Why Two Transformers with the Same kVA Rating Can Have Very Different Inrush Currents

What happens when a transformer is energised?

When a transformer is switched on, current is required to establish magnetic flux within the core.

During normal operation, the magnetising current needed to maintain this flux is relatively small. During energisation, however, the magnetic conditions within the core can temporarily be very different.

Depending on the switching conditions, the core may be driven towards magnetic saturation.

When this occurs, the magnetising impedance falls significantly and a large transient current can flow through the primary winding.

This is commonly referred to as magnetising inrush.

The magnitude and duration of this transient are influenced by both the transformer design and the electrical system supplying it.

Why doesn’t kVA determine energisation behaviour?

A transformer’s kVA rating describes its ability to transfer power continuously within its designed operating limits.

It does not completely describe what happens during switching.

Two transformers with the same rating can use different magnetic materials, core dimensions, winding arrangements and design margins.

These differences can significantly affect their behaviour during energisation.

1. Core design

The magnetic core is fundamental to transformer performance.

Core cross-sectional area, geometry, construction and magnetic material all influence how the transformer behaves as magnetic flux is established.

Two transformers with identical voltage and kVA ratings can therefore use different core designs and respond differently during switching.

2. Operating flux density

Transformer designers select an operating flux density based on several requirements including size, losses, efficiency, temperature rise and material characteristics.

A transformer operating closer to the saturation region of its magnetic material may have less magnetic headroom available during an unfavourable switching event.

Small differences in magnetic design can therefore produce significant differences in energisation performance.

3. Residual magnetic flux

When a transformer is switched off, the magnetic flux within the core does not necessarily return completely to zero.

A degree of residual or remanent flux may remain within the magnetic circuit.

When the transformer is energised again, that residual magnetism can either oppose or add to the newly established magnetic flux.

If the conditions combine unfavourably, the core can be pushed further towards saturation, increasing the initial current peak.

4. Switching point on the AC waveform

The exact point on the AC voltage waveform at which the transformer is energised also influences the transient response.

Switching the transformer at one point in the waveform may result in relatively modest energisation current.

Switching the same transformer at a different point can produce a substantially larger transient.

This is one reason why the same transformer does not necessarily behave identically every time it is switched on.

5. Winding and source impedance

The electrical impedance between the transformer and its power source also affects how much current can flow.

Factors include:

• Transformer winding resistance

• Leakage impedance

• Cable impedance

• Supply network impedance

• Available short-circuit capacity

A strong electrical supply with relatively low impedance can support a larger instantaneous current than a weaker source.

This means energisation behaviour should be considered as a transformer-and-system interaction, rather than simply a characteristic of the transformer itself.

Two 50 kVA transformers can still behave very differently

Consider two transformers rated at 50 kVA, with the same primary and secondary voltages.

Transformer A may use a larger magnetic core operating at a relatively conservative flux density.

Transformer B may use different core material, geometry, winding characteristics or magnetic loading.

Both transformers may comfortably deliver their rated 50 kVA continuously.

However, their behaviour during the first few cycles after energisation could be significantly different.

This becomes especially important when transformers are supplied from:

• Generators

• UPS systems

• Restricted electrical supplies

• Sensitive distribution networks

• Closely coordinated circuit breakers or fuses

• Systems where voltage disturbance must be limited

In these applications, the transformer’s rating plate alone may not provide enough information to understand how it will interact with the wider electrical system.

Considering energisation during transformer design

We are increasingly seeing customers ask for energisation performance to be considered during the transformer design and verification process.

This allows expected switching behaviour to be considered before manufacture rather than becoming an issue during commissioning.

Core material, core dimensions, operating flux density, winding design and electrical impedance can all form part of that assessment.

Where required, predicted energisation behaviour can then be reviewed alongside the customer’s:

• Protective devices

• Generator or UPS characteristics

• Available fault level

• Cable impedance

• Switching arrangement

• Wider system requirements

At R Baker, we have the transformer design capability and practical engineering experience to consider these requirements during the design process and develop bespoke transformers around the electrical system in which they will operate.

Because two transformers can have exactly the same kVA rating.

That doesn’t mean they will behave the same way when they are energised.

Need to understand how a transformer will behave during energisation? Our engineering team can assess core design, flux density, impedance and system requirements during the design stage. Contact us today.

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