May 2008
Intermediate to advanced
340 pages
9h 41m
English
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1.8 PER UNIT NORMALIZATION
In power engineering a normalization of numerical values is common practice. This so-called per unit normalization is similar to calculating with percentages. In the percentage system, the product of two quantities expressed as percentages must be divided by 100 to obtain the result in percentage. This drawback is circumvented in the per unit system. In the per unit (pu) system, 100 % corresponds to 1 pu. The per unit value of a certain quantity is defined as:
In power system analysis, the following quantities are of interest and frequently normalized: voltages, currents, impedances and powers. These quantities are interrelated and this means that a selection of two base values fixes immediately the base values for the other two. Therefore, selecting a base for the per unit calculation requires a selection of two base values only, and the other two base values can be calculated. Taking into account the constant voltage nature of the power system and the fact that the equipment is rated in Volt-Amperes, the base voltage and (apparent) power are usually the quantities to specify the base values, as shown in Table 1.4.
Example 1.14 Base quantities and per unit calculation
Consider Example 1.7 (p. 27). The calculation of the voltage at the sending end of the short transmission line will be repeated here in per unit quantities. The base voltage and base (apparent) ...
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