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perunitsystem - September 5 1999 TECHNICAL BULLETIN 007 The...

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September 5, 1999 TECHNICAL BULLETIN ° 007 The Per Unit System Overview Page 1 of 6 Overview The per unit system is based on the formula shown in Equation (1). actual quantity per unit base quantity = (1) Using this method, all quantities are expressed as ratios of some base value or values. In many respects the method is similar to the use of percentage values. When analyzing a real system, all parameters such as voltage, current, power, and impedance can be converted to per unit. If the bases are chosen wisely, the entire system may then be analyzed and the local results calculated. In the following discussion the subscript pu will indicate a per unit value, the subscript B will indicate a base value, and no subscript will indicate an actual value such as Amperes, Ohms, or Volts. Fundamental Procedure The per unit method can be described in five (5) basic steps: 1. Select the base(s) for your system. 2. Convert each known quantity (volts, amps, ohms, etc.) to per unit values. 3. Draw the system impedance diagram using the per unit values. 4. Perform the circuit analysis to determine the current magnitudes in per unit. 5. Convert the per unit currents and voltages from step 4 into Volts and Ohms. Selection of Base Quantities The most fundamental requirement of a per unit system is that all formulas and calculations must provide correct answers. In other words, if it works in actual values, it must work in per unit.
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