In short. When a transformer is switched on, its core can saturate and draw a current many times full load that decays over seconds. It is rich in harmonics and can depress generator voltage, trip overcurrent or differential relays, and shake the supply. The study estimates the inrush and checks the source and protection can cope.
What causes inrush
The core flux at switch-on depends on the voltage point on the wave and the residual flux left from the last de-energisation. If the new flux adds to residual flux, the core is driven into saturation and the magnetising current spikes. Peak inrush may reach several times to more than ten times the rated current, with a duration from several cycles to many seconds.
Factors that increase it
- Larger transformer rating relative to the source.
- High residual flux, especially after a trip at current zero.
- Switching at the unfavourable point on the voltage wave.
- Low source impedance (a stiff source gives a higher, longer inrush).
- Several transformers energised together, or energised onto one already live (sympathetic inrush).
Effects on generator systems
| Effect | Why |
|---|---|
| Voltage dip and distortion | High current, rich in harmonics, through the generator impedance |
| Frequency disturbance | Reactive and real power surge |
| Nuisance relay trips | Overcurrent and differential relays see the inrush |
| Mechanical stress | Forces on transformer windings and the generator |
| Harmonic resonance | With capacitors, harmonics may be magnified |
Study steps
- Collect transformer data: rating, impedance, core material, magnetisation curve if available.
- Estimate peak inrush and decay time from manufacturer data.
- Model the source impedance, generator or utility.
- Calculate the voltage dip and compare with limits. See voltage-dip studies.
- Check relay settings: instantaneous pickup above inrush, harmonic restraint on differential. See transformer protection.
- Choose a sequence for multiple transformers.
Mitigation
- Energise transformers one at a time with a delay between.
- Energise from the generator before other loads, or with minimal load connected.
- Energise from the low-voltage side where possible, since the inrush is then smaller in the upstream network.
- Use point-on-wave controlled switching or pre-insertion resistors for large units.
- Use soft-start strategies from the generator, such as ramping voltage.
- Set relays with sensible time delays or harmonic blocking.
Illustrative example
A site restarts on generator after an outage with three 1,000 kVA transformers on the same bus. Energising all at once draws a combined inrush that stalls the 1,500 kVA generator and trips its overcurrent relay. The sequence is changed to energise the transformers one at a time, with a few seconds between each, and the voltage dip stays within limits.
Why energising a transformer is not a small event
Switching on a transformer can draw a magnetising inrush of 8 to 12 times rated current, decaying over several cycles to a second or more, and rich in second harmonic. The magnitude depends on residual flux and the point on the voltage wave at closing.
| Effect | Consequence |
|---|---|
| Voltage dip on the source | Equivalent to a heavy, low power factor load |
| Protection pickup | Overcurrent trips if set too sensitive |
| Harmonic content | Second and third harmonic distortion |
| Mechanical force | Stress on windings |
Worked example (illustrative)
A 1,000 kVA, 11 kV/415 V transformer has rated primary current of about 52 A. With inrush of 10 times, the peak is about 520 A for roughly 100 ms. A feeder relay set at 150 A with a 50 ms delay may trip on energisation. Increase delay or raise the instantaneous setting above the inrush, subject to damage curve limits.
Generator supply
When a generator energises a transformer, the inrush voltage dip is more severe than on the grid. Typical measures:
- Energise the transformer first, before other loads.
- Use soft energisation with a pre-insertion resistor or controlled switching where justified.
- Raise generator voltage slightly before energisation if the design allows.
Study outputs
Inrush peak, duration, voltage dip and harmonic content. See transformer protection and generator sizing.
Common mistakes
- Instantaneous settings below inrush.
- Energising all transformers together.
- Ignoring harmonics in the generator voltage.
What to send us
Send transformer and generator data and the restart sequence. Send your application details. See also transformer sizing.
Questions engineers ask
How long does inrush last?
From several cycles to several seconds or more, depending on the transformer and source.
Why does the generator trip on energising?
Inrush can look like a fault to the relay, or can overload the engine briefly.
Does a no-load transformer cause inrush?
Yes. Inrush happens at energisation whether or not there is load.
Can inrush be eliminated?
Controlled switching can reduce it greatly, but not always remove it.
Values are illustrative. Use manufacturer inrush data for design.