Assignment No.4
Question No.1
The magnitude of the phase voltage of an ideal balanced three-phase Y-connected source is 125 V. The
source is connected to a balanced Y-connected load by a distribution line that has an impedance of 0.1 +
j0.8 . The load impedance is 19.9 + j14.2 . The phase sequence is acb (i.e. negative phase sequence). Use
the a-phase as reference and figure out the three line currents, the three phase voltages at the load and
the three line voltages at the load.
Ans. IaA = 5<-36.87 , IbB = 5<83.12, IcC = 5<-156.87, VAN = 122.3 <-1.36, VBN = 122.3 <118.64, VCN =
122.3 <-121.36, VAB = 211.71<-31.36 , VBC = 211.71<88.69, VCA = 211.71<-151.36.
Question No.2
A balanced delta-connected load has an impedance of 60 + j45. The load is fed through a line having an
impedance of 0.8 + j0.6. The phase voltage at the terminals of the load is 480 V. The phase sequence is
positive. Use VAB as reference
a. Calculate the three phase currents of the load.
b. Calculate the three line currents.
c. Calculate the three line voltages at the load.
Ans. IAB = 6.4<-36.87, IBC = 6.4<-156.87, ICA = 6.3<83.13. IaA = 11.09<-66.87 , IbB = 11.09<173.13, IcC =
11.09<53.13. 480<0, 480<-120, 480<120.
Question No.3
The c-phase voltage of a balanced three-phase Y-connected system is 450<-25 V. If the phase sequence
is negative , what is the value of VAB.
Ans. 779.42<65
Question No.4
The line voltage VAB at the terminals of a balanced three-phase delta-connected load is 4160<0 V. The
line current IaA is 69.28<-10 A.
a. Calculate the per-phase impedance of the load if the phase sequence is positive.
b. Repeat (a) for negative phase sequence.
Ans. 104<-20, 104<40.
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