Page "Per-unit system" Paragraph 1

A per-unit system provides units for power, voltage, current, impedance, and admittance.

Only two of these are independent, usually power and voltage.

All quantities are specified as multiples of selected base values.

For example, the base power might be the rated power of a transformer, or perhaps an arbitrarily selected power which makes power quantities in the system more convenient.

The base voltage might be the nominal voltage of a bus.

Different types of quantities are labeled with the same symbol ( pu ); it should be clear from context whether the quantity is a voltage, current, etc.