Per-unit values only help if everything in your study sits on the same base. Mix bases, and a perfectly good impedance suddenly looks wrong — not because the equipment changed, but because the ruler did.
The fix is one conversion, and it is worth memorizing:
Z on the new base = Z on the old base × (new MVA base / old MVA base) × (old kV base / new kV base)²
That is it. Scale by the MVA ratio, then correct for any change in voltage base, squared.
A worked example
Say a transformer nameplate gives 10% impedance on its own 20 MVA base. You are building a study on a common 100 MVA base, at the same voltage.
0.10 × (100 / 20) = 0.50 pu
So that same transformer is 0.50 pu on the study base. It has not become five times more impeding in the real world — the number just grew because the base grew. This is also why small units can look electrically "stiff" in big studies: their per-unit impedance balloons when you move them onto a much larger base.
Watch the voltage base too
The squared term is easy to forget, and it matters most when you cross a transformer with a different ratio. If the kV base changes, the impedance changes with the square of that ratio, even if the MVA base stays put. Check both bases before you trust a number.
A quick sanity check
If you want to check your own conversion, I use the free per-unit converter on GridCircuits — it shows the formula it is using, so you can see each step rather than just trusting an answer.
Before your next study, take one conversion and do it by hand with the formula above. If the hand calculation and the tool agree, you can trust the rest of the base changes in the model — and you will know exactly why the numbers moved.
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