8 24 2017 Per Unit Calculation(Student Version)

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  Per Unit Quantity ã The per unit quantity of voltage, current, power and impedance is found from dividing the actual quantity by a base value of that quantity.  −  =     ã Per unit value is denoted by ‘ p.u .’. ã The base value quantity typically follows transformer rating. ã The per unit values are the same irrespective of the sides of the transformer.  Base Value Selection (Single-phase circuit) ã Base values are real numbers , denote by subscript ‘ B ’. ã Voltage base values follow transformer voltage ratings. (   ) ã Only single base complex power in the system. ã Thus, the base values of real power, reactive power, and complex power are all the same real number.  1 =  1 =  1 ã Current base values are calculated from the base power and base voltage.    = 1   ã Impedance base values (same value for impedance, resistance, or reactance) are calculated from voltage and current.    =    =   1  Example ã A single-phase 20kVA, 480/120 V, 60 Hz transformer has an equivalent leakage impedance referred to 120-volt winding of    = 0.0525∠20 Ω. Using the transformer rating as base values, find per-unit leakage impedance.
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