What does Zbase represent in per-unit calculations?

• A. The base impedance determined by the actual impedance of the transformer.
• B. The base impedance used for normalization, defined by Vbase and Sbase.
• C. Zbase = Vbase^2 / Sbase.
• D. The short-circuit impedance of the line.

Answer: (C)

Zbase is the reference impedance used for normalization in per-unit calculations. You choose a system base voltage and a base apparent power, and the impedance that goes with those bases is Zbase = Vbase^2 / Sbase. This value is a scaling factor, not the actual impedance of any equipment. Any real impedance is converted to per-unit by dividing by Zbase, and conversely, per-unit values are converted back to ohms by multiplying by Zbase. The purpose is to keep impedances comparable across the system regardless of voltage level or size of equipment. This base isn’t the transformer’s actual impedance, and it isn’t the line’s short-circuit impedance; it’s the chosen reference impedance derived from the base voltage and base power. The explicit formula Zbase = Vbase^2 / Sbase gives the exact numerical value used for that normalization.