SNS COLLEGE OF ENGINEERING, COIMBATORE – 641 107.
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
TUTORIAL IV
POWER SYSTEM ANALYSIS
Date: 23.09.2015
1. The phase voltage across a certain load are given as Va=(176-j132)Volts, Vb=(-128-j96)Volts
and Vc=(-160+j100)Volts. Compute positive, negative and zero sequence component of
voltage. [Ans: Va1 = (163.24-j35.10)Volts, Va2=(50.1-j53.9)Volts, Vao = (-112-j128)Volts].
2. A balanced delta connected load is connected to a three phase system and supplied to it is
a current of 15 amps. If the fuse is one of the lines melts, compute the symmetrical
components
of
line
currents.
[Ans: Ia1= (7.5+j4.33)Amps, Ia2= (7.5+j4.33)Amps, Ia0= 0 Amps.]
3. The symmetrical components of phase voltages in a 3Φ unbalanced system are Va0 =
10∟180° Volts, Va1 = 50∟0° Volts and Va2 = 20∟90° Volts. Find the phase voltages.
[Ans:Va = 40+j20; Vb = -52.32-j 53.30; Vc = -17.68+j33.3]
4. Determine the symmetrical components of the unbalanced 3Φ currents Ia= 10∟0° Amps, Ib=
12∟230° Amps and Ic= 10∟130°. [Ans.: Ia1= 10.56∟0°; Ia2= 1.04∟37°; Ia0= 1.47∟-160°]
5. Draw the positive, negative and zero sequence network of the power system as shown in
fig.
6. A 50MVA, 11KV, synchronous generator has a sub transient reactance of 20%.The
generator supplies two motors over a transmission line with transformers at both ends as
shown in fig. The motors have rated inputs of 30 and 15 MVA, both 10KV, with 25% sub
transient reactance. The three phase transformers are both rated 60MVA, 10.8/121KV,
with leakage reactance of 10% each. Assume zero sequence reactance for the generator
and motors of 6% each. Current limiting reactors of 2.5 ohms each are connected in the
neutral of the generator and motor number 2. The zero sequence reactance of the
transmission line is 300 ohms. The series reactance of the line is 100 ohms. Draw the
positive, negative and zero sequence networks.