Ferroresonance or nonlinear resonance is a complex electrical phenomenon. It can cause overvoltages and overcurrents in an electrical power system and can pose a risk to transmission and distribution equipment and to operational personnel. It occurs when a circuit containing a nonlinear inductance is fed from a source that has series capacitance. One example of nonlinear inductance is the magnetic core of a wound type voltage transformer, but it may also arise due to the complex structure of a 3 or 5 limb three phase power transformer. The circuit series capacitance can be due to a number of elements, such as the circuit-to-circuit capacitance of parallel lines, conductor to earth capacitance, circuit breaker grading capacitance, busbar capacitance, or bushing capacitance, etc.
Ferroresonance should not be confused with linear resonance that occurs when inductive and capacitive reactances of a circuit are equal. In linear resonance the current and voltage are linearly related in a manner which is frequency dependent. In the case of ferroresonance it is characterised by a sudden jump of voltage or current from one stable operating state to another one. The relationship between voltage and current is dependent not only on frequency but also on a number of other factors such as the system voltage magnitude, initial magnetic flux condition of transformer iron core, the total loss in the ferroresonant circuit and the point on wave of initial switching.
In his paper "Examples of Ferroresonance in a High Voltage Power System", Jacobson claims that the term "ferroresonance" was coined by Bucherot in 1920 to describe the phenomenon of two stable fundamental frequency operating points coexisting in a series resistor, nonlinear inductor, capacitor circuit.