Ferroresonance in Power Systems: Causes, Effects, and Engineering Mitigation

Introduction

Ferroresonance is one of the most misunderstood and underestimated phenomena in medium-voltage (MV) and high-voltage (HV) power systems. Unlike short circuits or insulation failures, it is not a fault condition, yet it can lead to severe overvoltages, VT failures, insulation damage, and protection maloperation.

With the increasing use of long MV cables, compact substations, renewable energy plants, and lightly loaded networks, the risk of ferroresonance has significantly increased.

A common engineering question is:

Why does low inductance—especially during core saturation—lead to Ferroresonance?

This article explains the answer step by step.

What Is Ferroresonance? (Quick Recap)

It occurs when:

• A non-linear inductance, like an iron-core VT or transformer)
• Interacts with system capacitance of cables, overhead lines, and breaker grading capacitors
• Under low damping or lightly loaded conditions

Unlike linear resonance, ferroresonance produces:

• Sustained overvoltage
• Highly distorted, non-sinusoidal waveforms
• Multiple stable and unstable operating states
• Chaotic and unpredictable behaviour

Role of Inductance in Ferroresonance

The key enabler of ferroresonance is the collapse of inductance when an iron core enters magnetic saturation.

Iron-Core Magnetisation Is Non-Linear

What the magnetisation curve tells us:

• At low flux → inductance is high and nearly linear
• Near the knee point → core starts saturating
• Beyond saturation → small voltage increase causes large current rise

👉 Once saturated, the transformer no longer behaves like a linear inductor.

Why Low Inductance Triggers Ferroresonance

1. Inductance Drops Sharply During Saturation

• In normal operation, a VT presents high magnetising inductance
• During saturation:
  • Core permeability collapses
  • Effective inductance reduces drastically
  • Inductance becomes voltage-dependent and non-linear

🔑 Low inductance is not a design feature—it is a saturation condition

2. Resonant Frequency Shifts into the Power System Band

The resonant frequency of an LC circuit is:

• As L decreases, the resonant frequency increases
• During saturation, inductance changes dynamically
• Resonance may suddenly align with:
  • 50 Hz fundamental frequency
  • Sub-harmonics, e.g. 25 Hz
  • Super-harmonics

⚠️ This alignment allows energy buildup instead of decay.

3. Low Inductance Causes High Magnetising Current

• Lower inductance → higher magnetising current
• Higher current → deeper saturation
• Deeper saturation → even lower inductance

🔁 This forms a positive feedback loop:

Saturation → Low inductance → High current → More saturation

This loop sustains ferroresonant oscillations.

4. Low Inductance Reduces Natural Damping

• Ferroresonance often occurs in lightly loaded systems
• When inductance collapses:
  • Energy exchange between L and C increases
  • Very little energy is dissipated
• Oscillations do not die out naturally

This explains why ferroresonance can persist for hours or days after a switching event.

When inductance becomes low:

• Cable and breaker capacitances become electrically significant
• Even small capacitances store substantial energy
• Energy oscillates violently between:
  • Capacitance (electric field)
  • Saturated inductance (magnetic field)

⚡ This sustained LC energy exchange maintains ferroresonance.

Why Voltage Transformers Are Most Vulnerable

Voltage transformers are especially prone because:

• Small core size
• High operating flux density
• Designed for accuracy, not saturation margin

Even a minor overvoltage can push a VT into saturation, causing:

• Rapid inductance collapse
• Severe overvoltage on secondary
• Thermal and insulation failure

👉 This is why ferroresonance incidents are most commonly reported in single-phase VTs in MV systems.

Key Characteristics of Ferroresonance

• ⚠️ Highly non-linear and unpredictable
• 🔄 Multiple stable operating states
• 📈 Overvoltages of 2–5 pu or higher
• 🌊 Distorted or chaotic waveforms
• ⏳ Can persist indefinitely without damping

Ferroresonance Mitigation Guideline

1. Objective

The objective of this guideline is to prevent, suppress, or eliminate ferroresonance in medium- and high-voltage power systems by applying correct design practices, equipment selection, switching philosophy, and operational discipline.

2. Systems Most Vulnerable

High-risk systems have one or more of the following:

• Long MV underground cable feeders
• Lightly loaded or unloaded transformers
• Isolated or resistance-grounded neutral systems
• Single-phase or single-pole switching devices
• Voltage transformers connected to dead-ended cables
• Renewable energy plants (solar / wind collector networks)

3. Root Cause to Be Addressed

• Prevent deep saturation of voltage transformers
• Increase system damping
• Avoid resonance-triggering switching conditions

4. Voltage Transformer (VT) Selection Guidelines

4.1 VT Core Design

Recommended

• Five-limb core VTs
• Shell-type or ferroresonance-resistant designs

Avoid

• Single-limb VTs in isolated or cable-fed systems

4.2 VT Rating and Flux Margin

• Select VTs with:
  • High knee-point voltage
  • Continuous overvoltage withstands capability (≥1.9 pu)
• Avoid marginal designs operating close to saturation

5. VT Secondary Circuit Practices

5.1 Never Leave VT Secondary Open

• Always provide a permanent resistive burden
• Open-circuited VT secondary dramatically increases the risk

5.2 Use Damping Resistors or RC Networks

• Install:
  • Secondary damping resistors
  • RC damping circuits where specified
• Purpose:

• Increase losses
• Suppress sustained oscillations

6. Switching Philosophy and Operational Controls

6.1 Avoid Single-Pole Switching

• High-risk operations

• Single-phase breaker opening
• Staggered pole operation
• Isolator switching with VT connected
• Recommendation

• Use three-pole gang-operated breakers
• Ensure simultaneous phase operation

6.2 Avoid VT Energisation on Dead-Ended Cables

• Do not energise VT connected to long cables without a load
• Ensure Load or damping is present before energisation

7. Network Design & Grounding Philosophy

7.1 Neutral Grounding

• Prefer Resistance-grounded neutral systems
• Avoid completely isolated neutral systems

Grounding improves damping and reduces ferroresonance severity.

7.2 Cable Length Management

• Minimise MV cable length feeding VTs
• Avoid dead-end cable terminations at VT locations
• Break long cable runs with loads or damping points

8. Protection & Monitoring Measures

8.1 VT Supervision

• Implement VT fuse failure detection and VT supervision relays
• Monitor Abnormal voltage rise and Distorted waveforms

8.2 Alarms and Interlocks

• Provide alarms for:
  • VT secondary open condition
  • Abnormal voltage persistence
• Interlock switching operations where feasible

9. Special Considerations for Renewable Energy Plants

Renewable plants are highly susceptible due to:

• Long MV collector cables
• Frequent switching
• Light-load operation during start-up and shutdown

Mandatory actions

• Specify ferroresonance-resistant VTs
• Include damping in VT secondary as standard
• Perform risk assessment during the design stage

10. Commissioning & O&M Best Practices

10.1 Commissioning Checklist

✔ VT secondary burden connected
✔ No open-circuited VT secondary
✔ Correct grounding verified
✔ Three-pole breaker operation confirmed
✔ Cable lengths reviewed
✔ Protection supervision enabled

10.2 Operation & Maintenance

• Never isolate VT secondary during live operation
• Investigate:
  • Unusual humming
  • VT overheating
  • Voltage fluctuations without load
• Replace failed VT with ferroresonance-resistant design only

11. Engineering Design Checklist (Quick Reference)

✔ Identify VT + long cable combinations
✔ Avoid single-phase switching
✔ Ensure permanent VT damping
✔ Specify high overvoltage withstand VTs
✔ Select proper grounding philosophy
✔ Include ferroresonance study in MV design

Conclusion

Ferroresonance is not a rare event—it is a predictable outcome of poor damping, saturation, and switching practices.

It can prevented by design, not by protection.

By applying the above mitigation measures at the concept, design, commissioning, and operation stages, ferroresonance can be effectively eliminated from MV and HV power systems.

External Reference

https://en.wikipedia.org/wiki/Ferroresonance_in_electricity_networks?utm_source=chatgpt.com

https://library.e.abb.com/public/2e4528a2d55c5414c12572dd00247313/VT%20guard_presentation-ferrores_sales_version_eng.pdf?utm_source=chatgpt.com