In short. When a transformer is switched on, its core can draw a brief, large current called inrush, often several times its rated current and rich in harmonics. A generator has far less short-circuit capability than the grid, so inrush can collapse its voltage or trip protection. Screen the inrush against the source and use staging or soft energisation when needed.
What causes inrush
Switching on at a point on the voltage wave, with residual magnetism in the core, can drive the core into saturation. The magnetising current then peaks far above normal, then decays over many cycles. The size depends on the switching instant, remanence, transformer design and source impedance.
Typical magnitudes (orders of magnitude)
| Feature | Typical |
|---|---|
| Peak inrush | Roughly 5 to 12 times rated current, higher in some designs |
| Decay time | From tenths of a second to several seconds |
| Harmonic content | Strong second harmonic and other low orders |
| Source effect | Higher impedance sources reduce peak but prolong decay and depress voltage |
The manufacturer’s data gives the transformer’s inrush multiple and duration.
Why generators are sensitive
- Generator voltage dips sharply with large reactive current.
- Generator overcurrent and differential protection may mis-operate on inrush harmonics.
- The prolonged decay on a weak source can hold voltage low long enough to trip loads.
- Several transformers energised together add their inrushes.
Screening approach
- Obtain the transformer’s rated current and its inrush multiple from the manufacturer.
- Estimate the peak inrush and compare with the generator’s short-term capability.
- Compare with the setting of the generator and transformer protection.
- Check the voltage dip on the bus and on loads already connected.
- Consider the number of transformers and energise sequence.
Worked example (illustrative)
A 1,000 kVA, 415 V transformer has rated current of about 1,391 A. At an inrush multiple of 8, peak is about 11,100 A. A 1,000 kVA generator has a rated current of about 1,391 A, and a short-term capability maybe three times that, about 4,200 A. The inrush peak would exceed it, depressing the voltage. Mitigation: energise the transformer with downstream loads off, on a larger bus, via a pre-insertion resistor or a soft energisation scheme, or let the generator run unloaded at full voltage first. The numbers show why a transformer on a generator needs a specific check.
Mitigation options
- Staged energisation: one transformer at a time.
- Controlled switching at the optimum point on wave.
- Pre-insertion resistors or soft-start energisation.
- Soft-start of the generator voltage (ramped excitation).
- Sizing the generator or bus to tolerate the inrush.
- Harmonic restraint or blocking in relay settings.
Where else it matters
Black-start and restoration, ATS transfers and STS transfers onto transformers. See black-start restoration and energisation and inrush studies.
Screening inrush
Peak inrush ≈ K × rated current, with K typically 8 to 12 for distribution transformers, decaying over 0.1 to 1 second
| Transformer size | Typical peak (multiple of rated) | Duration |
|---|---|---|
| Below 500 kVA | 10 to 14 | About 0.1 s |
| 500 to 2,500 kVA | 8 to 12 | 0.1 to 0.5 s |
| Above 2,500 kVA | 5 to 8 | Up to a few seconds |
Indicative only; check the maker’s data.
Worked example (illustrative)
A 1,000 kVA transformer energised from a 1,250 kVA generator. Rated current on 415 V = 1,391 A. Peak inrush at 10 times = 13.9 kA. The generator subtransient reactance of 15 percent gives short-circuit current about 6.7 times rated, around 11.6 kA at its terminals. The inrush is comparable to the generator’s fault current, so voltage will collapse briefly. Energise the transformer with no downstream load, ensure relay settings tolerate the inrush, and consider energising from the high-voltage side where the source is stronger.
Mitigation options
- Energise unloaded transformers first, while the generator has headroom.
- Pre-insertion resistors or controlled switching for large units.
- Relay settings with inrush restraint or time delay.
See energisation studies and transformer protection.
Common mistakes
- Sizing a generator without considering the transformer inrush.
- Energising several transformers together.
- Protection settings that trip on inrush.
What to send us
Send the transformer ratings, the supply and the energisation scheme. Send your application details.
Questions engineers ask
How long does inrush last?
From a fraction of a second to several seconds, depending on the transformer and source.
Can inrush trip a generator?
Yes, through overcurrent, voltage collapse or protection operation on harmonics.
How do I reduce it?
Stage the energisation, use controlled switching or soft energisation, or size the supply for it.
Is it the same as the short-circuit current?
No. It is a magnetising effect, not a fault, but it can be of similar magnitude briefly.
Figures are illustrative. Obtain inrush data from the transformer manufacturer.