How it works
Choose a system base: Sbase = 100 MVA and Vbase at each voltage level following transformer turns ratios (11 kV on LV side, 220 kV on HV side). Base impedance Zbase = Vbase²/Sbase; on the HV side Zbase = 220²/100 = 484 Ω. If a transformer has 8% leakage reactance on its own rating (50 MVA, 11/220 kV), convert to system base: Znew = Zold × (Sbase_new/Sbase_old) × (Vbase_old/Vbase_new)². This formula appears in almost every per-unit numerical. The beauty is that transformer pu impedance is the same on both sides of the transformer — no need to refer to primary or secondary.
Key points to remember
Advantages of the pu system: transformer pu impedance is independent of which side it is referred to, pu values for equipment of the same type fall in a narrow range (transformer leakage reactance 0.05–0.10 pu, generator synchronous reactance 0.8–1.2 pu), and network equations become simpler. When multiple machines have different ratings, always convert all pu quantities to the common system base before adding or comparing. The actual value = pu value × base value — never forget to convert back at the end of the calculation.
Exam tip
The examiner always asks you to convert transformer impedance from its own MVA base to a new system base — write down Znew = Zold × (MVAnew/MVAold) × (kVold/kVnew)² and substitute carefully to avoid base confusion.