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Per Unit Impedance in 3Phase Systems
In a 3phase system, if
Z_{ohms} = linetoneutral impedance in ohms per phase and
There are cases in which transformers, generators, and transmission lines are operated at voltages somewhat different from rated value. Transformers are seldom operated at normal frequency with voltages that are substantially above rated values because of oversaturating the iron, which leads to excessive exciting current and to excessive core losses. However, transformers may operate at somewhat reduced voltages, as for example, 13.8kv transformers operating at 13.2 kv and at normal frequency. In changing the per unit value from one base to another, the change in voltage base must be taken into account. Thus, if
is the per unit value of Z_{ohms} impedance for voltampere and voltage bases of VA_{1} and V_{1} then the same impedance will have a per unit value of
for voltampere and voltage bases of VA_{2} and V_{2} from which it is evident that
Equation 6103a is valid for singlephase systems as well as for 3phase systems.


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