Per-Unit and Percent Values
Power systems operate at voltages for which kilovolt (kV) is the most convenient unit for expressing voltage. Also, these systems transmit large amounts of power, so that kilovolt-ampere (kVA) and megavolt-ampere (MVA) are used to express the total (actual or apparent) three-phase power. These quantities, together with kilowatts, kilovars, amperes, ohms, flux, and so on, are usually expressed as a per unit or percent of a reference or base value. The per-unit and percent nomenclatures are widely used because they simplify specification and computations, especially when different voltage levels and equipment sizes are involved.
This discussion is for three-phase electric systems ...