advertisement

Three-phase Circuits Balanced 3-phase systems Unbalanced 3-phase systems SKEE 1043 Circuit Theory Dr. Nik Rumzi Nik Idris 1 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, IN 0 The line or phase currents have to be calculated individually 2 Unbalanced 3-phase systems Ia A ZA In N C Ib Ic Ia VAN ZA VBN Ib ZB ZB B Ic VCN ZC ZC C Ia Ib Ic In What if the system is a 3-wire system (i.e. or Y)? Ia Ib Ic 0 For Y connected load with 3 wire system, mesh analysis can be used 3 Unbalanced 3-phase systems A a IAB Z1 Z3 ICA b B Z2 IBC C c VAB IAB Z1 V IBC BC Z2 VCA ICA Z3 4 Unbalanced 3-phase systems Power measurement for unbalanced system • Two-wattmeter method is still applicable. However only PT=P1+P2 is true. Reactive power CANNOT be obtained from √3(P2-P1) • For a Y connected, 4 wire system, three wattmeters (instead of two) are needed since In 0 • Again, the line (or phase) currents has to be calculated individually 5