Appendix D

The Per-Unit System

D.1 Introduction

Because of various advantages involved, it is customary in power system analysis calculations to use impedances, currents, voltages, and powers in per-unit values (which are scaled or normalized values) rather than in physical values of ohms, amperes, kilovolts, and megavoltamperes (or megavars, or megawatts).

A per-unit system is a means of expressing quantities for ease in comparing them. The per-unit value of any quantity is defined as the ratio of the quantity to an “arbitrarily” chosen base (i.e., reference) value having the same dimensions. Therefore, the per-unit value of any quantity can be defined as physical quantity

Quantity in per unit = Physical quantity Base value of quantity ...

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