Per unit quantities, like percentage quantities, are actually fractional quantities of a reference quantity. These have a lot of importance as per unit quantities of parameters tend to have similar values even when the system voltage and rating change drastically. The per unit system permits multiplication and division in addition to addition and subtraction without the requirement of a correction factor (when percentage quantities are multiplied or divided additional factors of 0.01 or100 must be brought in, which are not in the original equations, to restore the percentage values). Per-unit values are written with “pu” after the value.
For power, voltage, current and impedance, per unit quantity may be obtained by dividing by the respective base of that quantity.
Expressions such as Ohm’s Law can be applied for per unit quantities as well. Since Voltage, Current, Impedance and Power are related, only two Base or reference quantities can be independently defined. The Base quantities for the other two can be derived there from. Since Power and Voltage are the most often specified, they are usually chosen to define the independent base quantities.
For power, voltage, current and impedance, per unit quantity may be obtained by dividing by the respective base of that quantity.