What Is Ferroresonance in a Transformer?

Published 27 Aug 2026Updated 27 Aug 20266 min readAdvancedGrid
Quick answer — Ferroresonance is a non-linear resonance between a transformer (or its magnetising inductance) and the network capacitance (typically cable capacitance), which can produce sustained overvoltages and overcurrents far above normal. It is caused by the saturation of the transformer core and occurs under certain system configurations, energising sequences, or faults — and can damage the transformer, arresters and other equipment.
Contents
What is ferroresonance and why is it dangerous?

How it occurs

A series or parallel resonance forms between the transformer (non-linear inductance) and the line/cable capacitance. Under certain conditions — e.g. a cable with a transformer open or a particular breaker sequence — the voltage is not limited by linear analysis and can jump.

Symptoms

Severe, sometimes chaotic overvoltages and overcurrents, audible noise, and possible excessive currents that overheat the transformer or damage arresters.

Prevention

Avoid unfavourable configurations, use proper switching sequences, consider resistive loading, surge arresters and, where needed, a ferroresonance study.

References & standards

Related

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Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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