It is a basic principle of electromagnetic induction that an electric current flows in a closed coil placed within a varying magnetic field. This current produces a secondary magnetic field in its own turn. The two magnetic fields repel each other and consequently the conductors in the coil experience a force that is proportional to the product of the two field strengths.
In a transformer, the principal magnetic field is itself set up by the flow of current in the primary coil. The secondary current, and therefore the secondary magnetic field, is proportional to the primary current. Therefore the forces mutually experienced by the coils are proportional to the square of either current. This means that under short-circuit conditions, the forces experienced by the windings are two orders of magnitude higher than at rated currents.
In core type transformers, these forces act radially, tending to compress a coil and reduce its axial length. In shell type constructions the forces act at a perpendicular to the coil surface and tend to reduce its radial width.
If a power transformer is not designed and built properly, external short-circuits can cause significant weakening of its active parts, thus reducing its reliability, even if there is no immediate internal breakdown. Conductors can displace and stretch, coils can distort, bulge, buckle, telescope, tilt or rupture leading to broken insulation and thus to inter-turn short-circuits. Mechanical failures of the insulation can occur due to motion between conductors and spacers. Winding end supports can collapse.
Design and manufacturing practice
Of the many elements that go into the design and manufacture of power transformers in order to improve their short-circuit withstand capabilities, the choice of conductor material is the most important, as its mechanical properties, such as yield strength and modulus of elasticity, are critical to performance.
For this reason, good design practices, such as those of ABB, use copper with a minimum yield strength of 90 N/mm2 at 0.2% offset (in other words it would require a stress greater than 90 N/mm2 to cause a permanent strain of 0.2%). This figure goes as high as 280 N/mm2 and beyond for heavy-duty transformers with frequent short-circuits such as those used for arc furnaces. The use of the right grade of copper is considered by good designers as the best way of ensuring high short-circuit withstand capability in power transformers.
- Short Circuit Duty of Power Transformers: Giorgo Bertagnolli
- State of the Art on the Use of Copper and Aluminium Conductors in Distribution Transformers Manufacturing: R. Salustiano & M. L. B. Martínez Federal University of Itajubá– Lat-Efei