Ferroresonance and its causes

Ferroresonance is a non-linear resonance between a capacitance and an iron-cored (saturable) inductance, such as a transformer magnetising reactance. It produces sustained, distorted overvoltages and overcurrents, often 2–4 pu, with sudden jumps between operating states.

Why it happens

In a series circuit of capacitance $C$ and saturable inductance $L$, resonance needs $\omega L = 1/(\omega C)$. A linear inductor has a fixed $L$, but an iron-core inductance falls sharply when the core saturates. So for a wide range of $C$ there is some flux level at which $\omega L_{sat} \approx 1/(\omega C)$. The circuit can then "jump" into a high-voltage resonant state and stay there. Several stable states can exist for the same source voltage. Which one occurs depends on initial conditions such as residual flux and the switching instant.

  Vs ~---| C |---+
                 |
               L(sat)   (transformer
                 |       magnetising)
  ---------------+------- ground

Causes in power systems

1. Single-phase switching of an unloaded or lightly loaded transformer. One or two phases are open (fuse blown, breaker pole stuck, single-pole switching). The open phase is then fed through line or cable capacitance in series with the magnetising reactance.
2. Transformer fed through a long cable, or through the grading capacitors of open breakers. The cable capacitance or the breaker grading capacitance forms the series $C$.
3. Voltage transformers (PTs) in ungrounded or isolated-neutral systems. The PT magnetising reactance resonates with the phase-to-ground capacitance of the network.
4. Lightly loaded transformers on series-compensated lines.
5. Broken conductor faults, or loss of the ground connection on a wye-connected transformer.

Effects

• High, distorted overvoltages that can damage insulation and surge arresters.
• Overheating, noise and failure of PTs and transformers.

It is avoided by three-pole switching, loading the transformer or adding damping resistors, grounding the neutral, and avoiding long cable–transformer combinations with no load.