## electrical

### Switchgear and Protection MCQ Objective Questions and Answers Page - 6

1.
 To protect the power transformer  (Y – Y with neutral grounded)  against fault, what type of connection do the CTs have  ? A $\Delta&space;-Y$ connection B $\Delta&space;-&space;\Delta$   connection C $Y-Y$ connection D $Y-\Delta$ connection Answer : B Explanation :
2.
 In Merz-Price percentage differential protection of a  $\Delta&space;-Y$  transformer, the CT secondaries connection in the primary and secondary winding of the transformer would be in the form of A $\Delta&space;-&space;Y$ B $Y&space;-&space;\Delta$ C $\Delta&space;-&space;\Delta$ D $Y-Y$ Answer : B Explanation :
3.
 To prevent mal operation of differentially connected relay while energising a transformer, the relay restraining coil is biased with A Second harmonic current B Third harmonic current C Fifth harmonic current D Seventh harmonic current Answer : A Explanation :
4.
 The inrush current of a transformer at no load is maximum if the supply voltage is switched on A At peak voltage value. B At zero voltage value. C At half voltage value. D At  0.866  time voltage value. Answer : B Explanation :
5.
 Magnetising inrush phenomena occur in A Large transformer primary only, when the transformer is switched on. B Large transformer primary when a 3-phase fault occurs in the secondary. C Large transformer secondary only when the transformer is switched on in the primary. D None of the above. Answer : A Explanation :
6.
 In case of power transformer  : A Positive, negative and zero-sequence impedances are all equal. B Positive and zero-sequence impedances are equal but less than negative- sequence impedance. C Positive sequence impedance is greater than both negative- and zero- sequence impedances. D Positive sequence impedance is less than negative - sequence impedance but equal to zero- sequence impedance. Answer : A Explanation :
7.
 For the protection of transformers, harmonic restraint is used to guard against A Magnetising inrush current. B Unbalanced operation C Lightning D Switching overvoltage Answer : A Explanation :
8.
 The connections of  CTs  are opposite to that of transformer windings so that current in the pilot wires of two  are/have A Same phase. B Opposite in phase. C A phase difference of 600. D A phase difference of 900. Answer : A Explanation :
9.
 For preventing the maloperation of Merz-Price protection scheme on inrush of magnetizing current A The relay restraining coil is biased with second harmonic current. B Time lag is provided in the relay. C Relay sensitivity is reduced by employing a shunt. D All of the above. Answer : A Explanation :
10.
 Problems associated with differential protection  is/are A Magnetizing inrush current. B Mismatching characteristics of  CTs. C Charge of ratio as a result of tapping. D All of the above. Answer : D Explanation :
11.
 Which of the following sections can be employed for bus-bars  ? A Bars B Rods C Tubes D Any of the above Answer : D Explanation :
12.
 The material used for bus-bars should have A Low resistivity B Low cost C High softening temperature D All of the above Answer : D Explanation :
13.
 A bus-bar is rated by A Current only B Voltage only C Current, voltage and frequency D Current, voltage, frequency and short-circuit current Answer : D Explanation :
14.
 Single bus-bar arrangement A Is cheapest in initial as well as maintenance cost. B Provides simple operation and relaying system C Has the drawback that there will be complete shutdown when fault occurs on the bus itself. D All of the above Answer : D Explanation :
15.
 Double bus-bar arrangement makes use of two identical bus-bars and has the advantage(s) that A It does not require any bus coupler and permits switchover from one bus to another whenever desired. B It provides maximum flexibility and reliability. C Either bus-bar may be taken up for maintenance. D All of the above. Answer : D Explanation :
16.
 A bus coupler circuit breaker is utilised in a substation for A Joining the transmission line with station bus-bar. B Joining main and transfer bus in a substation. C Joining the generator with transfer. D Joining the neutral of the generator with earth. Answer : B Explanation :
17.
 Zero-sequence impedance of the transmission line is A Equal to the positive-sequence impedance of the line. B About 2.5-3 times the positive – sequence impedance of the line. C Less than positive – sequence impedance of the line. D None of the above. Answer : B Explanation :
18.
 Which is the pilotless protection method for feeder line  ? A Differential protection. B Carrier current protection. C Time graded protection. D None of these. Answer : B Explanation :
19.
 In the case of transmission line protection, overcurrent used is  : A Only up to 110 kV B Only up to 220 kV C Only up to 50 kV D Only up to 1100 kV Answer : C Explanation :
20.
 Which is the main relay for protecting up to  90%  of the transmission line-length in the forward direction  ? A Directional overcurrent relay. B Mho relay. C Carrier-current protective relay. D Impedance relay. Answer : B Explanation :
21.
 For complete protection of a 3-phase line A Three-phase and three-earth fault relays are required. B Three-phase and two-earth fault relays are required. C Two-phase and two-earth fault relays are required. D Two-phase and one-earth fault relays are required. Answer : D Explanation :
22.
 How many relay are used to detect interphase fault of a three-line system  ? A 1 B 2 C 3 D 6 Answer : C Explanation :
23.
 The transmission line distance production relay having the property of being inherently directional is A Impedance relay B MHO relay C OHM relay D Reactance relay Answer : B Explanation :
24.
 Time graded protection of a radial feeder can be achieved by using A Definite time relays B Inverse time relays C Both define and inverse time relays D None of the above Answer : C Explanation :
25.
 For protection of parallel feeders fed from one end, the relays required are A Non-directional relays at the source end and directional relays at the load end. B Non-directional relays at both ends. C Directional relays at source end Non-directional relays at load end. D Directional relays at both ends. Answer : A Explanation :
26.
 Pilot-wire protection scheme is most economical and provides high speed relaying for A Short length of lines up to  15 km. B Medium length of lines up to  60 km. C Long length of lines up to  200 km. D All of the above. Answer : A Explanation :
27.
 Adjustable resistors are connected in the pilot wires in order to A Change the phase of the current flowing through the relay. B Get equipotential points on pilot wires. C Reduce current flowing through the relay. D None of the above. Answer : B Explanation :
28.
 Signal mixing transformers are used for A Induction disc type overcurrent relay. B Direction sensitive distance relay. C Pilot-wire feeder protection. D Plain differential relay. Answer : C Explanation :
29.
 In a balanced voltage pilot-wire protection scheme, if the pilot-circuit gets short-circuited, the relay will A Fail to trip on internal faults. B Fail to trip on external faults. C Trip on full-load. D Trip instantaneously. Answer : C Explanation :
30.
 The frequency of carrier in the carrier current pilot scheme is in the range of A 1 kHz  to  10 kHz B 10 kHz  to  25 kHz C 25 kHz  to  50 kHz D 50 kHz  to  500 kHz Answer : D Explanation :
31.
 The frequency of carrier transmitted by microwave pilot is in the range of A 1,000 kHz  to  15,000 kHz B 2,000 kHz  to  5,000 kHz C 900 MHz  to  6,000 MHz D 10,000 MHz to  15,000 MHz Answer : C Explanation :
32.
 In carrier current protection the purpose of the wave-trap is for A Trapping power frequency waves. B Trapping high frequency waves entering into  generators/transformer unit. C Both (a) and (b) D None of the above Answer : B Explanation :
33.
 A line trap in carrier current relaying tuned to carrier frequency presents A High impedance to carrier frequency but low impedance to power frequency. B Low impedance to both carrier and power frequency. C High impedance to both carrier and power frequency. D Low impedance to carrier frequency but high impedance to power frequency. Answer : A Explanation :
34.
 A line trap in a long transmission line is used to A Improve the power factor. B Dampen the overvoltage oscillations. C Confine the carrier signals in the line. D Product the line against direct lightning stroke. Answer : C Explanation :
35.
 Distance protection scheme is preferred over graded time-lag over-current protection in HV and EHV lines because A It is faster in operation. B It is simple. C It is cheaper in cost. D All of the above. Answer : A Explanation :
36.
 Three step time-distance characteristic of distance relay can be had by A Changing taps on voltage transformer. B Separate measuring elements for zones 2 and 3. C Switching resistance in relay restraint circuit at pre-set time intervals by means of timer element. D Any of the above. Answer : D Explanation :
37.
 Lightning is a huge spark caused by electrical discharge taking place between A Clouds B Within the same cloud C Cloud and earth D Any of the above Answer : D Explanation :
38.
 Switching over-voltages are more hazardous than lightning surges in case of A Low voltage system B 33 kV system C EHV and UHV systems D All of these Answer : C Explanation :
39.
 The over-voltage surges in power system may be caused by A Lightning B Resonance C Switching D All of these Answer : D Explanation :
40.
 Requirements of protection power station buildings against direct stokes are A Interception B Conduction C Dissipation D All of these Answer : D Explanation :
41.
 The line insulation is _____________ the insulation level of the station equipment A Less than B Same as C More than D Not directly related with Answer : D Explanation :
42.
 When a wave propagates on a transmission line, it suffers reflection several times at A Tapping B Load end C Sending end D Sending another end Answer : D Explanation :
43.
 Which of the following statements is incorrect? A Station batteries are used to operate relay only B The lightning arresters are basically surge diverters C An impedance relay has maximum fault current when fault occurs near the relay D A high-speed relay has an operation of 1 to 2 cycles Answer : A Explanation :
44.
 Discrimination between main and back up protection is provided by the use of relays which are A Fact B Sensitive C Slow D None of the above Answer : C Explanation :
45.
 Induction cup relay is operated due to changes in A Current B Voltage C Impedance D All of the above Answer : D Explanation :
46.
 AC network analyser is used to solve problems of A Load flow B Load flow and short-circuit C Load flow and stability D Load flow, short-circuit and stability Answer : D Explanation :
47.
 Which of the following statements is incorrect? A Lightning arrestors are used before the switchgear B Shunt reactors are used as compensation reactors C The peak short current is (18 × $\sqrt{2}$) times the AC component D The MVA at fault is equal to base MVA divided by per unit equivalent fault reactance Answer : A Explanation :
48.
 Short-circuit currents are due to A Single phase to ground faults B Phase to phase faults C Two phases to ground faults D Any of these Answer : D Explanation :
49.
 To reduce short circuit fault currents ______________  are used. A Reactors B Resistors C Capacitors D None of the above Answer : A Explanation :