说明：  We desire to normalize the voltage equations, that is, we desire to express them in per-unit. The advantages of doing so are: • The per-unit system offers computational simplicity by eliminating units and expressing system quantities as dimensionless ratios • The numerical values of currents and voltages are related to their rated values irrespective of machine size. • It eliminates the need for using arbitrary constants and simplifies some of the mathematical expressions so that they may be expressed in terms of equivalent circuits. • Impedances, when given on the machine base, lie on a relatively narrow range so that errors can be easily detected.