Per-Unit System

$\text{per-unit value} = \frac{\text{actual value}}{\text{base value}}$

Choose a base power $S_b$ (e.g. 100 MVA) and a base voltage at each level; the other bases follow:

$Z_b = \frac{V_b^2}{S_b} \qquad I_b = \frac{S_b}{\sqrt{3}V_b}$

Changing base: $Z_{pu,new} = Z_{pu,old}\left(\dfrac{V_{old}}{V_{new}}\right)^2\dfrac{S_{new}}{S_{old}}$.

Benefits: transformers disappear from the per-unit diagram (ideal ratio becomes 1:1), equipment impedances fall into narrow, familiar ranges, and errors are easy to spot.