The ferroresonance phenomena is a non-linear series resonance that may result in transient currents of high amplitude and high harmonic content.

For medium voltage networks the following system conditions may result in ferro resonance conditions:

• Use of single pole insulated voltage transformers

• Ungrounded network condition (insulated neutral star point)

• Transient phenomena such as switching action to trigger the resonance

In the attached project "EMT Ferro Resonance Voltage Transformer" a ferroresonance phenomena occurs between the inductive (single pole) voltage transformers and the 11 kV cable after loss of the earthing transformer in this system due to opening of circuit breaker "CB". The circuit breaker is opened to clear a fault in the downstream system.