Usually the name plate ratings of these individual devices are different and hence their respective individual base values will be also different. In order to refer all per unit values to a common system base, it is necessary to change all device p.u. An example later will illustrate this procedure. It gives relative magnitude information. The p.u. system is very useful in simulating machine systems on analog, digital, and hybrid computers for steady-state and dynamic analysis.
Manufacturers usually specify the impedance of a piece of apparatus in p.u.
Hence, it can be used directly if the bases chosen are the same as the name plate rating. The p.u. value of the various apparatus lie in a narrow range, though the actual values vary widely. For complicated systems involving many transformers or different turns ratio, this advantage is a significant one in that a possible cause of serious mistakes is removed.