Per-Unit Base Impedance
Also known as kV squared over MVA · per unit system · base ohms · Z base · pu impedance base
Enter your known values, leave one input blank, and solves for the missing one. Try different units for next level excitement!
Learning zone
Power systems span 500 kV transmission down to 120 V receptacles, and every transformer in between rescales the volts, amps and ohms. The per-unit system kills that nuisance by expressing everything as a fraction of a chosen base. Pick a base voltage and a base MVA, and the base impedance follows: . On a 13.8 kV, 100 MVA base that is Ω, so an actual 3.8 Ω line reactance is 2.0 per unit.
The magic is what happens at a transformer. Choose bases that scale by the same turns ratio and the transformer disappears from the impedance diagram entirely — a 5% impedance is 0.05 pu on either side, no referring, no squared turns ratios, no arithmetic to get backwards. That single property is why every utility fault study, every stability program and every relay-setting calculation is done in per unit rather than ohms.
Two things trip people. First, kV²/MVA and V²/VA give the identical number of ohms, so mixing the pairs (kV with VA, say) throws the answer out by a million; this solver takes SI internally, so pick kV and MVA from the menus and it handles it. Second, a manufacturer's percent impedance is on the equipment's own base, not yours — moving a 5.75% impedance from a 1000 kVA nameplate onto a 100 MVA study base means multiplying by the ratio of the bases, and forgetting that step is the most common error in a hand-built fault study.
- = Base impedance (Ω)
- = Base voltage (kV)
- = Base apparent power (MVA) (MW)
- Base impedance — Series RLC Impedance, Series RL or RC Impedance
- Base voltage — Three-Phase Real Power, Single-Phase Real Power with Power Factor
- Base apparent power (MVA) — Three-Phase Apparent Power, Power Factor from Real and Apparent Power