Per Unit system is the simplest system of calculation in the electrical power system. It is the rational value of the actual value of machine with respect to base value fixed before.
P.U.= Actual value /Base Value
You see the electrical system equipment have very high ratings which are quite trouble for calculations. So we in order to make calculations easy we just take the rational values of the machines. As in P.U. system there is no “UNIT” used, it make calculations free from unit change and maintaining the units in the S.I. system.
In most cases calculations are done with respect to impedance which give the proportional analysis of whole system in the same reference.
P.U. system makes the computer calculations easy as the preset base values itself compare with the new actual values and gives the proper P.U. result.
MVA (PU) = MVA (actual) / MVA ( base)
KV (PU) = KV (actual) / KV ( base)
X (PU) = X ( actual) * [ MVA (base) / KV^2 (base) ] Where, X is the reactance.
AS you can see the MVA base value is constant throughout the whole system but the KVbase value is taken with respect to transformer ratings at the corresponding sides.
In this way the PER UNIT system works, I hope this information will guide you little. This topic is quite critical to understand but if you’re not satisfied with this explanation you can put your doubts here.