A healthy transformer can draw a surge larger than a fault for a few cycles, and protection must know the difference.
When a transformer is switched on, its core can be driven deep into saturation. The result is a short, lopsided surge of magnetising current that may trip breakers or relays that are not set correctly.
What Is Transformer Inrush Current?
Transformer inrush current is the large magnetising current drawn in the first cycles after a transformer is energised. Its peak can reach 8 to 12 times rated current, far above the normal no load current of a few percent.
Eaton explains that the surge is needed to establish flux in the core and decays within a few cycles. The decay rate depends on winding resistance and inductance, so large units can take several seconds to settle.
The waveform is very distorted and flows mostly in one polarity. That shape is the key to telling it apart from a real fault.
Inrush happens on the primary only, since the secondary is open or lightly loaded during switching. The same physics also affects current transformers when they saturate.
Why the Core Saturates
In steady state, flux lags voltage by 90 degrees and peaks at the voltage zero crossing. If the switch closes at that zero, flux starts from its residual value and swings to about twice the normal peak.
Modern cores run near 1.6 to 1.7 tesla, so doubling flux pushes them well past saturation. Eaton notes that the worst case needs maximum residual flux and the worst switching moment together, so it is rare.
Transformer Inrush Current Formulas
Rated current I = kVA ÷ (√3 × kV)
Peak inrush ≈ Multiple × √2 × I
Worked example, 1000 kVA, 0.415 kV secondary energised from LV side:
I = 1000 ÷ (1.732 × 0.415) = 1391 A
Multiple = 10
Peak inrush ≈ 10 × 1.414 × 1391 ≈ 19.7 kA for the first cycle
Manufacturers usually quote the multiple as a peak value relative to the rated RMS current. Use their data where possible, because core design changes the result.
Rated current comes from the nameplate, as shown in transformer kVA rating. Always check which side is switched, since the LV side draws a larger multiple.
Inrush vs Fault Current
| Feature | Inrush | Internal Fault |
|---|---|---|
| Waveform | Offset, peaky, one polarity | Near sinusoidal |
| 2nd harmonic | High, often above 15 percent | Low |
| Duration | Cycles to seconds, decaying | Until cleared |
| Differential current | Appears on one side only | Real imbalance |
| Correct response | Restrain | Trip fast |
Differential relays block tripping when the 2nd harmonic ratio exceeds a set level, commonly 15 to 20 percent. Our protective relays guide covers these functions.
Harmonic content is explained in more depth in power harmonics. Modern relays also use waveform shape and dead angle detection.
5 Proven Fixes for False Trips
Breaker curves must pass above the inrush point, a check shown in time current coordination. For smaller units, see MCCB rating selection.
Sympathetic inrush occurs when an already energised transformer draws extra current as a neighbour is switched in. It lasts longer and can confuse protection on both units, especially where two transformers share one busbar.
Effects on the Network
The voltage dip can reset sensitive loads, as explained in voltage sag and flicker. Energise large transformers before connecting sensitive equipment.
Peak Inrush Calculator
Use the result to confirm breaker instantaneous settings, CT ratings and genset capacity before commissioning day. Actual inrush is usually lower because the worst switching angle is rare.
- Get inrush data from the maker.
- Enable harmonic restraint.
- Coordinate breaker curves.
- Energise one unit at a time.
- Instantaneous pickup set too low.
- Ignoring residual flux after tests.
- Switching all feeders together.
- Undersized generator for energising.
Transformer Inrush Technical PDF
Inrush Theory Video
Transformer Inrush Current FAQ
Related Articles
- Transformer Vector Group
- Transformer Routine and Type Tests
- Protective Relays Explained
- Transformer Turns Ratio
- Transformer kVA Rating
External References
What We Learn Today
- Transformer inrush current peaks when the core saturates after switching near voltage zero.
- Inrush is offset, peaky and rich in 2nd harmonic.
- Correct settings and controlled switching stop false trips.