---
title: "Ferroresonance and its causes"
subject: "High Voltage Engineering"
chapter: 3
key: "semester_8/high_voltage_engineering:ferroresonance-causes"
source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/ferroresonance-causes
site: BCT Engineering
---

# Ferroresonance and its causes

High Voltage Engineering · Chapter 3: Overvoltages in Power System

- Asked in: 2080 Chaitra, 4 marks
- Video: [💥Ferroresonance in Distribution/Power Transformers💥](https://www.youtube.com/watch?v=2mlF6Y-b6J0) — Megavolt Homeschool
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/ferroresonance-causes
- Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-3

## Question

What is ferroresonance? Write about the causes of ferroresonance in power system.

## Short answer

Ferroresonance is a non-linear resonance between a capacitance and a saturable iron-core inductance, producing sustained overvoltages and distorted currents. Causes include single-phase switching of unloaded transformers, long cable- transformer combinations, voltage transformers in isolated-neutral systems, and broken conductor faults.

## Answer

**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.

```text
  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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is ferroresonance?
   A. Resonance between a capacitance and a fixed linear inductance
   B. Non-linear resonance between a capacitance and a saturable iron-core inductance
   C. Resonance of two capacitors
   D. Resonance at exactly 50 Hz only
   Answer: B. Ferroresonance involves a saturable inductance, so it can jump to a sustained high-voltage state. (Key point: Definition)

2. Why can resonance occur over a wide range of C in such a circuit?
   A. Inductance of iron core falls on saturation, so some flux level gives wL = 1/(wC)
   B. The capacitance changes with flux
   C. The source frequency changes
   D. The resistance becomes zero
   Answer: A. Saturation reduces L sharply, so a resonant point exists for many values of C. (Key point: Why it occurs)

3. Which is a typical cause of ferroresonance?
   A. Three-pole switching of a loaded transformer
   B. Single-phase switching of an unloaded transformer fed through cable capacitance
   C. Solidly grounded loaded generator
   D. Fully loaded transformer
   Answer: B. Open phases fed through cable or grading capacitance place C in series with the magnetising reactance. (Key point: Typical cause)

## Related in High Voltage Engineering

- [FACTS devices changing the power system](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Afacts-changes-power-system) — High Voltage Engineering, chapter 1
- [Vision of power system 25 years ahead](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Apower-system-vision-25-years) — High Voltage Engineering, chapter 1

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