POWER TRANSFORMER SHORT CIRCUIT FORCES BASICS AND TUTORIALS

SHORT CIRCUIT FORCES ON POWER TRANSFORMERS BASIC INFORMATION
What Are The Short Circuit Forces Acting On Power Transformers?


Forces exist between current-carrying conductors when they are in an alternating-current field. These forces are determined using :

F = B I sin x
where

F = force on conductor
B = local leakage flux density
x = angle between the leakage flux and the load current. In transformers, sin x is almost
always equal to 1.


Thus
B = uI
and therefore
F directly proportional to I^2

Since the leakage flux field is between windings and has a rather high density, the forces under shor tcircuit conditions can be quite high. This is a special area of transformer design. Complex computer programs are needed to obtain a reasonable representation of the field in different parts of the windings.

Considerable research activity has been directed toward the study of mechanical stresses in the windings and the withstand criteria for different types of conductors and support systems.

Between any two windings in a transformer, there are three possible sets of forces:

• Radial repulsion forces due to currents flowing in opposition in the two windings

• Axial repulsion forces due to currents in opposition when the electromagnetic centers of the two windings are not aligned

• Axial compression forces in each winding due to currents flowing in the same direction in adjacent
conductors

The most onerous forces are usually radial between windings. Outer windings rarely fail from hoop stress, but inner windings can suffer from one or the other of two failure modes:

• Forced buckling, where the conductor between support sticks collapses due to inward bending into the oil-duct space

• Free buckling, where the conductors bulge outwards as well as inwards at a few specific points on the circumference of the winding

Forced buckling can be prevented by ensuring that the winding is tightly wound and is adequately supported by packing it back to the core. Free buckling can be prevented by ensuring that the winding is of sufficient mechanical strength to be self-supporting, without relying on packing back to the core.

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