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1 Three-phase Circuits Balanced 3-phase systems Unbalanced 3-phase systems Dr. Nik Rumzi Nik Idris SKEE 1043 Circuit Theory

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2 Unbalanced 3-phase systems What is unbalanced 3-phase system? Sources that are unbalanced: unequal magnitude and unequal difference in phase angles Unequal load impedance only this will be considered in SKEE1043 This will result in unbalanced currents (even though the sources are balanced) hence for Y connected load, with 4 wire, I N 0 The line or phase currents have to be calculated individually

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3 Unbalanced 3-phase systems C IaIa ZAZA ZBZB ZCZC InIn IbIb IcIc N A B C What if the system is a 3-wire system (i.e. or Y)? For Y connected load with 3 wire system, mesh analysis can be used

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4 Unbalanced 3-phase systems b c a Z2Z2 Z1Z1 Z3Z3 A B C I AB I CA I BC

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5 Unbalanced 3-phase systems Power measurement for unbalanced system Two-wattmeter method is still applicable. However only P T =P 1 +P 2 is true. Reactive power CANNOT be obtained from √3(P 2 -P 1 ) For a Y connected, 4 wire system, three wattmeters (instead of two) are needed since I n 0 Again, the line (or phase) currents has to be calculated individually

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