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Transformer Impedance Calculator — %Z, Ohms, Per-Unit & Fault Level

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Transformer rating (kVA) Use the transformer nameplate apparent-power rating.Selected winding rated voltage (V) Use line-to-line voltage for three-phase.Transformer phase Nameplate impedance (%) Prefer the actual nameplate or factory-test value.

Calculated result

0.008611 Ω referred to 415 V

Per-unit impedance: 0.05 pu

Base impedance: 0.172225 Ω

Rated current: 1,391.21 A

Ideal terminal fault-current multiplier: 20×

Transformer-only symmetrical terminal fault current: 27.824 kA

Transformer fault level: 20 MVA

Show the working

1. Zpu = 5 ÷ 100 = 0.05 pu.
2. Zbase = V² ÷ S = 415² ÷ 1,000,000 = 0.172225 Ω.
3. Zohm = Zpu × Zbase = 0.05 × 0.172225 = 0.008611 Ω.
4. Irated = S ÷ (√3 × V) = 1,000,000 ÷ (√3 × 415) = 1,391.21 A.
5. Infinite-bus terminal screen: Isc = Irated ÷ Zpu = 1,391.21 ÷ 0.05 = 27.824 kA.

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