Short-circuit withstand: what happens inside a transformer during a 2-second fault

A short circuit on the low-voltage busbar is the most violent event a distribution transformer will ever experience. For the fraction of a second before the protection clears the fault, the windings carry currents of 20 to 25 times their rating and are pushed apart by forces of several tonnes. IEC 60076-5 requires the transformer to survive this – and defines how the ability is demonstrated.

The current

The symmetrical short-circuit current is set by the transformer's own impedance (for an infinitely strong network): I_sc = I_n / u_k. For u_k = 4 % that is 25 × I_n; for 6 % it is 16.7 × I_n. The first peak, with full DC offset, reaches about 2.5 times the symmetrical RMS value – for a 630 kVA / 0.42 kV unit that is a peak of roughly 55 kA on the LV side.

Two kinds of stress

	Thermal withstand	Dynamic withstand
Cause	I²R heating of the conductors	Electromagnetic forces between the windings
Duration in the standard	2 s (Category I–II, ≤ 2 500 kVA)	First peak, then decaying
Limit	Winding temperature ≤ 250 °C (Cu) / 200 °C (Al) after 2 s	No permanent deformation; reactance change < 2 % (oil) after test
Verification	Calculation per IEC 60076-5	Calculation or test in a high-power laboratory

Where the forces go

The leakage flux between the concentric HV and LV windings produces a radial force that tries to burst the outer winding outwards and crush the inner winding inwards, and an axial force that compresses the windings towards their centre. Axial forces are the dangerous ones: any asymmetry between the windings – a tap winding at one end, uneven shrinkage of the insulation – produces a net force that can shift the whole coil. Manufacturers control this with precise winding geometry, axial clamping under defined pressure, and drying under vacuum before impregnation.

How withstand is proven

1. Calculation: IEC 60076-5 allows demonstration by calculation, provided the manufacturer's method has been validated by tests on comparable designs.
2. Type test in an independent laboratory: the transformer is subjected to a series of short circuits (three per phase, each 0.5 s) at full asymmetric current; afterwards the reactance must not have changed by more than the permitted value and the routine tests are repeated. Laboratories such as KEMA, CESI or LTC run these tests.

Because a test destroys nothing but proves everything, it is usually carried out on one representative unit of a design family. Ask the supplier which family your rating belongs to and whether a test certificate exists for it.

Design features that matter

• Foil LV windings (common in PNACHO-D and in oil units up to ~1 000 kVA): the axial force is distributed evenly along the whole foil – inherently robust.
• Rigid axial clamping with pressed insulation and spring elements.
• Symmetrical placement of the tap winding.
• Round, self-supporting coils rather than rectangular ones.

PNACHO transformers are designed to IEC 60076-5 short-circuit Category I (≤ 2 500 kVA) with a 2-second thermal withstand; the impedance values on the PNACHO-O and PNACHO-D product pages let you calculate the fault current for your switchgear design.