Editing Per-unit system (section)

{{Short description|In power systems, expression of system quantities as fractions}}
{{More footnotes needed|date=September 2023}}
In the [[power systems|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. 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. This can be a pronounced advantage in power system analysis where large numbers of transformers may be encountered. 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. Conversion of per-unit quantities to volts, ohms, or amperes requires a knowledge of the base that the per-unit quantities were referenced to. The per-unit system is used in [[Power flow study|power flow]], [[short circuit]] evaluation, [[motor starting methods|motor starting]] studies etc.

The main idea of a per unit system is to absorb large differences in absolute values into base relationships. Thus, representations of elements in the system with per unit values become more uniform.

A per-unit system provides units for [[electric power|power]], [[voltage]], [[Current (electricity)|current]], [[Electrical impedance|impedance]], and [[admittance]]. 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. All quantities are specified as multiples of selected base values.  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.  The base voltage might be the nominal voltage of a [[Busbar|bus]].  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.