Pronounced Ferro-resonance. Ferroresonance is a potential source of Transient overVoltage. In three-phase Electric Circuits, single-phase Switching (by mistake), fuse blowing, or a broken Conductor can result in overvoltages when ferroresonance occurs between the magnetizing Impedance of a Transformer and the system Capacitance of the isolated phase or phases.
Ferroresonance is usually known as a series resonance. That is because the
necessary conditions arise when one or two open phases result in capacitance
being energized in series with the nonlinear magnetizing impedance of a
transformer. This can occur at all voltage levels. For example, the capacitance could be
as simple as a length of cable connected to the unGrounded winding of a pad-mounted transformer.
The greatest chance of ferroresonance occurring on Sho-Me’s system involves Magnetic Wound Potential Transformer (MPT) s. According to Doble when an MPT is in parallel with large grading Capacitor across the gap of a Circuit Breaker ferroresonance can occur. When ferroresonance occurs, something usually blows up (see Ferroresonance - Dangers).