Transformers are susceptible to damage by secondary short-circuit currents having magnitudes that can be many times rated load current. The damage results from the following effects:
• The I^2R losses in the winding conductors are increased by the square of the current. This increases the temperature rise of the windings.
Because protective devices limit the duration of short circuits (as opposed to overloads), the temperature rise of the winding can be calculated by dividing the total energy released by the I^2R losses by the thermal capacity of the conductor.
• The short-circuit currents exclude flux in the core and increase stray flux around the core. This stray flux induces currents in metallic parts other than the winding conductors, which can be damaged thermally.
• A short circuit applied to the secondary circuit of an autotransformer can substantially increase the voltage across the series winding and across the common winding through induction.
This not only presents the possibility of damaging the winding insulation by overvoltage, but will also drive the core into saturation and significantly increase core losses with potential damaging effects from temperature.
• Bushings and tap changers have current ratings that are usually only marginally greater than the rated load of the transformer.
Since fault currents are many times rated currents and these components have short thermal time constants, they can be seriously overloaded and thermally damaged.
• Stray flux in the vicinity of current-carrying conductors produces mechanical forces on the conductors.
When a short circuit is applied to a transformer, there is a significant increase in stray flux, resulting in greater mechanical forces on the windings, leads, bushings, and all other current-carrying components.
These components, especially the windings, must be braced to withstand these forces.