If the base apparent power Sbase is doubled while the voltage base remains the same and Zactual unchanged, how does Zpu change?

If the base apparent power Sbase is doubled while the voltage base remains the same and Zactual unchanged, how does Zpu change?

• A. Zpu remains the same.
• B. Zpu doubles.
• C. Zpu halves.
• D. Zpu increases by the square root of 2.

Answer: (B)

The main idea is how per-unit impedance changes when the base power changes while the voltage base stays the same. In per-unit terms, Zpu = Zactual / Zbase, and Zbase = Vbase^2 / Sbase. With the voltage base fixed and the actual impedance unchanged, doubling the base apparent power makes Zbase drop by half (since Zbase is inversely proportional to Sbase). If Zbase halves and Zactual stays the same, then Zpu doubles. So the per-unit impedance increases by a factor of two.

The main idea is how per-unit impedance changes when the base power changes while the voltage base stays the same. In per-unit terms, Zpu = Zactual / Zbase, and Zbase = Vbase^2 / Sbase. With the voltage base fixed and the actual impedance unchanged, doubling the base apparent power makes Zbase drop by half (since Zbase is inversely proportional to Sbase). If Zbase halves and Zactual stays the same, then Zpu doubles. So the per-unit impedance increases by a factor of two.