What Is Transformer Inrush Current?
Quick answer — Magnetising inrush is the large, decaying current that flows when a transformer is energised, caused by the sudden build-up of flux in the core. It can be many times the rated current (often 8-12x, more for large or weakly damped units) and contains a strong second harmonic and DC component. It can stress the transformer and, if not filtered, trip the differential protection.
What is transformer inrush current and why does it happen?
Why it occurs
On energisation the flux in the core is determined by the voltage and the residual flux from the last de-energisation. If the residual flux and the required flux oppose, the core can temporarily saturate, drawing a very large magnetising current.
Its shape
The current is initially high, decays over several cycles or seconds, is asymmetric (DC offset), and is rich in the second harmonic and harmonics.
Handling it
Differential protection is usually given a harmonic (often second-harmonic) restraint or a time delay to avoid a false trip on inrush. The inrush is a normal, non-fault condition.
References & standards
- IEC 60076-1
- IEEE C57.12.90
Related
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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