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

2. 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

3. 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

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

5. 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