There are several reasons for using a per-unit system: Different types of quantities are labeled with the same symbol ( pu) it should be clear whether the quantity is a voltage, current, or other unit of measurement. The base voltage might be the nominal voltage of a bus. For example, the base power might be the rated power of a transformer, or perhaps an arbitrarily selected power which makes power quantities in the system more convenient. All quantities are specified as multiples of selected base values. With the exception of impedance and admittance, any two units are independent and can be selected as base values power and voltage are typically chosen. Thus, representations of elements in the system with per unit values become more uniform.Ī per-unit system provides units for power, voltage, current, impedance, and admittance. The main idea of a per unit system is to absorb large differences in absolute values into base relationships. The per-unit system is used in power flow, short circuit evaluation, motor starting studies etc. Conversion of per-unit quantities to volts, ohms, or amperes requires a knowledge of the base that the per-unit quantities were referenced to. Moreover, similar types of apparatus will have the impedances lying within a narrow numerical range when expressed as a per-unit fraction of the equipment rating, even if the unit size varies widely. This can be a pronounced advantage in power system analysis where large numbers of transformers may be encountered. Calculations are simplified because quantities expressed as per-unit do not change when they are referred from one side of a transformer to the other. In the power systems analysis field of electrical engineering, a per-unit system is the expression of system quantities as fractions of a defined base unit quantity.
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