Instructions.—(1) Attempt any five questions. All questions carry equal marks.
Answers must be written in English or Hindi. However all
the answers should be in one language only.
Parts of the same question, if any, must be answered together
without answers to other questions intervening.
Each question must be started on fresh page.
Marks will be deducted if handwriting is not easily legible.
Answers will be evaluated on the basis of logic, brevity and
clarity in exposition.
Use of ordinary calculators with six functions and twelve
digits log tables is permitted.
1. A series R-L circuit with R 50? and L 10 H has a constant voltage
100 Volts applied at t=0 by closing a switch. Find—
equation for i,VR and VL
current at t 0.5 sec
time at which VR VL
2. A 25kVA transformer supplies a load of 12 kW at power factor of 0.6 lagging.
Find the of full load, the transformer is carrying. If additional unity
power factor loads are to be served with the same transformer, how many
kW may be added before the transformer is at full load
3. Define infinite bus-bar. Discuss the effect of variation of excitation of a
synchronous machine connected to such a system.
Discuss the division of load active and reactive components) between
two alternators when they are running in parallel
4. A 5000 kVA, 10000 1500 rpm, 50Hz alternator runs on parallel with
other machine, its synchronous reactance is 0.20 per unit. Find for no
load and full load at 0.8 pf lagging, the synchronizing power per unit
mechanical angle of phase displacement and calculate the synchronizing
torque if the mechanical displacement is 0.5
5. A single phase overhead line consist of a pair of conductors 10 mm in
diameter spaced uniformly 2 meters apart in air. Find the capacitance
per km.
If the line is 30 km long and its one end is connected to 50 kV, 50Hz
system, what is the charging current with the other end open circuited
6. What is meant by capacitance grading of cable
Write a short note on pressure cable.
What is the purpose of inter-sheath in cable
7. A 3-wire DC system with 400 V between outers supplies lighting loads of
1200 amp and 1040 amp on the positive and negative side and the motor
load of 400 kW across the outers.
Calculate the load on the main generators and on each of the balancer machines
assuming that at this load each balancer machine has a loss of 6 kW
8. Discuss the methods of improving the state stability and transient stability
of a system.
A power system has a maximum steady state stability limit equal to 100
MW. If it is supplying a load of 50 MW, what is the sudden increase in load
permissible if the transient stability of the system is to be maintained
[Turn over
CON 327 4
9. Assume that a generator with constant internal voltage is subjected to
different types of faults and that all the resistances are negligible. Also,
assume that the absolute values of fault currents for the different types
of faults are as follows
1000 amp
line to line, 1400 amp
line to ground, 2220 amps.
If the positive sequence voltage to neutral is 2000 volts, what are the positive,
negative and zero sequence impedances
10. 10 Electrical power is to be supplied to a 25 Hz system from a 50
Hz system through a motor-generator set consisting of two directly coupled
synchronous machines.
What is the minimum number of poles which the motor may have
What is the minimum number of poles which the generator may have
At what speed in rpm will the set specified in and operate