Different bases in a short-circuit calculation
A fictional network calculation uses a transformer rated one megavolt-ampere with a six-percent leakage impedance on its own rating. The study adopts a two-megavolt-ampere power base while keeping the voltage base unchanged at the transformer terminals. The exercise treats the source as an ideal infinite bus behind that transformer impedance and ignores motor contribution and upstream impedance. One analyst keeps the impedance at zero point zero six per unit after changing base. Another converts it before calculating an ideal balanced fault. The team must show why a base change alters the numerical per-unit impedance without changing the physical equipment. No real fault level, breaker rating, arc-flash boundary or safe work procedure is supplied.
1. Convert the impedance to the new base.
Read the worked answer
With voltage base unchanged, Zpu,new=0.06*(2/1)=0.12per unit. The physical impedance is unchanged; its normalized numerical value changes because the base impedance changes.
2. Find the ideal fault current in per unit and state the limitation.
Read the worked answer
With an ideal1per-unitprefaultvoltageandonlythe0.12per-unittransformerimpedance, the modeled current magnitude is1/0.12≈8.333per unit on the new current base. Real studies need actual sources, impedance, connections and relevant operating cases; this result is not a protection setting.