Part 1 of the short-circuit series. The same LV board fault worked two ways — the quick MVA method and the IEC 60909 per-unit method — and why they differ by 7.6 %.
| Network | 250 MVA, 11 kV grid (R/X = 0.1) → 1.6 MVA, 11/0.4 kV Dyn transformer, uk = 6 %, Pk = 15 kW |
| Fault | Three-phase bolted fault on the 0.4 kV LV board |
| LV tolerance | +6 % (cmax = 1.05 at LV, 1.10 at 11 kV) |
Each element is turned into its own short-circuit power and the powers are combined like resistors in parallel.
In ohms at 0.4 kV: Z = 0.000973 + j0.006409 Ω (Zbase = 0.4²/100 = 0.0016 Ω).
The MVA method ignores the voltage factor c and the transformer correction KT, and treats impedances as pure reactances. IEC 60909 deliberately uses cmax = 1.05 for LV with +6 % tolerance, giving the maximum current needed for equipment ratings. The MVA method is a good sense check, but the equipment duty should be taken from IEC 60909.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| MVA method | Sense check | 34.8 kA | ✓ PASS |
| IEC 60909 maximum Ik″ | Used for equipment rating | 37.41 kA | ✓ PASS |
| LV board rating | ≥ 37.41 kA | 50 kA | ✓ PASS |
Key insight: Over 85 % of the impedance comes from the transformer. For LV boards fed by a transformer, the transformer impedance sets the fault level; the grid strength matters much less.
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Open IEC 60909 Short-Circuit Calculator calculator →IEC 60909-0 Table 1 gives cmax = 1.10 above 1 kV, and 1.05 (or 1.10 with a +10 % tolerance) at low voltage. The grid impedance is calculated with the factor of its own network.
It corrects the transformer impedance for the real operating conditions before the fault, so the equivalent voltage source method gives the right maximum current.