Transformer Inrush Current
When specifying or designing a transformer, much of the initial focus naturally falls on the steady-state requirements:
- Voltage.
- Current
- Power rating.
- Losses
- Temperature rise.
- Insulation
But one of the most demanding conditions for the wider electrical system can occur before the transformer has even begun normal operation: Energisation
When a transformer is first switched on, the magnetising inrush current can be many times greater than its normal operating current.
Depending on transformer design and the conditions at the instant of energisation, the first current peak can commonly reach around 10–15 times rated current, with significantly higher values possible in some transformer designs and ratings.
For smaller transformers, first peaks approaching 20–25 times rated current can occur under unfavourable energisation conditions.
What causes transformer inrush current?
Transformer inrush is primarily associated with the magnetic behaviour of the core during energisation.
The magnitude of the current can be influenced by:
- The point on the AC voltage waveform at which the transformer is energised
- Residual magnetic flux remaining within the core
- Core material and saturation characteristics
- Normal operating flux density
- Core geometry and construction
- Primary winding resistance and reactance
- Supply impedance
- Supply voltage and frequency
- Connected load conditions
One of the most significant conditions occurs when the residual magnetic flux within the core and the applied voltage drive the core further towards magnetic saturation.
Once the core moves into saturation, the magnetising impedance falls considerably and a large transient current can flow.
Why does this matter?
A transformer may operate perfectly well at its rated load but still create problems during the first few cycles after switch-on.
High transformer inrush current can interact with:
- Circuit breakers
- Fuses
- Contactors
- Protection relays
- Generators
- UPS systems
- Distribution networks
- Other equipment connected to the same supply
If the energisation characteristic has not been considered, the result can be nuisance operation of protection or an unexpected voltage disturbance on the supply.
This is why transformer protection cannot always be considered purely from the normal full-load current.
For example, a transformer with a rated primary RMS current of 20 A experiencing a first peak equivalent to 10 times rated current could momentarily produce a current peak of approximately 200 A.
At 15 times rated current, that becomes approximately 300 A.
And under more severe conditions, the peak could be higher again.
Importantly, these are peak transient values, not continuous RMS operating currents.
The current peak is only part of the story
A good transformer inrush assessment should not simply state a maximum current.
The magnitude, waveform and decay characteristic all matter.
The largest current peak normally occurs immediately after energisation, with the transient reducing over the following cycles as the magnetic conditions within the transformer stabilise.
The resulting current-versus-time characteristic can then be considered alongside the operating characteristics of the upstream protective devices.
This allows engineers to examine whether protection will successfully ride through legitimate transformer energisation while still providing the required fault protection.
Assessing inrush before testing
Where sufficient transformer design information is available, expected energisation behaviour can be assessed during the design stage.
An inrush assessment can consider:
- Expected peak magnetising current
- Credible worst-case switching conditions
- Residual flux conditions
- Magnetic saturation behaviour
- Current decay
- Source impedance
- Interaction with circuit breakers or fuses
- Protection coordination
- Requirements for subsequent physical testing
The important point is that:
Transformer inrush is not simply a transformer characteristic. It is a transformer-and-system interaction.
The transformer design, electrical supply, source impedance, switching arrangement and protection system all influence what happens during those first few cycles.
An increasingly important design requirement
We are seeing customers request transformer inrush assessment much more frequently as part of the design and verification process.
Rather than simply specifying the transformer rating, customers increasingly want to understand what the transformer will demand from their electrical system during energisation.
This is particularly important where the transformer is connected to a restricted supply, generator, UPS system or tightly coordinated protection arrangement.
And this is where transformer design experience becomes particularly valuable.
Core material, core area, operating flux density, magnetic construction, winding impedance and other design parameters can all influence energisation behaviour.
We have the transformer design capability and engineering experience to consider inrush requirements during the design process, assess expected energisation characteristics and develop transformer designs around the requirements of the wider electrical system.
Because sometimes the most demanding part of a transformer’s operation isn’t running continuously at full load.
It’s what happens during the first few cycles after you switch it on.
To learn more about our services click on the links below for our social media feeds.