Power transformer with inferior core

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.

Dear sir,

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?

BEST REGARD

M.S.J

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.

BEST REGARDS

M.S.J

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...

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

Calculation of forces

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.