Transformer Internal Forces
During normal operation, internal structures and windings are subjected to mechanical forces due to the magnetic forces. These forces are illustrated in figure 8. By designing the internal structure
very strong to withstand these forces over a long period of time, service life can be extended.
However, in a large transformer during a “through fault” (fault current passing through a transformer), forces can reach millions of pounds, pulling the coils up and down and pulling them apart 60 times per second. Notice in figure 8 that the internal low-voltage coil is being pulled downward, while the highvoltage winding is pulled up, in the opposite direction. At the same time, the right-hand part of the figure shows that the high- and lowvoltage coils are being forced apart. Keep in mind that these forces are reversing 60 times each second. It is obvious why internal structures of transformers must be built incredibly strong. Many times, if fault currents are high, these forces can rip a transformer apart and cause electrical faults inside the transformer itself. This normally results in arcing inside the transformer that can result in explosive failure of the tank, throwing flaming oil over a wide area. There are protective relaying systems to protect against this possibility, although explosive failures do occur occasionally.
Figure 8 – Transformer Internal Forces