10 Per unit in 3-phase systems

10 Per unit in 3-phase systems - Lecture 12 Per unit in 3...

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Lecture 12 Per unit in 3 φ systems In 1 φ systems we defined 2 base quantities for each 1 φ device: V B , S B . For example, I = 1.1 pu implies that it is 10% above the rated current. In addition we have seen that the transformers can be replaced by solid wires yielding the following relations, 1_ 2_ p up u p u VV II = = Hence the analysis is simpler without the transformers. The rationale behind the definition of the base quantities was to maintain KCL, KVL, and Ohm’s laws invariant to the new set of functions. Therefore if VI Z = in SI units and p u pu Z = . In 3 φ systems the same rationale is applied. Consider the 3 φ power consumed by a 3 φ balanced load. We have seen that 3 3c o s PV I φ = . Figure 1 - 3f load Recall that, 0, 120, 120, , 120, 120 ab c c φφ =∠ −− −+ . Note that V is the rms value of the phase to ground voltage. Be aware that in 3 φ systems voltages are rated phase to phase 3 = . Example. A 3 φ motor is rated at 1000 V implies that V φφ = 1000V, therefore 1000/ 3 V = Volts.
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10 Per unit in 3-phase systems - Lecture 12 Per unit in 3...

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