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Transformer riddle no.2 – Power transformer with inferior core

The core material and some other things shall be supplied by a foreign company that they work under its license. While boycotting duration they got to use inferior material in core transformer construction. Whereas they didn't have adequate base knowledge of transformer design, they followed all construction algorithms that were dictated by foreign company. They thought this instruction deviation only influenced the power losses of transformers, but in practice they found other important problem about short circuit withstand of transformers. The power transformers windings sometimes were damaged mechanically due to a sever short circuit fault current.


What is your opinion? How core material selection influenced the transformer withstand against short circuit current?
Author : Hamid
 
#1
Thu, September 13, 2007 - 11:34
The capability to withstand short circuit forces is a function of the design, materials, and construction technique. It's not clear that the core material would play a direct role in short circuit strength, since the forces are primarily between the windings themselves. The alignment of primary versus secondary windings is quite critical to prevent excessive axial forces and the coils must also be sufficiently preloaded during manufacture via rigid clamping.

Was the damage to an inner winding (i.e., bending/crushing) or was there radial shifting of the outer winding from tensile force (hoop stress)? Does the transformer use circular or rectangular shaped windings - circular windings can withstand short circuit forces with less deformation. Was there twisting of disk winding conductors? Were the windings made from copper or aluminum alloy, and were they properly specified and/or heat treated? Was the pressboard material prestabilized, high density and within specification (for percent compression) to provide sufficient rigidity? Do you have any pictures of the damaged areas? Unfortunately, identification and correcting the problem(s) would require significantly more information that you have provided.
 
Author : BertHickman
 
#2
Thu, September 13, 2007 - 11:36
Thank you very much for your usefule information.

I think we must focuses to leakage and linkage flux conceptions in transformer system.

What is your opinion?
 
Author : Hamid
 
#3
Thu, September 13, 2007 - 11:38

quote:
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I would think that if the core was of an impedance which was too low or high, then the inrush and available fault current would be incorrect.

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Correct me if I'm wrong but isn't the core material the soft iron laminated core of the transformer and not the windings?
Jim 
Author : JimmyDee
 
#4
Thu, September 13, 2007 - 11:39

Besides of eddy current losses there are also hysteresis losses in a TX core.

Eddy currents are circulating currents within the magnetic laminations, caused by poorly aligned stacks of soft iron laminations. Also the surfaces may be rusted or rough and cause additional losses.

Hysteresis losses are losses created by the positive and negative cycles of the 50 or 60 Hz sinewave. The quality of the iron (good BH curve) are the important factor here and heat is created in the back and forth going magnetic field.

Copperlosses are directly proportional to the load on the TX (I²R)

Windings may get damaged when due to excessive heat the paper insulation fails, loosely packed winding coils vibrate and may schafe through the insulation material, or deteriation of oil causes acidity and leakage currents to increase. Poor connections at the off load tapchanger, may cause arcing creating excess carbon traces in the oil and ethylene gas.
Also sustained overloading of a TX will do damage.

Don't know what country your TX is but poorly designed TX's will fail prematurely.

 
Author : RODALCO
 
#5
Thu, September 13, 2007 - 11:45
I remember one of our physics tests at school many years ago. Our teacher had provided one solenoid with a magnetic bar in the center; He put one metallic ring on magnetic bar and connected terminal of the coil to 50HZ, 220V power supply. Suddenly, we observed a wonderful phenomenon. The metallic ring jumped up and stood over the magnetic coil.

We had to explain the reason of that phenomenon. I think if our teacher had replaced solenoid magnetic rod with an other inferior core, we would have observed a higher jumping.

Indeed, the applied force to the metallic ring is similar to the transformer winding stresses which meet short circuit fault currents. 
Author : Hamid
 
#6
Thu, September 13, 2007 - 11:46

Actually if your teacher had replaced the core with an inferior one, the ring would not have gone as high, since core losses would have caused a reduction in the flux "seen" by the ring. The ring would also not go as high if the core had a lower permeability either since more flux would escape the core before it could pass through the ring (i.e., giving you a lower coupling coefficient and higher leakage inductance).

The forces seen within your transformer arise from the same basic effect (Lenz's Law), but the directions of the forces are considerably more complex due to the more complicated geometry of the windings and flux paths through, and outside of the core, especially if the core itself saturates during the current fault. Because of the closed magnetic path, the effective coupling between windings is considerably higher than the case above with the floating ring, so the fault current can be considerably greater - potentially sufficient to exceed the yield strength of the winding or the surrounding insulating materials.

In what way(s) was the core material inferior? Did it have lower relative permeability, lower saturation flux, higher electrical conductivity, was it made from thicker laminations, or did the material have a larger area hysteresis loop than desired?? Simple core loss alone does not explain the winding damage problem. Lower permeability or lower saturation flux might...

 
Author : BertHickman
 
#7
Thu, September 13, 2007 - 11:48
In regard to the ring jumping, I think the reduction of linkage flux isn't caused to reduction forces; because the mutual forces between bobbin windings and metallic ring is proportional to rating of system leakage flux. Indeed the metallic ring prefer to move toward place where the less linkage flux is linked it (according to lens's law), also without leakage flux, that different linkage flux region is not occurred.

For transformer winding forces you can refer to related text similar to following handbook

The J & P

Transformer Book

Twelfth edition

A PRACTICAL TECHNOLOGY OF THE POWER TRANSFORMER

Martin J. Heathcote, CEng, FIEE

Newnes

OXFORD BOSTON JOHANNESBURG MELBOURNE NEW DELHI SINGAPORE

Page 232 of this transformer book is here

The precise magnitude of the short-circuit forces depends very much upon
the leakage flux pattern, and the leakage flux pattern also determines such
important parameters as the leakage reactance and the magnitude of the stray
losses. Manufacturers nowadays have computer programs based on finite
element analysis which enable them to accurately determine the leakage
flux throughout the windings. These computer programs can be very simply
extended for the calculation of short-circuit forces to enable manufacturers to
accurately design for these. Occasionally, however, it might be necessary to
make a longhand calculation and in this case the following, which is based
on an ERA Report Ref. Q/T134, 'The measurement and Calculation of Axial
Electromagnetic Forces in Concentric Transformer Windings', by M. Waters,
BSc, FIEE., and a paper with the same title published in the Proceedings of
the Institution of Electrical Engineers, Vol. 10, Part II, No. 79, February 1954,
will be of assistance.
 
Author : Hamid
 
#8
Tue, October 9, 2007 - 08:59
I think the problem of short-circuit of windings is due to poor Core-Laminations cutting, Finish & Edge-protection while filling core strips, which caused poor insulation of windings
at sharp edges.

Of  cource poor core material may over-heat the windings but cannot short-circuits of windings

Haajee Pakistan
 
Author : Haajee
 
#9
Tue, June 23rd, 2009 - 14:41
During Short circuit test the core ( made of inferior material ) is quite likely has saturated which has reduced the impedence of the coil s resulting flow of  higher current that is  responsible for the failure of the coils .
-Mohapatra-
 
Author : R K Mohapatra - From: India
 
#10
Tue, June 23rd, 2009 - 15:44
The leakage flux around the core will be very high ( max) during Short Ckt test , whuich will have a factor of overhaeting , more at corners and edges .Hopwevr thge basic fcator is early saturation of core which gives rise to reduction of impedence that generates high ( very high ) current in the winding causing the failures
 
Author : R K Mohapatra - From: India
 
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