RBI, BANK ,VIZAG STEEL ,SAIL, GENCO , TRANSCO, NTPC, BHEL, ISRO, UPSC, IES IMPORTANT QUESTIONS MCQ

AC MACHINES 1

2021-03-14T10:14:00.002+05:30

AC MACHINES

1. Silicon content in iron lamination's is kept within 5% as it
a) increases hysteresis loss
b) increases the cost
c) makes the material brittle
d) reduces the curie point.
ans. c

2. If the iron core of a transformer is replaced by an air core, then the hysteresis losses in the transformer will
a) increase
b) decrease
c) remain unchanged
d) become zero
ans. d

3. Which of the following will improve the mutual coupling between primary and secondary circuits?
a) Transformer oil of high breakdown voltage
b) High reluctance magnetic core
c) Winding material of high resistivity
d) low reluctance magnetic core
ans. d

4. In order to reduce the hysteresis loss
a) core may be laminated.
b) silicon steel may be used as the core material
c) core may be constructed with any permanent magnet material such as Alnico.
d) core may be impregnated with varnish.
ans. b

5. Why is the core of the transformer built up of lamination?
a) To reduce eddy current loss.
b) For the convenience of fabrication.
c) No specific advantage.
d) For increasing the permeability.
ans. a

6. When are the eddy-current losses in a transformer reduced?
a) If laminations are thick.
b) If the number of turns in the primary winding is reduced.
c) If the number of turns in the secondary winding is reduced.
d) If laminations are thin.
ans.d

7. The primary and secondary winding's are interlaced for
a) easiness of coil making
b) reduced leakage
c) reduced cost
d) uniform heating.
ans. b

8. When a transformer winding suffers a short-circuit, the adjoining turns of the same winding experience
a) an attractive force
b) a repulsive force
c) no force
d) all the above
ans. a

9. Under short-circuit condition, the windings of a transformer having 5% impedance will experience a radial force
a) 5 times of full load value
b) 20 times of full load value
c) 25 times of full load value
d) 400 times of full load value
ans. d

10. The function of oil in a transformer is
a) to provide insulation and cooling.
b) to provide protection against lightning
c) to provide protection against short circuits.
d) to provide lubrication.
ans. a

AC MACHINES 2

2021-03-14T09:55:00.001+05:30

AC MACHINES

1. Conservator ( for a transformer) is used

a) to take up the expansion of oil due to temperature rise.
b) to act as an oil storage
c) for better cooling of transformer
d) none of the above

ans. a

2. Sludge formation in transformer oil is due to which one of the following?

a) Ingress of dust particles and moisture in the oil.
b) Appearance of small fragments of paper, varnish, cotton, and other organic materials in the oil.
c) Chemical reaction of the transformer with oil insulating materials.
d) Oxidation of transformer oil.

ans. d

3. A Buchholz relay is used for

a) protection of transformer against all internal faults.
b) protection of transformer against external faults.
c) protection of transformer against both internal and external faults.
d) protection of induction motors.

ans. a

4. Which is the arrangement of windings in a core type single phase transformer?

a) Half lv inside and half hv outside on each core limb.
b) Lv on one core limb and hv on the other limb.
c) Sandwiched lv and hv discs on each core limb.
d) Half hv side and half hv outside on each core limb.

ans. a

5. Addition of tubes to the transformer tank improves heat dissipation capacity because of

a) additional cooling surface
b) additional dissipation by radiation only.
c) additional dissipation by convention only
d) additional dissipation by radiation and convention both.

ans. a

6. Transformer is rated in kVA instead of kW because

a) load pf is often not known.
b) kVA is fixed where kW depends on load pf.
c) total transformer loss depends on the volt-amperes.
d) it has become customary.

ans. c

7. Which three-phase connection can be used in a transformer to introduce a phase difference of 30 degrees between its output and corresponding input line voltages?

a) Star-Star
b) Star-Delta
c) Delta-Delta
d) Delta-ZigZag

ans. b

8. A 3-phase transformer has a rating of 20 MVA,220 kV(star)- 33kV(delta) with leakage reactance of 12%. The transformer reactance (in ohms) referred to each phase of lv delta connected side is

a) 23.5
b) 19.6
c) 18.5
d) 8.7

ans. b

9. The percentage resistance and percentage reactance of a 10kVA, 400V/200V,3-phase transformer are 2% and 10% respectively. If the constant losses in the machine are 1%., the maximum possible percentage efficiency of the transformer is

a) 98.32
b) 97.25
c) 96.85
d) 96.12

ans. b

10. A 3-phase,delta-star transformer is supplied at 6000 V on the delta connected side.The terminal voltage on the secondary side when supplying full-load at 0.8 lagging power factor at 415V. The equivalent resistance and reactance drops for the transformer are 1% and 5% respectively. The turns ratio of the transformer is

a) 14
b) 24
c) 42
d) 20

ans. b

AC MACHINES 3

2021-03-14T09:16:00.000+05:30

AC MACHINES

1.The supply to one terminal of delta-wye connected 3-phase core type transformer which is on no-load fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be
a) 230,230,115
b) 230,115,115
c) 345, 115,115
d) 345,0,345
ans. b

2. In a transformer fed from a fundamental frequency voltage source, the source of harmonics is the
a) overload
b) poor insulation
c) iron loss
d) saturation of the core.
ans. d

3. The magnetizing current in a transformer is rich in
a) 3rd harmonic
b) 5th harmonic
c) 7th harmonic
d) 13th harmonic
ans. a

4. Which of the following conditions is/are necessary for the parallel operation of two transformers?
a) percentage impedance should be the same
b) polarities of both secondaries should be the same.
c) voltage ratings should be the same.
d) All the above
ans. d

5. Two transformers of identical voltages but of different capacities are operating in parallel, for satisfactory load sharing
a) impedance's must be equal
b) per unit impedance's must be equal
c) per unit impedance's and X/R ratio must be equal
d) impedance's and X/R ratio must be equal.
ans.c

6. Two transformers are to be operated in parallel such that they share the load in proportion to their kVA ratings. The rating of the first transformer is 500 kVA and its pu leakage impedance is 0.05 pu. If the rating of the second transformer is 250 kVA, its pu leakage impedance is
a) 0.20
b) 0.10
c) 0.05
d) 0.025
ans. b

7. An auto-transformer having a transformation ratio of 0.8 supplies a load of 10 kW. The power is transformed inductively from the primary to secondary is
a) 10 kW
b) 8 kW
c) 2 kW
d) zero
ans. c

8. The coils are to be connected to obtain a single-phase,400V/1000V, auto-transformer to drive a load of 10kva. Which of the given options should be exercised to realize the required auto-transformer?
a) Connect A and D; Common B
b) Connect B and D; Common C
c) Connect A and C; Common B
d) Connect A and C; Common D.
ans. a

9. In the auto-transformer obtained in question 48, the current in each coil is
a) Coil-1 is 25 A and Coil-2 is 10A
b) Coil-1 is 10 A and Coil-2 is 25A
c) Coil-1 is 10 A and Coil-2 is 15A
d) Coil-1 is 15 A and Coil-2 is 10A
ans.d

10. Constant current transformer is of ____ type.
a) core
b) shell
c) core or shell
d) None of these
ans. b

AC MACHINES 4

2021-03-14T08:53:00.000+05:30

AC MACHINES

1. Tappings of a transformer are provided

a) at the phase end of the lv side.
b) at the phase end of hv side.
c) at the neutral end of hv side.
d) at the middle of hv side.

ans. d

2. Tappings are usually provided on the HV winding of a transformer because it

a) is easily accessible physically
b) has to handle low currents
c) has a large number of turns which allows smoother variation of voltage
d) all of the above

ans. d

3.In a constant voltage transformer (CVT), the output voltage remains constant due to

a) capacitor
b) input inductor
c) saturation
d) tapped winding's

ans. c

4. The rating of transformer _____ when the supply frequency is increased

a) remains unchanged
b) decreases
c) increases
d) None of these

ans. c

5. Distributed winding and short chording employed in ac machines will result in

a) increase in emf and reduction in harmonics
b) reduction in emf and increase in harmonics
c) increase in both emf and harmonics
d) reduction in both emf and harmonics

ans. d

6. It is desirable to eliminate the 5th harmonic from the phase voltage of an alternator. The coils should be short-pitched by an electrical angle of

a) 30 degrees
b) 36 degrees
c) 72 degrees
d) 18 degrees

ans. b

7. A 4-pole, 3-phase, double layer winding is housed in a 36 slot stator for an ac machine with 60 degrees phase spread. Coil span is 7 slot pitches. The number of slots in which top and bottom layers belong to different phases is

a) 24
b) 18
c) 12
d) 0

ans. a

8. The armature of a single-phase alternator is completely wound with T-single turn coils distributed uniformly. The induced voltage in each turn is 2V (RMS). The emf of the whole winding is

a) 2 T volts
b) 1.11 T volts
c) 1.414 T volts
d) 1.273 T volts

ans. d

9. A 3-phase alternator is wound with a 60-degree phase-spread armature winding and develops 300 kVA. If the armature is reconnected utilizing all the coils for single-phase operation with a phase spread of 180 degrees, the new rating of the machine is

a) 100 kVA
b) 200 kVA
c) 250 kVA
d) 300 kVA

ans. b

10. The phase sequence of the three-phase alternator will reverse if

a) the field current is reversed keeping the direction of rotation the same.
b) the field current remains the same but eh direction of rotation is reversed.
c) the field current is reversed and the number of poles is doubled.
d) the number of poles is doubled without reversing the field current.

ans. b

AC MACHINES 5

2021-03-14T08:39:00.000+05:30

AC MACHINES

61. A 500 MW 3-phase Y-connected synchronous generator has a rated voltage of 21.5 kV at 0.85 pf. The line current when operating at full load rated conditions will be
a) 13.43 kA
b) 15.79 kA
c) 23.25 kA
d) 27.36 kA
ans. b

62. A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is
a) magnetizing
b) demagnetizing
c) cross-magnetizing
d) ineffective
ans. b

63. A field excitation of 20 A in a certain alternator results in an armature current of 400A in short circuit and a terminal voltage of 200V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200 A is
a) 1 V
b) 10 V
c) 100 V
d) 1000 V
ans. d

64. A 100 kVA ,415V(line) , star-connected synchronous machine generates rated open circuit voltage of 415 V at a field current of 15 A.The short-circuit armature current at a field current of 10 A is equal to the rated armature current. The per unit saturated synchronous reactance is
a) 1.731
b) 1.5
c) 0.666
d) 0.577
ans. a

65. A single-phase ,2000 V alternator has armature resistance and reactance of 0.8 ohms and 4.94 ohms respectively. The voltage regulation of the alternator at 100 A load at 0.8 leading power-factor is
a) 7 %
b) -8.9 %
c) 14 %
d) 0 %
ans. b

66. Which one of the following is not correct with respect of synchronous machines?
a) In salient pole machines , the direct-axis synchronous reactance is greater than the quadrature axis synchronous reactance.
b) The damper bars help the motor to self-start.
c) Short-circuit ratio is the ratio of field current required to produce the rated voltage on the open circuit to the rated armature current.
d) The V-curve of a synchronous motor represents the variation in armature current with field excitation at a given output power.
ans. c

67.Higher synchronous reactance is preferred in the present day alternators because one can have
a) reduced sub transient current
b) reduced harmonic currents
c) reduced transient currents
d) higher voltage regulation with the load.
ans. c

68. A stand-alone engine-driven synchronous generator is feeding a partly inductive load. A capacitor is now connected across the load to completely nullify the inductive current. For this operating condition
a) the field current and fuel input have to be reduced
b) the field current and fuel input have to be increased
c) the field current has to be increased and fuel input left unaltered
d) the field current has to be reduced and fuel input left unaltered.
ans. d

69. A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half, with field current held constant, its new power factor will be
a) unity
b) leading
c) lagging
d) dependent on machine parameters
ans. b

70. A synchronous machine connected to a power system grid bus-bar is operating as a generator. To make the machine operate as a motor, the
a) direction of rotation is to be reversed
b) phase-sequence is to be changed
c) field excitation is to be decreased.
d) mechanical input is to be less than the losses at the shaft.
ans. d

AC MACHINES 6

2021-03-14T07:59:00.000+05:30

AC MACHINES

1. A synchronous generator connected to an infinite bus is over-excited. Considering only the reactive power, from the point of view of the system, the machine acts as
a) capacitor
b) inductor
c) resistor
d) none of the above
ans. a

2. A synchronous motor operates at 0.8 pf lagging. If the field current of the motor is continuously increased
a) the power factor decreases up to a certain value of the field current and thereafter it increases
b) the armature current increases up to a certain value of the field current and thereafter it decreases
c) the power factor increases up to a certain value of the field current and thereafter it decreases
d) the armature current decreases up to a certain value of filed current and thereafter it increases
e) both c and d
ans. e

3. A cylindrical rotor synchronous motor is switched on the dc supply with its field winding's shorted on themselves. It will
a) not start
b) start but not run at synchronous speed
c) start as an induction motor and then run as synchronous motor
d) start and run as a synchronous motor
ans. b

4. Two 3-phase Y-connected alternators are to be paralleled to a set of common busbars. The armature has a per phase synchronous reactance of 1.7 ohm and negligible armature resistance. The line voltage of the first machine is adjusted to 3300 V and that of the second machine is adjusted to 3200 V. The machine voltages are in phase at eh instant they are paralleled. Under this condition, the synchronizing current per phase will be
a) 16.98 A
b) 29.41 A
c) 33.96 A
d) 58.82 A
ans. a

5. A power station consists of two synchronous generators A and B ratings of 250 MVA and 500 MVA with inertia 1.6 pu and 1 pu respectively on their own base MVA ratings. The equivalent per unit inertia constant for the system on 100 MVA common base is
a) 2.6
b) 0.615
c) 1.625
d) 9.0
ans. d

6. A 3-phase,4-pole, squirrel cage induction motor to
a) increase the strength of rotor bars.
b) reduce the magnetic hum and locking tendency of the rotor
c) economize on the copper to be used.
d) provide ease of fabrication.
ans. b

7. The rotor of 3-phase, 5 kW, 400 V, 50 Hz, slip ring induction motor is wound for 6 poles while its stator is wound for 4 poles. The approximate average no-load steady-state speed when this motor is connected to 400 V, 50 Hz supply is
a) 1500 rpm
b) 500 rpm
c) 0 rpm
d) 1000 rpm
ans. c

8. The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with a 3-phase sinusoidal voltage having phase sequence A-B-C. For counterclockwise rotation of the motor, the phase sequence of the power supply should be
a) B-C-A
b) C-A-B
c) A-C-B
d) B-C-A and C-A-B
ans. b

9. If a 400V,50Hz star-connected, 3-phase, squirrel cage induction motor is operated from a 400V, 75Hz supply, the torque that the motor can now provide while drawing rated current from the supply.
a) reduces
b) increases
c) remains the same
d) increases or reduces depending upon the rotor resistance.
ans. a

10. A 3-phase squirrel cage induction motor has a starting torque of 150% and a maximum torque of 300% with respect to rated voltage and rated frequency. Neglect the stator resistance and rotational losses. The value of slip for maximum torque is
a) 13.48%
b) 16.42%
c) 18.92 %
d) 26.79%
ans. d

AC MACHINES 7

2021-03-14T07:51:00.000+05:30

AC MACHINES

1.The power input to a 415 V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 49 kW. The stator losses are 1 kW and friction and windage losses total 2 kW. the efficiency of the motor is
a) 92.5%
b) 92%
c) 90%
d) 88%
ans. c

2. A 400 V, 15 kW, 4-pole, 50 Hz, Y-connected induction motor has a full-load slip of 4%. The output torque of the machine at full load is
a) 1.66 Nm
b) 95.5 N
c) 99.47 Nm
d) 624.73 Nm
ans. c

3. A 400 V,50 Hz, 30 hp, a 3-phase induction motor is drawing 50 A current at 0.8 power factor lagging. The stator and rotor copper losses are 1.5 kW and 900 W respectively. The friction and windage losses of 1050 W and the core losses are 1200 W. The air-gap power of the motor will be
a) 23.06 kW
b) 24.11 kW
c) 25.01 kW
d) 26.21 kW
ans. c

4. No-load test on a 3-phase induction motor was conducted at different supply voltages and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. This intersection point yields
a) core loss
b) stator copper loss
c) stray load loss
d) friction and windage loss
ans. d

5.What is the shunt resistance component in the equivalent circuit obtained by a no-load test of an induction motor representative of?
a) windage and frictional losses only
b) core losses only
c) core, windage, and frictional losses
d) copper losses
ans. b

6. The synchronous speed for the seventh space harmonic MMF wave of a 3-phase,8-pole,50 Hz induction machine is
a) 107.14 rpm in forward direction
b) 107.14 rpm in reverse direction
c) 5250 rpm in forward direction
d) 5250 rpm in reverse direction
ans.a

7. In an induction motor, if the air-gap is increased
a) speed will reduce
b) efficiency will improve
c) power factor will be lowered
d) breakdown torque will reduce
ans. c

8. Unbalanced supply voltage is given to a 3-phase, delta-connected induction motor will cause
a) zero sequence currents
b) less heating of the motor
c) negative sequence component current
d) all the above
ans. c

9. An induction motor is fed from a balanced three-phase supply at rated voltage and frequency through a bank of three single-phase transformers connected in delta-delta. One unit of the bank develops a fault and is removed. Then
a) single phasing will occur and the machine fails to start
b) single phasing will not occur but the motor terminal voltages will become unbalanced and the machine can be loaded to the extent of 57.7% of its rating
c) the machine can be loaded to the extent of 57.7% of its rating with balanced supply at its terminals
d) the machine can be loaded to the extent of 66 2/3% of its rating with balanced supply at its terminals
ans. a

10. An induction motor having full-load torque of 60 Nm when delta connected develops a starting torque of 120 Nm. For the same supply voltage, if the motor is changed to star-connection, the starting torque developed will be
a) 40 Nm
b) 60 Nm
c) 90 Nm
d) 120 Nm
ans. a

AC MACHINES 9

2021-03-14T07:47:00.000+05:30

AC MACHINES

1. A single phase induction motor with only the main winding excited would exhibit the following response at synchronous speed
a) rotor current is zero
b) rotor current is non-zero and is at slip frequency
c) forward and backward totaling fields are equal
d) forward rotating field is more than the backward rotating field.
ans. d

2. A 230 V, 50 Hz, 4 pole, single-phase induction motor is rotating in the clockwise direction at a speed of 1425 rpm. If the rotor resistance at standstill is 7.8 Ohm, then the effective rotor resistance in the backward branch of the equivalent circuit will be
a) 2 ohm
b) 4 ohm
c) 78 ohm
d) 156 ohm
ans. b

3. For single phase capacitor start induction motor which of the following statements is valid?
a) The capacitor is used for power factor improvement
b) The direction of rotation can be changed by reversing the main winding terminals.
c) The direction of rotation cannot be changed.
d) The direction of rotation can be changed by interchanging the supply terminals.
ans. b

4. In a single-phase induction motor driving a fan load, the reason for having a high resistance rotor is to achieve
a) low starting torque
b) quick acceleration
c) high efficiency
d) a reduced size
ans. b

5. The type of single-phase induction motor having the highest power factor at full load is
a) shaded pole type
b) split-phase type
c) capacitor -start type
d) capacitor-run type.
ans. d

6. The torque-speed characteristics of a repulsion motor resembles which of the following dc motor characteristic?
a) separately excited
b) shunt
c) series
d) compound
ans. c

7. Ratio of the rotor reactance X to the rotor resistance R for a 2-phase servo motor
a) is equal to that of normal induction motor
b) is less than that of a normal induction motor
c) is greater than that of a normal induction motor
d) may be less or greater than that of a normal induction motor.
ans. b

8. In a stepper motor, the detent torque means
a) minimum of the static torque with the phase winding excited
b) maximum of the static torque with the phase winding excited
c) minimum of the static torque with the phase winding unexcited
d) may be less or greater than that of a normal induction motor.
ans. b

9. For a given stepper motor, the following torque has the highest numerical value
a) detent torque
b) pull-in torque
c) pull-out torque
d) Holding torque
ans. c

10. The following motor definitely has a permanent magnet rotor.
a) DC commutator motor
b) Brushless dc motor
c) Stepper motor
d) Reluctance motor
ans. c

AC MACHINES 8

2021-03-14T07:43:00.000+05:30

AC MACHINES

91. The following starting method for an induction motor is inferior from the point of view of poor starting torque per ampere of the line current drawn
a) Direct-on-line starting
b) Auto transformer method of starting
c) series inductor method of starting
d) star-delta method of starting
ans. c

92. A 3-phase induction motor is started by direct-on-line switching at the rated voltage. If the starting current drawn is 6 times the full-load current, and the full -load slip is 4%, then the ratio of the starting developed torque to the full load torque is approximately equal to
a) 0.24
b) 1.44
c) 2.40
d) 6.00
ans. b

A three-phase squirrel cage induction motor has a starting current of seven times the full load current and a full load slip of 5%
93. If an autotransformer is used to reduced voltage starting to provide 1.5 per unit starting torque, the auto-transformer ratio (%) should be
a) 57.77 %
b) 72.56%
c) 78.25%
d) 81.33%
ans. c
94. If a star-delta starter is used to start this induction motor, the per-unit starting torque will be
a) 0.607
b) 0.816
c)1.225
d) 1.616
ans. b
95. If a starting torque of 0.5 per unit is required then the per unit starting current should be
a) 4.65
b) 3.75
c) 3.16
d) 2.13
ans. c

96. An ac induction motor is used for a speed control application. It is driven from an inverter with a constant v/f control. The motor nameplate details
V: 415 V
Ph: 3
f: 50 Hz
N: 2850 rpm
The motor is run with the inverter output frequency set at 40Hz, and with half the rated slip. The running speed of the motor is
a) 2400 rpm
b) 2280 rpm
c) 2340 rpm
d) 2790 rpm
ans.

97. The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of supply voltage to supply frequency (V/F) constant. At a rated frequency of 50 Hz and a rated voltage of 400 V, its speed is 1440 rpm. find the speed at 30 Hz, if the load torque is constant.
a) 882 rpm
b) 864 rpm
c) 840 rpm
d) 828 rpm
ans. c

98. A 3-phase induction motor is driving a constant torque load at rated voltage and frequency. If both voltage and frequency are halved, the following statements relate to new conditions if stator resistance, leakage reactance, and core loss are ignored.
P. The difference between synchronous speed and actual speed remains the same
Q. The air-gap flux remains the same
R. The stator current remains the same
S. The PU slip remains the same
among the above, correct statements are
a) all
b) P ,Q,R
c) Q, R, S
d) P, S
ans. b

99. A 3-phase squirrel cage induction motor has full-load efficiency of 0.8 and maximum efficiency of 0.9. It is operated at a slip of 0.6 by applying a reduced voltage. The efficiency of the motor at this operating point is
a) less than 0.4
b) greater than 0.6
c) in the range of 0.8+- 0.1
d) none of the above
ans. c

100. A 3-phase slip-ring induction motor is fed from the rotor side with the stator winding short-circuited. The frequency of the currents in the short-circuited stator is
a) slip frequency
b) supply frequency
c) frequency corresponding to rotor speed
d) zero
ans. a

AC MACHINES 10

2021-03-14T07:39:00.000+05:30

AC MACHINES

1.Which one of the following types of motors is most suitable for a computer printer drive?
a) Reluctance motor
b) Hysteresis motor
c) Shaded pole motor
d) Stepper motor
ans. d

2. A 3-phase, 3 stack, variable reluctance step motor has 20 poles on each rotor and stator stack. The step angle of this step motor is
a) 3 degrees
b) 6 degrees
c) 9 degrees
d) 18 degrees
ans. b

AC MACHINES 11

2021-03-14T07:36:00.000+05:30

AC MACHINES

11. The core flux of a practical transformer with a resistance load
a) is strictly constant with load changes
b) increases linearly with load
c) increases as the square root of the load
d) decreases with the increase of load
ans: a

12. In a transformer, zero voltage regulation at full load is
a) not possible
b) possible at unity power factor load
c) possible at leading power factor load
d) possible at lagging power factor load
ans: c

13. A single-phase transformer has a maximum efficiency of 90% at full load and unity power factor. Efficiency at half load at the same power factor is
a) 86.7%
b) 88.26 %
c) 88.9%
d) 87.8%
ans: d

14. The main purpose of performing open-circuit on a transformer is to measure its
a) copper loss
b) core loss
c) total loss
d) insulation resistance
ans: b

15. The desirable properties of transformer core material are
a) low permeability and low hysteresis loss
b) high permeability and high hysteresis loss
c) high permeability and low hysteresis loss
d) low permeability and high hysteresis loss
ans: c

16. The all-day efficiency of a transformer depends primarily on
a) it's copper loss
b) the amount of load
c) duration of load
d) both b and c
ans: d

17. The use of higher flux density in the transformer design
a) reduces the weight per KVA
b) increases the weight per KVA
c) has no relation with the weight of the transformer
d) none of the above
ans: a

18. What does the use of higher flux density in the transformer design
a) Increase the weight per KVA
b) Decrease in weight per KVA
c) Reduced iron losses
d) Reduced copper losses
ans. b

19. Cores of large power transformers are made from which one of the following?
a) Hot-rolled steel
b) Cold-rolled non-grain-oriented steel.
c) Cold-rolled grain-oriented steel
d) Ferrite.
ans. c

20. Grain-oriented laminated sheet steel in transformer reduces
a) copper loss
b) eddy current loss
c) hysteresis loss
d) none of the above
ans. c

AC BASICS AND NETWORKS 1

2021-03-10T17:30:00.000+05:30

AC BASICS AND NETWORKS

11. Unit of reactive power is
a) VA
b) watt
c) VAR
d) ohm
ans: c

12. An ac source of 200V RMS supplies active power of 600 W and reactive power of 800 VAR. The RMS current drawn from the source is
a) 10 A
b) 5 A
c) 3.75 A
d) 2.5 A
ans: b

13. With the increase in applied frequency, the dielectric loss in the material will
a) increase
b) decrease
c) remains constant
d) becomes zero
ans: a

14. In an ac series RLC circuit, the voltage across R and L is 20 V, the voltage across L and C is 9 V and voltage and voltage across RLC is 15 V, What is the voltage across C?
a) 7V
b) 12 V
c) 16 V
d) 21 V
ans: a

15. The quantity (2 + j9) represents the resistive and reactive components of current drawn by an ac circuit. The phase angle between the applied voltage and current is
a) 41.4'
b) 53.13'
c) 36.87'
d) 46.86'
ans: c

16. Under the condition resonance in the R-L-C series circuit, the pf of the circuit is
a) 0.5 lagging
b) 0.5 leading
c) unity
d) zero
ans: c

17. In a series RLC circuit at resonance, the magnitude of voltage developed across the capacitor
a) is always zero
b) can never be greater than the input voltage
c) can be greater than the input voltage however, it is 90 degrees out of phase with the input voltage
d) can be greater than the input voltage and is in phase with the input voltage.
ans: c

18. In A series R-L-C circuit, the maximum voltage across the capacitor occurs at a frequency
a) double the resonant frequency
b) equal to the resonant frequency
c) below the resonant frequency
d) none of the above
ans: c

19. In a series R-L-C circuit, the voltage across inductance will be maximum
a) at the resonant frequency
b) just after resonant frequency
c) just before resonant frequency
d) just before and after resonant frequency
ans: b

20. For a series R-L-C circuit, the power factor at the lower power frequency is
a) 0.5 lagging
b) 0.5 leading
c) unity
d) 0.707 leading
ans: d

AC BASICS AND NETWORKS 2

2021-03-09T22:26:00.000+05:30

AC BASICS AND NETWORKS

Multiple-choice questions

21. Q-factor of as series R-L-C circuit possessing resonant frequency of 10 Hz and bandwidth of 5 Hz is
a) 0.5
b) 2
c) 2.5
d) 50
ans: b

22. When Q-factor of a circuit is high , then
a) power factor of the circuit is high
b) impedance of the circuit is high
c) bandwidth is large
d) none of the above
ans: b

23. A high Q coil has
a) large bandwidth
b) high losses
c) low losses
d) flat response
ans: c

24. A coil is tuned to resonance at 500 kHz with a resonating capacitor of 36 pf at 250 kHz , the resonance is obtained with a resonating capacitor of 160 pf .What is the self-capacitance of the coil?
a) 2.66 pf
b) 5.33 pf
c) 8 pf
d) 10.66 pf
ans: b

25. A R-L-C resonant circuit has a resonance frequency of 1.5 MHz and a bandwidth of 10kHz . If c= 150pf, then the effective resistance of the circuit will be
a) 29.5 ohm
b) 14.75 ohm
c) 9.4 ohm
d) 4.7ohm
ans: d

POWER SYSTEMS 1

2012-11-04T22:08:00.006+05:30

POWER SYSTEMS

11. In thermal power plants, the pressure in the working fluid cycle is developed by
a) Condenser
b) Superheater
c) feed water
d) turbine
ans: d

12. Which material is used in controlling chain reaction in av nuclear reactor?
a) Thorium
b) Heavy water
c) Boron
d) Beryllium
ans: b

13. When a fixed amount of power is transmitted, the efficiency of transmission increases when
a) Voltage decreases, power factor remains constant
b) voltage increases, power factor increase
c) voltage decreases, power factor decreases
d) voltage constant, power factor decreases
ans: b

14. The surge impedance of 0.400km long OH line is 400 ohm for 200 km length of the same line, the surge impedance will be
a) 200 ohm
b) 800 ohm
c) 400 ohm
d) 100 ohm
ans: c

15. The insulation level of a 400 EHV OH transmission line is decided on the basis of
a) lightning voltage
b) switching overvoltage
c) corona inception voltage
d) radio and TV interference
ans: b

16. Bundled conductors are employed to improve the
a) appearance of the transmission line
b) mechanical stability of the line
c) decrease system stability
d) increase the short circuit current
ans: c

17. An OH line having a surge impedance of 400 ohms is connected in series with an underground cable having a surge impedance of 100 ohm. If a surge of 50 Kv travels from the line end towards the line cable junctions, the value of the transmitted voltage wave at the junction is
a) 30 kV
b) 20 kV
c) 80 kV
d) -30 Kv
ans: b

19. The load-carrying capability of a long AC transmission line is
a) always limited by the conductor size
b) limited by stability considerations
c) reduced at low ambient temperatures
d) decreased by the use of bundled conductors of single conductors.
ans: b

20. Corona losses are minimized when
a) corona size is reduced
b) smooth conductor is released
c) sharp points are provided in the hardware
d) current density in conductors is reduced
ans: b

POWER SYSTEMS 2

2012-11-04T22:08:00.005+05:30

POWER SYSTEMS

21. The corona loss on a particular system at 50HZ is 1kw/km/ph. The corona loss at 60 HZ is
a) 1kw/ph/km
b) 0-83 kw/km/ph
c) 1.2 kw/km/ph
d) 1.13 kw/km/ph
ans: d

22. A loss-less radial transmission line with surge impedance loading
a) takes negative VAR at sending end and zero VAR at receiving end
b) take positive VAR at sending end and zero VAR at receiving end
c) has flat voltage profile and unity power factor at all points along with it
d) has sending end voltage higher than receiving end voltage and unity power factor at sending end
ans: c

23. Consider along, two wirelines composed of solid round conductors. The radius of both conductors is 0.25 cm and the distance between their centers is 1m. If this distance is doubled, then the inductance per unit length
a) doubles
b) halves
c) increase but does not double
d) decreases but not halve
ans: c

24. A long wire composed of a smooth round conductor runs above and parallel to the ground. A high voltage exists between the conductor and the ground. The max electric stress occurs at.
a) the upper surface of the conductor
b) the lower surface of the conductor
c) the ground surface
d) none of the above
ans: b

25. Bundled conductors are mainly used in high voltage OH transmission lines to
a) reduce transmission line losses
b) increase the mechanical strength of the line
c) reduce corona
d) reduce sag
ans: c

ANALOG ELECTRONICS 1

2012-11-04T22:06:00.002+05:30

11. Notch is also called as
a) band-pass filter
b) low-pass filter
c) high-pass filter
d) narrow-band reject filter
ans: d

12. The filter which exhibits ac equiripple in the pass-band and monotonic characteristic in stop band is
a) Buttorworth
b) Chebyshev
c) Bessel
d) Elliptic
ans: a

13. A multiplier which can accept input voltages of either polarity and can produce output voltage of both polarities is called
a) Bi-quadrant multiplier
b) four quadrant multiplier
c) two quadrant multiplier
d) one quadrant multiplier
ans: c

14. A multiplier is derived using two logarithmic amplifiers and and antilog amplifier . One more section required is
a) multiplier
b) adder
c) subtractor
d) divider
ans: b

15. A frequency multiplier usually operates in
a) push-pull
b) class A
c) class B
d) class C
ans: d

16. FSK demodulation is an application of
a) PLL
b) timer
c) BPF
d) none of the above
ans: a

17. An oscillator whose frequency is changed by a variable dc voltage is known as
a) a crystal oscillator
b) a VCO
c) an Armstrong oscillator
d) a piezoelectric device
ans: b

18. The integrated circuit chip NE/SE 566 is a
a) Schmitt trigger
b) power amplifier
c) video amplifier
d) voltage controlled oscillator
ans: d

19. For a step input , the output of an integrator is
a) a pulse
b) a triangular waveform
c) a spike
d) a ramp
ans: d

20. The damping factor of a second order active low pass filter to give butterworth response is
a) 1.732
b) 1.414
c) 0.707
d) none of the above
ans: b

ELECTRONIC COMPONENTS AND DEVICES 1

2012-11-03T17:08:00.003+05:30

11. The depletion region or space charge region or transition region in a semiconductor P-N junction diode has
a) electron and holes.
b) positive ions and electrons.
c) positive ions and negative ions.
d) no ions, electron or holes.
ans. c

12. In a p+ n junction diode under reverse bias, the magnitude of electric field is maximum at
a) the edge of the depletion region on the p-side.
b) the edge of the depletion region on the n-side.
c) the p+n junction.
d) the centre of the depletion region on the n-side.
ans. c

13. A p-n junction has a built-in potential of 0.8 V. The depletion layer width at a reverse bias of 1.2V is the 2 micrometer. For a reverse bias of 7.2V , the depletion layer width will be
a) 4 micrometer
b) 4.9 micrometer
c) 8 micrometer
d) 12 micrometer
ans. a

14. At 300 K for a diode current of 1mA , a certain germanium diode requires a forward bias of 0.1435 V, whereas a certain silicon diode requires a forward bias of 0.718 V, Under the conditions stated above , the closest approximation of the ratio of reverse saturation current in germanium diode to that in silicon diode is
a) 1
b) 5
c) 4 x 10³
d) 8 x 10³
ans. c

15. A silicon PN junction at a temperature of 20⁰C has a reverse saturation current of 10 pico-Amperes (pA). The reverse saturation current at 40⁰C for same bias is approximately
a) 30 pA
b) 40 pA
c) 50 pA
d) 60 pA
ans. b

16. Which of the following is not associated with a p-n junction ?
a) Junction Capacitance
b) Charge Storage-Capacitance
c) Depletion Capacitance
d) Channel Length Modulation.
ans. d

17. As the temperature is increased, the voltage across a diode carrying a constant current
a) increases
b) decreases
c) remains constant.
d) may increase or decrease depending on the doping levels in the junction.
ans. b

18. Choose proper substitutes of X and Y to make the following statement correct. Tunnel diode and Avalanche diode are operated in X bias and Y bias respectively
a) X- reverse ,Y-reverse
b) X-reverse, Y -forward
c) X-forward, Y- reverse
d) X-forward, Y-forward
ans. c

19. Consider the following assertions.
S1- For zener effect to occur, a very abrupt junction is required.
S2- For quantum tunneling to occur , a very narrow energy barrier is required.
which of the following is correct?
a) Only S2 is true.
b) S1 and S2 are both true but S2 is not a reason for S1
c) S1 and S2 are both true and S2 is a reason for S1.
d) Both S1 and S2 are false.
ans. a

20. A dc power supply has a no-load voltage of 30 V, and a full-load voltage of 25 V at a full load-current of 1A. Its output resistance and load regulation , respectively are
a) 5 ohm and 20 %
b) 25 ohm and 20%
c) 5 ohm and 16.7%
d) 25 ohm and 16.7 %
ans. b

ELECTRONIC COMPONENTS AND DEVICES

2012-11-02T21:16:00.001+05:30

1. The color code of a 1 kiloohm resistance is
a) black , brown , red
b) red, brown, brown
c) brown, black, red
d) black, black , red
ans. c

2. An electron with velocity u is placed in an electric field E and magnetic field B. The force experienced by the electron is given by
a) -eE
b) -eu X B
c) -e(u X E + B)
d) -e(E + u X B)
ans. d

3. The primary reason for the widespread use of Silicon semiconductor device technology is
a) abundance of Silicon on the surface of Earth.
b) larger bandgap of Silicon in comparison to Germanium.
c) favourable properties of Silicon-dioxide (Sio2).
d) lower melting point.
ans. a

4. A Silicon wafer has 100 nm of oxide on it and is inserted in a furnace at a temperature above 1000 degree centigrade for further oxidation in dry oxygen. The oxidation rate is
a) independent of current , oxide thickness and temperature.
b) independent of current, oxide thickness but depends on temperature.
c) slows down as the oxide grows.
d) zero as the existing oxide prevents further oxidations.
ans. d

5. The band gap of silicon at room temperature is
a) 1.3 eV
b) 0.7 eV
c) 1.1 eV
d) 1.4 eV
ans. c

6. n-type silicon is obtained by doping silicon with
a) Germanium
b) Aluminium
c) Boron
d) Phosporous
ans. d

7. Which of the following is true ?
a) A silicon wafer heavily doped with boron is a p+ substrate.
b) A silicon wafer lightly doped with boron is a p+ substrate.
c) A silicon wafer heavily doped with Arsenic is a p+ substrate.
d) A silicon wafer lightly doped with Arsenic is a p+ substrate.
ans. a

8. The concentration of minority carriers in an extrinsic semiconductor under equilibrium is
a) directly proportional to the doping concentration.
b) inversely proportional to the doping concentration.
c) directly proportional to the intrinsic concentration.
d) inversely proportional to the intrinsic concentration.
ans. b

9. Under low level injection assumption, the injected minority carrier current for an extrinsic semiconductor is essentially the
a) diffusion current
b) drift current
c) recombination current
d) induced current
ans. a

10. The majority carriers in an n-type semiconductor have an average drift velocity in a direction perpendicular to a uniform magnetic field B. The electric field E induced due to Hall effect acts in the direction .
a) v X B
b) B X v
c) along v
d) opposite to v
ans. b

SWITCHGEAR AND PROTECTION 3

2012-11-01T23:11:00.001+05:30

31. A 3-phase 11kV generator feeds power to a constant power unity power factor load of 100 MW through a 3-phase transmission line. The line-to-line voltage at the terminals of the machine is maintained constant at 11 kV. The per unit positive sequence impedance of the line based on 100 MVA and 11 KV is j0.2. The line-to-line voltage at the load terminals is measured to be less than 11kV. The total reactive power to be injected at the terminals of the load to the increase the line-to-line voltage at the load terminals to 11 kV is
a) 100 MVAR
b) 10.1 MVAR
c) -100 MVAR
d) -10.1 MVAR
ans. d

A generator feeds power to an infinite bus through a double circuit transmission line. A 3-phase fault occurs at the middle point of one of the lines. The infinite bus voltage is 1 pu, the transient internal voltage of the generator is 1.1 pu and the equivalent transfer admittance during fault is 0.8 pu. The 100 MVA generator has an inertia constant of 5 MJ/MVA and it was delivering 1.0 pu power prior of the fault with rotor power angle of 30⁰ . The system frequency is 50 Hz.

32. The initial accelerating power (in pu) will be
a) 1.0
b) 0.6
c) 0.56
d) 0.4
ans. c
33. If the initial accelerating power is Xpu, the initial acceleration in electrical degrees/sec , and the inertial constant in MJ-sec/electrical degrees respectively will be
a) 31.4X,18
b) 1800X, 0.056
c) X/1800 , 0.056
d) 3/3.14, 18
ans. b

34. In load-flow analysis , the load connected at a bus is represented as
a) constant current drawn from the bus.
b) constant impedance connected at the bus.
c) voltage and frequency dependent source at the bus.
d) constant real and reactive power drawn from the bus.
ans. d

35. Gauss-Siedel interative method can be used for solving a set of
a) linear differential equations only.
b) linear algebraic equations only.
c) both linear and nonlinear algebraic equations only.
d) both linear and nonlinear differential equations only.
ans. b

36. The Gauss-Siedel load flow method has the following disadvantages. Tick the incorrect statements
a) Unreliable convergence
b) slow convergence
c) choice of slack bus affects convergence.
d) A good initial guess for voltages is essential for convergence.
ans. b

37. A power system consists of 300 buses out of which 20 buses are generator bus, 25 buses are ones with reactive power support and 15 buses are the ones with fixed shunt capacitors. All the other buses are load buses. It is proposed to perform a load flow analysis in the system using Newton-Rapson method. The size of the Newton-Rapson Jacobian matrix is
a) 553 X 553
b) 540 X 540
c) 555 X 555
d) 554 X 554
ans. b

38. If the reference bus is changed in two load flow buses with same system data and power obtained for reference bus taken as specified P and Q in the latter run
a) the system losses will be unchanged but complex bus voltages will change.
b) the system losses will change but complex bus voltages remain unchanged.
c) the system losses as well as complex bus voltages will change.
d) the system losses as well as complex bus voltages will be unchanged.
ans. a

SWITCH GEAR AND PROTECTION 2

2012-10-29T07:17:00.000+05:30

21. A lightning stroke discharges impulse current of 10 kA (peak) on a 400 kV transmission line having surge impedance of 250 ohm. The magnitude of transient over voltage travelling waves in either direction assuming equal distribution from the point of lightning strike will be
a) 1250 kV
b) 1650 kV
c) 2500 kV
d) 2900 kV
ans. a

22. The insulation strength of an EHV transmission line is mainly governed by
a) load power factor
b) switching overvoltages
c) harmonics
d) corona
ans. b

23. Steady-state stability of a power system is the ability of the power system to
a) maintain voltage at the rated voltage level
b) maintain frequency exactly at 50Hz
c) maintain a spinning reserve margin at all times.
d) maintain synchronism between machines and on external tie lines.
ans. d

24. The load carrying capability of a long ac transmission line is
a) always limited by the conductor size
b) limited by stability considerations.
c) reduced at low ambient temperatures.
d) decreased by the use of bundled conductors of single conductors.
ans. b

25. An 800 kV transmission line is having per phase line inductance of 1.1 mH/km and per phase line capacitance of 11.68 nF/km. Ignoring the length of the line , its ideal power transfer capability in MW is
a) 1204 MW
b) 1504 MW
c) 2085 MW
d) 2606 MW
ans. c

26. An 800 kV transmission line has a maximum power transfer capacity of P. If it is operated at 400 kV with the series reactance unchanged, then maximum power transfer capacity is approximately
a) P
b) 2P
c) P/2
d) P/4
ans. d

27. A round rotor generator with internal voltage E1=2.0 pu and X=1.1 pu is connected to a round rotor sysnchronous motor with internal voltage E2= 1.3 pu and X= 1.2 pu. The reactance of the line connecting the generator supplies 0.5 pu power, the rotor angle difference between the machines will be
a) 57.42 degrees
b) 1 degree
c) 32.58 degrees
d) 122.58 degrees
ans. c

28. The pu parameters for a 500 MVA machine on its own base are: inertia , M= 20 pu ; reactance ,X= pu.
The pu values of inertia and reactance on 100 MVA common base, respectively, are
a) 4, 0.4
b) 100, 10
c) 4, 10
d) 100,0.4
ans. d

29. A power station consists of two synchronous A and B of ratings 250 MVA and 500 MVA with inertia 1.6 and 1.0 pu respectively on their own base MVA ratings. The equivalent pu inertia constant for the system on 100 MVA common base is
a) 2.6
b) 6.15
c) 1.625
d) 9
ans. d

30. A shunt reactor of 100 MVAr is operated at 98% of its rated voltage and at 96% of its rated frequency. The reactive power absorbed by the reactor is
a) 98. MVAr
b) 10.402 MVAr
c) 96.04 MVAr
d) 100 MVAr
ans. d

SWITCH GEAR AND PROTECTION 1

2012-10-28T18:41:00.002+05:30

11. In an inverse definite minimum time, electromagnetic type over-current relay , the minimum time feature is achieved because of
a) Saturation of the magnetic circuit
b) proper mechanical design.
c) appropriate time-delay element
d) electromagnetic damping.
ans.a

12. In a biased differential relay the bias is defined as a ratio of
a) number of turns of restraining and operating coil.
b) operating coil current and restraining coil current.
c) fault current and operating coil current.
d) fault current and restraining coil current.
ans. b

13. A 75 MVA , 10 kV synchronous generator has Xd=0.4 pu.The Xd value (in pu) to a base of 100 MVA ,11 kV is
a) 0.578
b) 0.279
c) 0.412
d) 0.44
and d

14. The plug setting of a negative sequence relay is 0.2 A. The current transformer ratio 5:1. The minimum value of line to line fault current for the operation of the relay is
a) 1 A
b) 1/1.732 A
c) 1.732 A
d) 0.2/1.732 A
ans. a

15. The neutral of 10 MVA , 11kV alternator is earthed through a resistance of 5 ohms. The earth fault relay is set to operate at 0.75 A. The CT's have a ratio of 1000/5. What percentage of the alternator winding is protected?
a) 85 %
b) 88.2 %
c) 15 %
d) 11.8 %
ans. b

16. For the protection f transformers, harmonic restraint is used to guard against
a) magnetizing inrush current
b) unbalanced operation
c) lighting
d) switching over voltage
ans. a

17. Voltage phasors at the two terminals of a transmission line of length 70 km have a magnitude of 1.0 per unit but are 180 degrees out of phase. Assuming that the maximum load current in the line is 1/5th of minimum 3-phase fault current. Which one of the following transmission line protection schemes will NOT pick up for this condition?
a) Distance protection using mho relays with zone-1 set 80% of the line impedance.
b) Directional over current protection set to pick up at 1.25 times the maximum load current.
c) Pilot relaying system with directional comparison scheme.
d) Pilot relaying system with segregated phase comparison.
ans. a

18. In a 3-step distance protection , the reach of the three zones of the relay at the beginning of the first line typically extends into
a) 100% of the first line, 50 % of the second line and 20% of the third line.
b) 80 %of the first line, 50 % of the second line and 20% of the third line.
c) 80% of the first line, 50 % of the second line and 10% of the third line.
d) 50% of the first line, 50 % of the second line and 20% of the third line.
ans. b

19. An extra high voltage transmission line of length 300 km can be approximated by a lossless line having propagation constant beta =0.00127 radians per km. Then the percentage ratio of line length to wavelenth will be given by
a) 24.24%
b) 12.12 %
c) 19.05 %
d) 6.06%
ans. d

20. An overheade line having a surge impedance of 400 ohm is connected in series with an underground cable having a surge impedance of 100 ohm. If a surge of 50 kV travels from the line end towards the line-cable junctions, the value of the transmitted voltage wave at the junction is
a) 30 kV
b) 20 kV
c) 80 kV
d) -30 kV
ans. b

SWITCHGEAR AND PROTECTION

2012-10-26T10:34:00.001+05:30

1. For a fault at the terminals of synchronous generator, the fault current is maximum for a
a) 3-phase fault
b) 3-phase to ground fault
c) line-to-ground fault
d) line-to-line fault
ans. c

2. A generator is connected through a 20 MVA , 13.8 /138 kV step down transformer, to transmission line. At the receiving end of the line a load is supplied through a step down transformer of 10 MVA ,138/69 kV rating. A 0.72 pu load, evaluated on load side transformer ratings as base values, is supplied from the above system. For system base values of 10 MVA and 69 kV in load circuit, the value of the load (in per unit) in generator circuit will be
a) 36
b) 1.44
c) 0.72
d) 0.18
ans. a

3. A 3-phase generator rated at 110 MVA , 11 kV is connected through circuit breakers to transformer. The generator is having direct axis sub-transient reactance Xd''=19% ,transient reactance Xd'= 26% and synchronous reactance =130%.The generator is operating at no load and rated voltage when a three phase short circuit fault occurs between the breakers and the transformer. The magnitude of initial symmetrical rms current in the breakers will be
a) 4.44 kA
b) 22.20 kA
c) 30.39 kA
d) 38.45 kA
ans. c

4. The severity of line-to-ground and 3-phase faults at the terminals of an unloaded synchronous generator is to be same. If the terminal voltage is 1.0 pu and Z1=Z2= j 0.1 pu, Z0= j0.05 pu for the alternator, then the required inductive reactance for neutral grounding is
a) 0.0166 pu
b) 0.05 pu
c) 0.1 pu
d) 0.15 pu
ans. b

5. The parameters of a transposed overhead transmission line are given as Self reactance Xs=0.4 ohm/km
and mutual inductance Xm=0.1 ohm/km . The positive sequence reactance X1 and zero sequence reactance respectively , in ohm/km are
a) 0.3, 0.2
b) 0.5,0.2
c) 0.5,0.6
d) 0.3,0.6
ans. d

6. A 500 MVA ,50Hz , 3-phase turbo-generator produces power at 22kv, Generator is Y-connected and its neutral is solidly grounded. Its sequence reactances are X1=X2=0.15 and X0=0.05 pu. It is operating at rated voltage and disconnected from the rest of the system (no load). The magnitude of the sub-transient line current for single line to ground fault at the generator terminal in pu will be
a) 2.851
b) 3.333
c) 6.667
d) 8.553
ans. d

At a 220 kV substation of a power system , it is given that the 3-phase fault level is 4000 MVA and single-line to ground fault level is 5000 MVA. Neglecting the resistance and the shunt susceptances of the system.
7. The positive sequence driving point reactance at the bus is
a) 2.5 ohm
b) 4.033 ohm
c) 5.5 ohm
d) 12.1 ohm
ans. d
8. The zero sequence driving point reactance at the bus is
a) 2.2 ohm
b) 4.84 ohm
c) 18.18 ohm
d) 22.72 ohm
ans. d
9. A 20MVA ,6.6kV , 3-phase alternator is connected toa 3-phase transmission line .The per unit positive-sequence , negative-sequence, and zero-sequence impedences of the alternator are j0.1, j0.1 and j0.04 respectively. The neutral of the alternator is connected to ground through an inductive reactor of j0.05 pu. The per unit positive negative and zero sequence impedences of the tranmission line are j0.1,j0.1 and j0.3 respectively.All per unit values are based on the machine ratings . A solid ground fault occurs at one phase of the far end of the tranmission line. The voltage of the alternator neutral with respect to ground during the fault is
a) 513.8 V
b) 889.9 V
c) 1112.0 V
d) 642.2 V
ans. d

10. Keeping in view the cost and overall effectiveness, the following circuit breaker is best suited for capacitor bank switching
a) vaccum
b) air-blast
c) SF6
d) oil
ans. a

TRANSMISSION AND DISTRIBUTION OF ELECTRICAL POWER 2

2012-10-25T10:45:00.003+05:30

21. A lossless transmission line having Surge Impedance Loading(SIL) of 2280 MW is provided with a uniformly distributed series capacitive compensation of 30% .Then SIL of the compensated transmission line will be
a) 1835 MW
b) 2280 MW
c) 2725 MW
d) 3257 MW
ans. b

22. Out of considerations listed below
1. no distance limitation related to steady-state stability
2. no reactive power requirement from the system at the two terminals
3. no substantial effect on fault level of the two systems at the terminals inspite of the interconnection
4. no corona problems
a) all the above
b) 1 and 3
c) 3 and 4
d) 2 and 4
ans. a

23. In a dc transmission line
a) it is necessary for the sending end and receiving end to be operated in synchronism.
b) the effects of inductive and capacitive reactances are greater than in an ac transmission line of the same rating.
c) there are no effects due to inductive and capacitive reactances
d) power transfer capability is limited by stability considerations.
ans. c

24. HVDC transmission is preferred to EHV AC because
a) HVDC terminal equipment are inexpensive
b) VAR compensation is not required in HVDC system
c) system stability can be improved
d) harmonics problem is avoided
ans. c

25. High Voltage DC (HVDC) transmission is mainly used for
a) bulk power transmission over very long distances.
b) interconnecting two systems with the same nominal frequency .
c) eliminating reactive power requirement in the operation.
d) minimizing harmonics at the converter stations.
ans. a

26. Choose the appropriate auxiliary components of HVDC transmission system from the following
1. DC line inductor .
2. AC line inductor .
3. Reactive power sources.
4. Distance relays on dc line.
5. Series capacitance on AC line.
a) 1 and 2
b) 1 and 3
c) 2 and 4
d) 4 and 5
ans. b

27. Bulk power transmission over long HVDC lines are preferred an account of
a) low cost HVDC terminals.
b) no harmonic problems
c) minimum line power losses
d) simple protection
ans. c

29. An HVDC link consists of rectifier, inverter , transmission line and other equipments. Which one of the following is true for this link?
a) The transmission line produces/supplies reactive power.
b) The rectifier consumes reactive power and the inverter supplies reactive power from /to the respective connected ac systems
c) Rectifier supplies reactive power and the inverter consumes reactive power to/from the respective connected ac systems.
d) Both converters (rectifier and inverter) consume reactive power from the respective connected ac systems.
ans. b

TRANSMISSION AND DISTRIBUTION OF ELECTRICAL POWER 1

2012-10-25T07:11:00.002+05:30

11. The concept of electrically short, medium and long line is primarily based on the
a) nominal voltage of the line
b) physical length of the line
c) wavelength of the line
d) power transmitted over the line
ans. b

12. For a fixed value of complex power flow in a transmission line having a sending end voltage V, the real power loss will be proportional to
a) V
b) V square
c) 1/V square
d) 1/V
ans. b

13. A transmission line has equal voltages at the two ends , maintained constant by two sources. A third source is to be provided to maintain constant voltage (equal to end voltages) at either the mid point of the line or at 755 of the distance from the sending end. Then the maximum power transfer capabilities of the line in the original case and the other two cases respectively will be in the following ratios
a) 1:1:1
b) 1:2:1/0.75
c) 1:2:4
d) 1:4:16
ans. a

14. A single phase transmission line and a telephone line are both symmetrically strung one below the other , in horizontal configurations , on a common tower. The shortest and longest distances between the phase and the telephone conductors are 2.5 m and 3 m respectively. The voltage(volt/km) induced in the telephone circuit , due to 50 Hz current of 100 amps in the power circuit is
a) 4.81
b) 3.56
c) 2.29
d) 1.27
ans. d

15. A long wire composed of a smooth round conductor runs above and parallel to the ground( assumed to be large conducting plane) . A high voltage exists between the conductor and the ground. The maximum electric stress occurs at
a) the upper surface of the conductor
b) the lower surface of the conductor
c) the ground surface
d) midway between the conductor and ground
ans. b

16. The total reactance and total susceptance of a lossless overhead EHV line, operating at 50Hz , are given by 0.045 and 1.2 pu respectively. If the velocity of wave propagation is 3x10^5 km/s, then the approximate length of the line is
a) 122 km
b) 172 km
c) 222 km
d) 272 km
ans. b

17. An extra high voltage transmission line of length 300 km can be approximated by a lossless line having propagation constant Beta= 0.00127 radians per km. The the percentage ratio of line length to wavelength will be given by
a) 24.24 %
b) 12.12 %
c) 19.05 %
d) 6.06 %
ans. d

18. A surge of 20 kV magnitude travels along a lossless cable towards its junction with two identical lossless overhead transmission lines. The inductance and the capacitance of the cable are 0.4 mH and 0.5 microF per km. The inductance and capacitance of the overhead transmission lines are 1.5 mH and 0.015 microF per km. The magnitude of the voltage at the junction due to surge is
a) 36.72 kV
b) 18.36 kV
c) 6.07 kV
d) 33.93 kV
ans. a

19. Serier capacitive compensation in EHV transmission lines is used to
a) reduce the line loading
b) improve the stability of the system
c) reduce the voltage profile
d) improve the protection of the line
ans. b

20. At an industrial substation with a 4 MW load, a capacitor of 2 MVAR is installed to maintain the load power factor at 0.97 lagging. If the capacitor goes out of service , the load power factor becomes
a) 0.85
b) 1.00
c) 0.80 lag
d) 0.90 lag
ans. c

TRANSMISSION AND DISTRIBUTION OF ELECTRICAL POWER

2012-10-25T06:26:00.001+05:30

1.Corona losses are minimized when
a) conductor size is reduced
b) smooth conductor is used
c) sharp points are provided in the line hardware
d) current density in the conductors is reduced.
ans. b

2. Consider a long ,two-wire line composed of solid round conductors. The radius of both conductors is 0.25 cm and the distance between their centres is 1m. If this distance is doubled, then the inductance per unit length
a) doubles
b) halves
c) increases but does not double
d) decreases but does not halve
ans. c

3. A 3-phase transmission line has its conductors at the corners of an equilateral triangle with side 3m . The diameter of each conductor is 1.63 cm. The inductance of the line per phase per km is
a) 1.232 mH
b) 1.182 mH
c) 1.093 mH
d) 1.043 mH
ans. c

4. For single-phase over head line having solid copper conductors of diameter 1 cm spaced 60 cm between centers , the inductance in mH/km is
a) 0.05 + 0.2 log 60
b) 0.2 log 60
c) 0.05 + 0.2 log 60/0.5
d) 0.2 log 60/0.5
ans. c

5. The conductors of a 10 km long , single phase, two wire line are seperated by a distance of 1.5 m.The diameter of each conductor is 1 cm . If the conductors are of coppers, the inductance of the circuit is
a) 50.0 mH
b) 45.3 mH
c) 23.8 mH
d) 19.6 mH
ans. c

6. The inductance of a power transmission line increases with
a) decrease in line length
b) increase in diameter of conductor
c) increase in spacing between the phase conductors
d) increase in load current carried by the conductors
ans. c

7. Bundled conductors are employed to improve
a) appearence of the transmission line
b) mechanical stability of the line
c) current carrying capacity of the line
d) corona performance of the line
ans. d

8. Bundled conductors are mainly used in high voltage overhead transmission lines to
a) reduce transmission line losses
b) increase mechanical strength of he line
c) reduce corona
d) reduce sag
ans. c

9. A 3-phase overhead transmission line has its conductors horizontally spaced with spacing between adjacent conductors equal to'd'. If now the conductors of the line are rearranged to form an equilateral traingle of sides equal to 'd' then
a) average capacitance and inductance will increase
b) average capacitance will decrease and inductance will increase
c) average capacitance will increase and inductance will decrease
d) surge impedance loading of the line increases
ans. c

10.For equilateral spacing of conductors of an untransposed 3-phase line, we have
a) balanced receiving-end voltage and no communication interference
b) unbalancing receiving-end voltage and no communication interference
c) balanced receiving-end voltage and communication interference
d) unbalanced receiving-end voltage and communication interference
ans. d