Transformer inrush current: why a 630 kVA unit can draw 10 kA at switch-on

A transformer that trips its own upstream protection the moment it is energised has almost always been sized and wired correctly. The culprit is magnetising inrush – a transient that has nothing to do with the load and everything to do with the instant the switch closes.

Where inrush comes from

The magnetic flux in the core lags the applied voltage by 90°. If the circuit breaker closes at the voltage zero crossing, the flux must start from zero and swing to twice its normal peak within the first half cycle. Add the residual flux left in the core from the last switch-off, and the core saturates. A saturated core has almost no inductance, so the only thing limiting the current is the winding resistance and the network impedance. The result: a current peak of 8 to 12 times rated current, rich in second harmonic, decaying over 0.1 to 1 s depending on the size of the transformer.

Typical values

Rating	Rated HV current at 21 kV	Peak inrush (≈ 10 × In, MV side)	Decay time
250 kVA	6.9 A	≈ 70 A	0.1 – 0.2 s
630 kVA	17.3 A	≈ 170 A	0.2 – 0.4 s
1 600 kVA	44 A	≈ 440 A	0.5 – 1 s
2 500 kVA	68.7 A	≈ 690 A	up to 1 s

Referred to the LV side of a 630 kVA / 0.42 kV unit that is a peak of around 8–9 kA – hence the title.

What makes it worse

• Low-loss cores. Modern grain-oriented steel operates closer to saturation, so Tier 2 transformers have somewhat higher inrush than 1990s units of the same rating.
• Switching at voltage zero on one phase – unavoidable with a three-pole switch; one phase is always near the worst case.
• Residual flux from a previous de-energisation under load.
• A strong MV network (low source impedance).

Protection settings that work

1. MV fuses: choose the rating from the fuse manufacturer's transformer tables, not from the rated current. A 630 kVA / 20 kV unit typically takes a 40–50 A fuse-link, roughly 2.5 × In, precisely to ride through inrush.
2. Overcurrent relays (50/51): the instantaneous element above 12 × In or with a short time delay (≥ 0.1 s); the time-delayed element with a curve that tolerates 10 × In for 0.1 s.
3. Second-harmonic restraint: numerical relays detect the 2nd harmonic content typical of inrush (> 15 %) and block tripping – enable it.
4. Controlled switching: for repeatedly switched transformers (PV plants every morning, test fields) a point-on-wave switching device closes each pole at the optimum instant and reduces inrush to about 1–2 × In.

Photovoltaic plants

PV transformers are energised from the MV side every day when the plant wakes up – or remain energised at night, paying no-load losses. Both strategies are common. If you switch daily, specify controlled switching or make sure the operator's relay uses harmonic restraint; if you stay energised, choose a unit with the lowest possible no-load losses, such as the PNACHO-O PV versions.