It is convenient to select the base values of KVA and KV, and calculate the base values of current and impedance from the selected base KVA and KV. Voltage, current, kilovoltamperes and impedance are so related that selection of base values for any two of them determines the base values of the remaining two. Once the independent variables amongst the four are defined, the other ones can easily be calculated from the knowledge of the independent variables. Out of the four quantities mentioned above, only two are independent.
There is a relation between these quantities, viz: kilovoltamperes(KVA), Kilovolt (KV), current (I) and impedance (Z). Quantity in per unit= (actual quantity)/(base value of quantity) Thus, per unit system of any quantity can be defined as: The base values are generally indicated by a subscript b and per-unit value by subscript p.u. Both the actual value and the base value are expressed in the same unit so that the per-unit value comes out with no dimension. The arbitrarily selected value is known as the base value. The per-unit value of a quantity is the ratio of the actual value of that quantity to an arbitrary chosen value of that quantity. Wikimedia Creative Commons:Electricity Grid Schematicįortunately, power system engineers have seen through all the difficulties such as transformation of all impedances to a single voltage level, and decided to come up with the per-unit system such that various physical quantities such as power, voltage, current and impedance are expressed as decimal, fraction or multiples of base quantities.īy adopting the per-unit system of representation, the different voltage levels in the line of the power system collapses, and a power network involving generators, transformers, and lines (of different voltage levels) reduces to a system of simple impedances.
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