach in US to take care about this by the k-factor rating, which is some kind of integral description of the effects involved. But for large transformers, especially converter transformers, it has been common for a long time to calculate the additional losses based on the indiviaul harmonic currents and the details of the transformer construction. That's the way I prefer.

So, if the the cost increase was just the answer to an increased RMS current, your supplier is fooling you. But you should provide more detailed information on the harmonics to him.

But if the new quote is already based on the spectrum of harmonic currents your supplier has done a good job.