Ferroresonance is a nonlinear resonance phenomenonthat affects power networks. Circuit must contain at least below circuit elements as it can be definedas ferroresonance:

• Non-linear saturable inductance (example; transformer)
• Capacitor
• Resistor

In a circuit consists of these elements; by changingthe values of an electrical component, the values of current and voltage values on the terminals at other components change suddenly.

Ferroresonance begins some switching events such as; load rejection, energizing or de-energizing of transformer, switching of circuit breaker. The causes risk to it can be listed as follows:

• Idling or operation on low load on the systems grounded neutral point
• Insulation faults
• Overloading or idling of voltage transformer
• Switching one or two phases
• Dissymmetry caused by connection errors
• Switching transformers or capacitors
• Connecting low load power transformers to network with short circuit load
• Long and / or capacitive cable feeding a transformer
• Streak of lightning on transmission lines