The Advantages of Per-Unit Computations

Making computations for electric systems in terms of per-unit values simplifies the work greatly. A real appreciation of the value of the per-unit method comes through experience. Some of the advantages of the method are summarized briefly below :

1. Manufacturers usually specify the impedance of a piece of apparatus in percent of per unit on the basis of the nameplate rating.

2. The per-unit impedances of machines of the same type and widely different rating usually lie within a narrow range, although the ohmic value differ materially for machines of different ratings. For this reason when the impedance is not known definitely, it is generally possible to select from tabulated average values a per-unit impedance which will be reasonably correct. Experience in working with per-unit values brings familiarity with the proper values of per-unit impedance for different types of apparatus.

3. When impedance in ohms is specified in an equivalent circuit, each impedance must be referred to the same circuit by multiplying it by the squire of the ratio of the rated voltages of the two sides of the transformer connecting the reference circuit and the circuit containing the impedance. The per-unit impedance, once expressed on the proper base, is the same referred to either side of any transformer.

4. The way in which transformers are connected in three-phase circuits does not affect the per-unit impedances of the equivalent circuit although the transformer connection does determine the relation between the voltage bases on the two sides of the transformer.

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