Deck 5: Electrical Protection and Safety: Part B

Question 1

For the protection of a large squirrel cage induction motor against single phase normally

A)An over-current relay is used
B)Differential relay is used
C)Directional relay is used
D)Negative sequence current sensitive relay is used

Answer 1

Negative sequence current sensitive relay is used

Question 2

The most commonly used method for the protection of three phase feeder is

A)Time graded protection
B)Differential protection
C)Reverse power protection
D)None of the above

Answer 2

Differential protection

Question 3

Which is the pilotless protection method for feeder line ?

A)Differential protection
B)Carrier current protection
C)Time credit protection
D)None of the above

Answer 3

Carrier current protection

Question 4

Time graded protection of a radial feeder can be achieved by using

A)Definite time relay
B)Inverse time relay
C)Both definite and inverse time relays
D)None of the above

Question 5

Pilot-wire protection scheme is the most economical and provides high speed relaying for

A)Short length of lines upto 15 km
B)Medium length of lives up to 60 km
C)Long length of lines up to 200 km
D)All of the above

Question 6

Problems associated with differential protection is/are

A)Magnetising inrush current
B)Mismatching characteristics of CTs
C)Charge of ratio as a result of tapping
D)All of the above

Question 7

For the protection of transformers, harmonic restraint is used to guard against

A)Magnetising inrush current
B)Unbalanced operation
C)Lightning
D)Switching over-voltage

Question 8

The types of faults that occur in electric motors are

A)Stator
B)Rotor
C)Overload
D)All of the above

Question 9

The main function of under voltage protective device generally employed with a motor starter is to

A)Open the supply circuit on failure of power supply
B)Control the motor voltage
C)Prevent the opening of supply circuit
D)None of the above

Question 10

The bias factor S in unit protection synchronous generators

A)Lies between .05 and .1 pu
B)Is less than .05 pu
C)Lies between .1 to .25 pu
D)Is greater than .025 pu

Question 11

In a balanced voltage pilot wire protection scheme if the pilot circuit gets opened, the relay will

A)Fail to trip on internal faults
B)Trip on full load
C)Trip instantaneously on external faults
D)None of the above

Question 12

In a balanced voltage pilot-wire protection scheme, if the pilot circuit gets shortcircuited, 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

Question 13

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

Question 14

The frequency of carrier transmitted by microwave pilot is in the range of

A)1000 kHz to 1500 kHz
B)2000 kHz to 5000 kHz
C)900 kHz to 6000 kHz
D)10000 kHz to 15000 kHz

Question 15

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

Question 16

As soon as a fault develops in a generator stator, it is essential to suppress field excitation, otherwise

A)Terminal voltage will drop
B)It may lead to loss of synchronism
C)It will continue to supply power to the stator winding fault
D)All of the above

Question 17

Distance protection scheme is preferred over graded lime-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

Question 18

Requirements of protection power station buildings against direct strokes are

A)Interception
B)Conduction
C). Dissipation
D)All of the above

Question 19

The protection against direct lighting strokes and a high-voltage steep waves is provided by

A)Ground wires
B)Lightning arresters
C)Lightning arresters and ground wires
D)Earthing of neutral

Question 20

For protection of rotating machines against lightning surge .............is used

A)Lightning arrester
B)Capacitor
C)Combination of lightning arrester and a capacitor
D)Lightning conductor and arrester

Question 21

In between the generating station and consumers a number of transformation and switching stations exist, these are called the

A)Switchgears
B)Substations
C)Intermediate substations
D)Transformation stations

Question 22

With large reactance interconnector between two power stations

A)The power stations may fall out of step due to large angular displacement between the power stations
B)Power will be transferred with minimum power losses
C)Power will be transferred with voltage fluctuations and noise
D)None of the above

Question 23

In a isolated neutral system, when a single line to ground fault occurs

A)Persistent arcing grounds will be developed
B)Voltage in the healthy phases rise to full line value causing isolation breakdown
C)The capacitive current in the faulty phase rises to 3 times its normal value
D)All of the above

Question 24

Tap changing transformers are employed for

A)Stepping down the voltage
B)Stepping up the voltage
C)Supplying low-voltage current to instruments
D)Both stepping up and stepping down the voltage

Question 25

Field failure in an alternator occurs due to

A)Failure of exciter
B)Faulty field circuit breaker
C)In rush of large currents
D)Both A and B