Code No.:EE603PC

ELECTRICAL AND ELECTRONICS ENGINEERINGSWITCHGEAR AND PROTECTION

UNIT 3I. Choose the correct alternative:1. A large-size alternator is protected against overloads by providing [ ]

a) Over current relays. b) Temperature sensitive relays

c) A combination of (a) and (b). d) A combination of (b) and (c).2. System frequency and system voltage respectively control [ ]

a) Active power, reactive power b) Active power, active powerc) Reactive power, reactive power d) Reactive power, active power

3. Reactance used in case of generator is [ ]a) Synchronous reactance b) Transient reactancec) Sub-transient reactance d) All of these

4. In a generator, 3ɸ, short circuit current is limited by [ ]a) Transient reactance b) Synchronous reactancec) Sub-tranisent reactance d) All of these

5. 3ɸ, short circuit current will be [ ]a) Infinity at infinite time b) Maximum at infinite timec) Zero at infinite time d) 10 mA at infinite time

6. To limit the current chopping in vacuum ,the contact material employed should have the properties of [ ]a) Low conductivity and high vapour pressure b) Low conductivity and low vapour pressure.c) High conductivity and high vapour pressure. d) High conductivity and low vapour pressure.

7. Keeping in view the cost and the overall effectiveness, which of the following circuit breaker is best suited for capacitor bank switching?

[ ]

a) Vacuum circuit breaker b) Air blast CBc) SF6 d) Oil CB

8. For rural electrification in India, which circuit breaker is generally used? [ ]a) Oil b) SF6

c) Vacuum d) Air blast9. If the time of operating of a relay for unity TMS is 10secs., the time of operation for 0.5 TMS will be [ ]

a) 20 secs. b) 5 secsc) 10 secs. d) None of the above

10. Rate of rise of restriking voltage depends [ ]a) Type of circuit breaker. b) Inductance of the system only.c) Capacitance of the system only. d) Inductance and capacitance of the system

II. Fill in the blanks:

1. The most commonly used system for the protection of generator is________________________________.

2. Automatic protection is generally not provided for _______________________________ of an alternator

3. The chief cause of over speed in an alternator is the_________________________________________.

4. Earth relays have ________________________________current settings

5. Overload protection is generally not provided for __________________________________

6. The most important stator winding fault of an alternator is________________________ fault

7. Balanced earth fault protection is generally provided for _________________________generators

8. An earth fault current is generally __________________________________than short circuit current

9. Merz-Price circulating current principle is more suitable for_______________________ than transformers

10. When input to the prime mover of generator fails the alternator runs as a___________________________

UNIT 1I. Choose the correct alternative:1. A circuit breaker is [ ]

a) power factor correcting device b) a device to neutralize the effect of transientsc)  a waveform correcting device d) a current interrupting device.

2. The function of protective relay in a circuit breaker is [ ]

a) to each any stray voltages b) to close the contacts when the actuating quantity reaches a certain predetermined value

c) to limit arcing current during the operation of circuit breaker d) to provide additional safety in the operation of

circuit breaker.3. A fault is more severe from the view point of RRRV if is a [ ]

a) Short line fault. b) Medium length line fault.

c) Long line fault. d) None of these.

4. The fault clearing time of a circuit breaker is usually [ ]

a) few minutes b) few seconds

c) one second d) few cycles of supply voltage

5. Which of the following circuit breakers is preferred for EHT application [ ]

a) Air blast circuit breakers b) Minimum oil circuit breakers

c) Bulk oil circuit breakers d) SF6 circuit breakers.6. t [ ]

a) 100 mm Hg b) 1 kg/cm2

c) 20 to 30 kg/cm2 d) 200 to 300 kg/cm2 

7. While selecting a gas for circuit breaker, the property of gas that should be considered is [ ]

a) high dielectric strength b) non-inflammability

c) non-toxicity d) all of the above

8. The acting contacts for a circuit breakers are made of [ ]a) Stainless steel b) Hard pressed carbonc) Porcelain d) Copper tungsten alloy.

9. Which circuit breaker is generally used in railway traction [ ]

a) SF6 gas circuit breaker b) Air break circuit breaker

c) Vacuum circuit breaker d) Minimum oil circuit breaker10. The heat produced at the contact point, due to passage of current, will least depend on [ ]

a) contact resistance b) time during which the current flows

c) current flowing d) temperature of the surrounding medium.

11 For the contact and their material, which of the following should have low valuea) Contact resistance b) Thermal capacityc) Thermal conductivity d) All of the above.

12  Minimum arcing voltage will be least in case ofa) carbon b) graphitec) tungsten d) Silver.

13 The arc voltage produced in the circuit breaker is alwaysa) in phase with arc current b) leading the arc current by 90°c) Lagging the arc current by 90°. d) All of the above

14 Sparking between contacts can be reduced bya)  inserting resistance in the line b) inserting a capacitor in series with the contactsc) Inserting a capacitor in parallel with the contacts. d) None of the above

15 Sparking occurs when a load is switched off because the circuit has a) high inductance b) high capacitance

c) high resistance. d) None of the above16  If the power factor is zero, the active recovery voltage will be

a) minimum b) 0.5c) 0.707 d) maximum.

17 In a circuit breaker the current which exists at the instant of contact separation is known asa) re striking current b) surge currentc) breaking current d) recovery current

18  Breaking capacity of a circuit breaker is usually expressed in terms ofa) Amperes b) Voltsc) MW d) MVA.

19 In a circuit breaker the lime duration from the instant of the fault to the extinction of arc is known asa) operation time b) total clearing timec) lag time d) lead time

20  For a high speed circuit breaker the total clearing time is nearlya)  1 to 2 cycles b) 5 to 10 cyclesc) 10 to 15 cycles d) less than 50 cycles.

II. Fill in the blanks:

1. A circuit breaker opens_______________________________________when a fault occurs on the system

2. A circuit breaker can _________________________________________the circuit immediately after automatic operation

3. When the contacts of circuit breaker are opened on the occurrence of a fault, an____________is struck

4.An A.C. Circuit is more easily interrupted than a d.c.,circuit because alternating current provides

_________________________

5 If the length of the arc increases its resistance is_______________________________________________

6. If the dielectric strength of the medium between contacts builds up more rapidly than the restriking voltage, then the arc will be__________________________________________________.

7. In an oil circuit breaker___________________________________is used as the arc quenching medium

8.The quantity of oil needed for arc control in oil circuit breakers is ____________________________than

that plain break oil circuit breakers.

9. Current Chopping mainly occurs in____________________________________________________.

10. Capacitive current breaking results in__________________________________________________.

11. In minimum oil circuit breakers ___________________________________is used for insulation purposes

12. Forced blast circuit breakers have________________________________ speed of circuit interruption.

13. Cross blast type air-blast CB is directed at ____________________________to the path of the arc.

14. Since the dielectric strength of SF6 gas is ____________________times that of air, such breakers can interrupt much larger currents.

15. Making capacity=_____________________________*symmetrical breaking capacity

16. ______________________________________is the phenomenon of current interruption before the natural current zero is reached.

17. If L=4mH;C=0.001µF,a current chop of magnitude 50A would induce a voltage of_______________ KV

18. ____________________________________is the normal frequency r.m.s.voltage that appears across the contacts of the circuit breaker after final arc extinction.

19. ___________________________________is the voltage that appears across the contacts of the circuit breaker during the arcing period.

20. ____________________________________is the transient voltage the appear across the contacts at or near current zero during arcing period.

UNIT 2I. Choose the correct alternative:1. The actuating quantity for the relays may be [ ]

a) Magnitude b) Frequencyc) Phase angle d) Any of the above

2. IDMT relays are used to protect the power transformers against [ ]a) External short-circuit b) Over loadsc) Internal short-circuits d) Both A and B

3. In distance protection, the relay measures [ ]

a) Negative sequence impedance of the light from relay up to the fault point b) Positive sequence impedance of the line from relay

up to the fault point

c) Self impedance of the line from relay up to the fault point d) Zero sequence impedance of the line from relay up

to the fault point4. Plug setting of a electromagnetic relay can be altered by varying [ ]

a) Number of ampere turns b) Air gap of magnetic pathc) Adjustable back stop d) None of these

5. The most efficient torque producing actuating structure for the induction type relays is [ ]a) Shaded pole structure b) Watt hour meter structurec) Induction cup structure d) Single induction loop structure

6. What do protective relays provide? [ ]

a) Provide additional safety to the circuit breaker in its operation. b) Close the contacts when the actuating quantity

attains a certain predetermined value.

c) Limit the arcing current during the circuit breaker operation. d) Earth or ground any stray voltage

7. The relay operating speed depends upon [ ]a) Spring tension b) Rate of flux build upc) Armature core air gap d) All of the above

8. An efficient and a well designed protective relaying should have [ ]a) Good selectivity and reliability b) Economy and simplicityc) High speed and selectivity d) All of the above

9. Burden of protective relay is the power [ ]a) Required to operate the circuit breaker b) Absorbed by the circuit of relayc) Developed by the relay circuit d) None of the above

10. Directional relays are based on flow of [ ]a) Power b) Currentc) Voltage wave d) All of the above

11 A differential relay measures the vector difference between a) Two currents b) Two voltages c) Two or more similar electrical quantities d) None of the above

12 Induction cup relay is operated due to changes in a) Current b) Voltageb) Impedance d) All of the above

13 ______________relays are used for phase fault on long transmission linea) Impedance b) Reactancec) Either of the above d) None of the above

14 ______________relays are used for phase fault on short transmission linea) Induction type b) Reactance c) Impedance d) None of the above

15 The relay with inverse time characteristic will operate within a) 1.5Sec b) 5 to 10 Secc) 5 to 20 Sec d) 20 to 30 Sec

16 The IDMT relays are used for over current and earth fault protection of transformers against a) Heavy loads b) Internal short circuitsc) External short circuits d) All of the above

17 Admittance relay is ____________________relaya) Impedance b) Directionalc) Non-directional d) None of the above

18 If the fault occurs near the impedance relay the V/I ratio will be

a) Constant for all distances b) Lower than that of if fault occurs away from the relay

c) higher than that of if fault occurs away from the relay d) None of the above

19 The torque produced in induction type relay(shaded pole structure)is

a) Inversely proportional to current b) Inversely proportional to square of currentc) Proportional to the current d) proportional to square of current

20 Over voltage protection is recommended for a) Hydro-electric generators b) Steam turbine generatorsc) Gas turbine generators d) All of the above

II. Fill in the blanks:

1. A relay performs the function of__________________________________________________________.

2. The relay operating coil is supplied through_________________________________________________.

3. A 1VA relay is ____________________________________sensitive than a 3 VA relay

4. The minimum relay coil current at which the relay operates is called______________________________.

5 Induction relays _______________________________ be used with d.c., quantities

6. An over current relay having a current setting of 125% is connected to a supply circuit through a CT of ratio 400/5A.The pick-up value will be_____________________________A.

7.The pick-up value of a relay is 7.5A and the fault current in the relay coil is 30A.Its Plug-Setting

multiplier is ___________________________________________________.

8. Back up protection functions when ________________________________________________________.

9. Most of the relays on service on electric power system are ___________________________relays

10. Induction relays are used with__________________________________ quantities

11. Back-up protection is generally employed for protection against________________________faults

12. Back-up protection is generally of _____________________________type.

13. _________________________________________is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

14. Pick-up current=

15. Plug-Setting Multiplier=

16. A _________________________ relay is one that operates when the phasor difference of two or more similar electrical quantities exceeds a pre determined value.

17. An impedance is relay is a_______________________________________

18. We do not require any protection against prime mover failure in case of____________________________

19. For the protection of stator winding of an alternate against internal fault involving ground the relay used is a__________________________________________________

20. Purpose of backup protection is______________________________