Why we need per unit system in power system?

Per unit system simplifies calculations and makes analysis easier.This is because :

• pu impedance of the transformer is the same referred to either side.
• The method of connection of the transformer does not affect the pu impedance of the transformer.
• Manufacturers mention impedance of the apparatus in pu value based on the rated power and voltage.The rated impedance can be used directly in any analysis if the base chosen is the same as that on the nameplate.
• The ohmic values of impedances of the machines may vary widely based on the constructing materials and ratings of the machines. However the pu impedances lie in a narrow range. Thus, if actual values are not known, we can get a good approximate by using pu impedances.
• Tools of circuit analysis can be directly applied to circuits with components in pu values.
• For simulating the steady state and transient models in computers, the pu method is very handy.

Per unit system is a method used in power system analysis to express the voltage, current, kVA and impedance in per unit of base (or reference) values of these quantity.
The per unit value of any quantity is defined as the ratio of actual value of the quantity in any unit to the base value in the same unit.
The quantity involved in power system are kVA, voltage, current and impedance. For power system calculation when equipment circuit are drawn the different component will have different voltage which are connected together in the system through transformer and inter-connection. Each apparatus is rated in kVA and it's impedance in ohm or in percentage value (which cannot be used as a factor of 100 and has to be computed) referring to its rated kVA and rated voltage.
The direct calculation is very complex and infact impossible due to very large network of power system.
The per unit method simplifies the calculation as all the quantities are only expressed in term of p.u.
This method is particularly suited where there is a change in voltage i.e, in transformer circuit