EEL303: Power Engineering I - Tutorial 7
√
1. A voltage of 100 2sin(314t + α) is applied to a series RL circuit by closing a switch.
R=10ohm and L=0.1H.
(a) What is the maximum possible value of DC component of current that can occur.
(b) If the switch is closed at an instant such that the DC component of current is zero,
what is the instantaneous value of voltage at the time of closing of a switch.
(c) If the switch is closed when the instantaneous voltage is zero, find the instantaneous
current 5 seconds later.
[Ans: (a)4.2915A (b)134.75V (c)-4.089A]
′
′′
2. A generator rated 100MVA, 18KV with reactances of xd = 19%, xd = 26%, xd = 130%
is connected to a transformer. The transformer is rated 100MVA, 240 star/18 delta KV,
x = 10%. If a three phase short circuit occurs on the high voltage side of the transformer
at rated voltage and no load, find
(a) The initial symmetrical RMS current in the transformer winding on the high voltage
side.
(b) The initial symmetrical RMS current in the line on low voltage side.
[Ans: (a)829.5A (b)11,060A]
3. A generator is connected through a transformer to a synchronous motor. Reduced to
the same base, the p.u sub-transient reactances of the generator and motor are 0.15 and
0.35 respectively. and leakage reactance of transformer is 0.1p.u. A three phase fault
occurs at terminals of motor when terminal voltage of generator is 0.9p.u and output
current of generator is 1.0p.u at 0.8pf leading. Find sub-transient current in p.u in fault,
in generator and in motor. Use terminal voltage of generator as reference phasor and
′′
′′
obtain solution using thevenin’s theorem. [Ans: Ig = 0.48-j3.24 p.u; Im = -1.03-j3.34
p.u]
4. The one-line diagram of three phase power system is as shown: Each generator is repre-
Figure 1:
sented by an emf behind the sub-transient reactance. All impedances are expressed in
p.u on common MVA base. All resistances and shunt capacitances are neglected. The
generators are operating on no-load at their rated voltage with their emfs in phase. A
three phase fault occurs at bus 3 through fault impedance Zf =j0.19p.u.
Electrical Engineering Dept - IIT Delhi
EEL303: Power Engineering I - Tutorial 7
(a) Using Thevenin’s theorem obtain the impedance to the point of fault and fault
current in p.u.
(b) Determine the bus voltages and line currents during fault.
[Ans: (a) Z33 =j0.2102; If = -j2.4988 p.u (b) V1f = 0.925p.u; V2f = 0.925p.u; V3f =
f
f
f
0.4748p.u; I12
= 0; I13
= -j1.5007p.u; I33
= -j1.0004p.u]
Electrical Engineering Dept - IIT Delhi