ABSTRACT
Power systems operate at voltages where 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 three-phase power. The per unit and percent nomenclatures are widely used because they simplify specification and computations, especially where different voltage levels and equipment sizes are involved. Percent is 100 times per unit. Both are used as a matter of convenience or of personal choice and it is important to designate either percent or per unit. The per unit value of any quantity is the ratio of that quantity to its base value, the ratio expressed as a non-dimensional decimal number. Currents in amperes and impedances in ohms should be referred to a specific voltage base or to primary or secondary windings of transformers.
