SWITCH GEAR AND PROTECTION 1

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

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