Category: Power Systems Objective Questions

Power Systems Multiple Choice Questions on “Geometric Mean Distance and Geometric Mean Radius – 1”.

1. Which of the system would require more insulation, a 400 kV ac line or 400 kV dc line?
A. 400 kV ac
B. 400 kV dc
C. Both will need same level of insulation
D. Data is insufficient to decide

Answer: A
Clarification: In ac system, the peak value attained will be 400*√2= 564 kV.

2. The amount of dc compensation needed by a 400 kV dc line for transmitting the data over 300 km having resistance as 5 ohms?
A. 400 var
B. 564 var
C. 600 VAR
D. None of the mentioned

Answer: A
Clarification: A dc line does not require any reactive compensation.

3. Steady state problem is not very serious problem in dc transmission line system due to which of the following systems?
A. No reactive compensation
B. Only resistance drop
C. Stable voltage regulation
D. No reactive compensation, resistance drop, voltage regulation

Answer: A
Clarification: Due to all the above reasons, mainly the reactive power compensation there is very less variation in stability.

4. DC line transmission can be used to provide _______ link.
A. asynchronous
B. synchronous
C. lossless
D. none of the mentioned

Answer: A
Clarification: As dc link provides to connect to different frequencies of the operating buses. It can transfer power frequency independent.

5. (i) Low short circuit current is required in dc system
(ii) Fast control of the converter switches cannot effectively control the damping of the oscillations.
Choose the most appropriate.
A. (i) and (ii) both are true
B. Only (ii) is false
C. Only (i) is false
D. Both statements are false

Answer: B
Clarification: It’s an advantage the using converters in dc link can control the fast damping of the oscillations which occur in ac systems.

6. The reactive power demand of the converters in HVDC can be provided by ___________
A. AC filters
B. Shunt Capacitors
C. SVC
D. AC Filters, Shunt Capacitors and SVC

Answer: D
Clarification: Reactive power demand by the HVDC system can be fulfilled by all the mentioned equipment’s.

7. As compared to the HVDC line, the corona and radio interference in a HVAC line are ____
A. more
B. lesser
C. same
D. cannot be determined

Answer: A
Clarification: The corona and radio interference losses are more in ac system.

8. What of the below mentioned statements are incorrect as compared to the HVDC system?
A. Back to back connection is possible
B. More corona losses
C. Distance limitation
D. Extra reactive power compensation

Answer: B
Clarification: Out of all the corona losses are less in HVDC system.

9. It is required to implement the HVDC system using the thyristors in large numbers. Why it becomes necessary to use a large number of thyristor switches?
A. Due to low voltage rating of thyristor switches
B. Due to low current rating of thyristor switches
C. Due to lack of response for open circuit fault
D. Cannot be determined

Answer: A
Clarification: Thyristors have low voltage rating and to achieve high voltage, a series of switches that have to used.

10. The corona loss on a particular system at 50 Hz is 1 kW/phase per km. The corona loss on the same system in case of DC transmission will be ____________
A. Zero
B. 0.5 kW/phase/km
C. 0.33 kW/phase/km
D. 0.6 kW/phase/km

Power Systems Question Bank on “Comparison of Faults in Three Phase System – 2”.

1. The zero sequence depiction of the unbalancing occurring at the terminals of the induction motor will be most likely _____________

Answer: A
Clarification: The zero sequence components will not have any phase displacement and they are co- phasal in nature.

2. Complete the given phasor diagram by assuming that operator ‘a’ is unity magnitude and counter clockwise rotation of 120 degrees.
A. 1-a
B. a-1
C. 1
D. –a³

Answer: A
Clarification: Based on the phasor calculations the unknown vector is found using balanced condition of the three vectors A+B+C = 0 and 1+a+a²=0. Then the third vector is 1-a.

3. The unknown vector in the given figure is ______ if we assume the system is balanced with a as unity magnitude and counter clockwise rotation of 120 degrees.
A. a
B. 0
C. a³
D. a⁴

Answer: A
Clarification: 1+a+ a² = 0.

4. For a balanced three phase system having a vector representation as mentioned in the figure, complete the vector X and Y.
A. X=1-a² ; Y=a²-a
B. bX=a² -a; Y=1-a²
C. X=a² -1; Y=1-a
D. X=a² -1; Y=a-1

Answer: A
Clarification: This question can be attempted by using the options. For a balanced system the net voltage will be zero. So option a and b are the shortlisted options. Out of these applying the rotation criteria, option a is the best match.

5. For a balanced three phase system having a vector representation as mentioned in the figure, complete the vector X and Y.
A. X=√3∠30° ; Y = √3∠-90°
B. X=√3∠-30° ; Y = √3∠90°
C. X=√3∠-30° ; Y = √3∠-150°
D. X=√3∠30° ; Y = √3∠150°

Answer: A
Clarification: This question can be attempted by using the options. For a balanced system the net voltage will be zero. So option a and b are the shortlisted options. Out of these applying the rotation criteria, option a is the best match in polar form.

6. The unknown vector in the given figure is ______ if we assume the system is balanced with a as unity magnitude and counter clockwise rotation of 120 degrees.
A. 1∠120°
B. 1∠30°
C. 1∠0°
D. 1∠-120°

Answer: A
Clarification: 1+a+ a² = 0; a= 1∠120°.

7. Complete the given phasor diagram by assuming that operator ‘a’ is unity magnitude and counter clockwise rotation of 120 degrees.
A. √3∠-30°
B. √3∠30°
C. √3∠-60°
D. √3∠90°

Answer: A
Clarification: Based on the phasor calculations the unknown vector is found using balanced condition of the three vectors A+B+C = 0 and 1+a+a²=0. Then the third vector is 1-a. Converting it in the polar form, it is √3∠-30°.

8. In the cylindrical rotor alternator, the sub transient and negative sequence reactances are same.
A. True
B. False

Answer: A
Clarification: It is true because the saliency offered is zero in the cylindrical pole machines.

9. The zero sequence impedance of a synchronous machine is independent of the pitch of the armature coils.
A. True
B. False

Answer: B
Clarification: The zero sequence impedance does depend on the pitch of the armature coils of the synchronous machine.

Power Systems Multiple Choice Questions on “Fuel Combustion and Combustion Equipments”.

1. Which of the following is not an advantage of stoker firing?
A. Zero capital investment
B. Greater combustion capacity
C. Uniform feeding of coal into furnance
D. Saving in labour cost
Answer: A
Clarification: Stokers are the mechanical arrangement which is used to feed coals into the furnace. Due to the mechanical equipment combustion capacity increases with uniform feeding of coal of into furnace, also labour cost is reduced. Stoker firing requires installation of mechanical stoker hence, capital investment requires.

2. In case of underfeed stokers fuel is supplied in the furnace _________
A. below the point of air admission
B. above the point of air admission
C. beside the point of air admission
D. by shovels
Answer: A
Clarification: Mechanical stokers are of two types namely underfeed stokers and overfeed stokers. The two differ in manner of feeding of coal below or above the level at which primary air is admitted in the furnace. In case of underfeed stokers fuel is supplied into the furnace below the point of air admission.

3. Which of the following is most advantageous and most widely method of solid fuel firing?
A. Stoker firing
B. Underfeed firing
C. Spreader stoker firing
D. Pulverised fuel firing
Answer: D
Clarification: The advantages of using pulverised fuel outweigh the disadvantages and all the modern power plants uses pulverised fuel firing method. In pulverised fuel firing better combustion is achieved. Pulverization is a means of exposing a large surface area to the action of oxygen and consequently helping the combustion.

4. What is the main disadvantage of spreader stoker?
A. Poor control
B. Difficult operation
C. More unwanted fly ash
D. Dangerous operation
Answer: C
Clarification: In spreader stoker the coal is not fed into the furnace by means of grate. Coal is fed by means of hopper, and function of grade is only to collect and remove the ash out of furnace. Spreader stokers can burn any type of coal from lignite to samianthracite, whether they are free burning or cocking. Their disadvantage is that fly ash is more.

5. Which do you mean by pulverization?
A. Burning of crushed coal
B. Burning of uncrushed coal
C. Crushing of coal into small partials
D. Breaking coal into small particsl
Answer: C
Clarification: Pulverization is crushing of coal into fine particles. This exposes large surface area to the action of oxygen and consequently helps the combustion. Solids fuels can be used in a powderd and burn like oil and gas.

6. What is the amount of air required to burn the pulverised coal?
A. Less than air required to burn lumped coal
B. More than air required to burn lumped coal
C. More than air required to burn lumped coal
D. No air required to burn the lapped coal
Answer: A
Clarification: The surface area of pulverised coal is increased in almost the ratio of 400:1, therefore high rates of combustion are possible. Moreover a smaller quantity of air is required than when the fuel is burnt in lumped form.

7. Capital cost of pulverised fuel firing is less than hand firing.
A. True
B. False
Answer: B
Clarification: Hand firing does not require any capital investment. For pulverised fuel firing separate coal preparation plant is required which makes the installation expensive.

8. What is the main requirement of good coal burner?
A. Minimum operating and maintenance cost
B. Production of uniform flame with complete combustion
C. The burner should be easy to handle and control
D. Attainment of proper designed
Answer: B
Clarification: Main requirement of good coal burner is complete combustion with production of uniform flame. Minimum operating and maintenance cost, simplicity in operation, attainment of proper design all are requirements of complete combustion.

9. Coal burners are employed to fire the pulverised coal along with primary air in the furnace.
A. True
B. False
Answer: A
Clarification: Coal burner’s fire pulverised coal with stream of primary air into the furnace. Secondary air is then admitted separately below the burner, around the burner or elsewhere.

10. Spread stockers can be employed for boiler capacitors of _________
A. 1000 kg/hr – 14000 kg/hr
B. 586 kg/hr – 1086 kg/hr
C. 1500 kg/hr – 40000 kg/hr
D. 70,000 kg/hr – 140,000kg/hr
Answer: D
Clarification: In spreader stoker coal is fed from the coal hopper into the path of the rotor by means of a conveyor and rotor throws the coal into the furnace. Grate collects the ash and moves it out of the furnace.

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Power Systems Quiz on “Geometric Mean Distance and Geometric Mean Radius – 2”.

1. The power transmission capability of bipolar lines is approximately ____________
A. same as that of 3-ph single circuit line
B. same as that of 3-ph double circuit line
C. twice to that of 3-ph single circuit line
D. half to that of 3-ph single circuit line

Answer: A
Clarification: Power transmission by bipolar dc line is same as that three phase single line transmission.

2. HVDC system has charging current but no skin effect while transferring the power through it.
A. True
B. False

Answer: B
Clarification: Skin effect as well as the charging effect, both are absent in dc transmission system.

3. As depicted in the figure, the power will be transferred in the system from A to B by a HVDC link as shown in the figure. The voltages are indicated in the figure and I >0, then _______
A. Vab<0, Vcd<0, Vab>Vcd
B. Vab<0, Vcd>0, Vab<Vcd
C. Vab>0, Vcd<0, Vab>Vcd
D. Vab>0, Vcd>0, Vab>Vcd

Answer: D
Clarification: The given situation is such as the current is more than zero, so a positive current will flow from higher voltage to lower voltage.

4. As depicted in the figure, the power is transferred in the system from B to A by a HVDC link as shown in the figure. The voltages are indicated in the figure and I <0, then ______
A. Vab<0, Vcd<0, Vab>Vcd
B. Vab<0, Vcd>0, Vab<Vcd
C. Vab>0, Vcd>0, Vab<Vcd
D. Vab>0, Vcd>0, Vab>Vcd

Answer: C
Clarification: The given situation is such as the current is less than zero, so a positive current will flow from higher voltage to lower voltage i.e. from cd to ab.

5. A dc line carries as compared to ac line _____________
A. more power
B. less power
C. same power
D. can’t be decided

Answer: A
Clarification: A dc line carries more power as compared to ac line.

6. Back to back HVDC is used to ___________
A. increase the transmission capability
B. decrease line losses
C. provide a stable connection
D. reduce the voltage drop

Answer: C
Clarification: Back to Back connection in HVDC will provide a stable connection.

7. HVDC transmission would require which of the following equipment’s?
(i) Pulse converter
(ii) AC filter
(iii) DC filter
(iv) DC generator
A. (i)
B. (i), (ii)
C. (ii), (iv)
D. (i), (iii)

Answer: A
Clarification: Only pulse converter is needed by HVDC transmission system.

8. As compared with the HVAC lines, dc transmission system is free from ___________
A. inductance
B. capacitance
C. phase displacement
D. all of the mentioned

Answer: D
Clarification: All the factors which involve frequency will be absent in dc system as in f= 0 in dc.

9. Mostly the high voltage transmission is provided by overhead lines due to _____________
A. low cost
B. low losses
C. easy installation
D. all of the mentioned

Answer: A
Clarification: Due to economic criteria, the high voltage transmission is done by overhead lines.

10. Consider the below statements and choose the most appropriate.
(i) Guy wire is galvanised.
(ii) Guy wire should possess high conductivity.
(iii) At installation it makes an angle of 40-60 degrees with earth.
A. (i), (ii) and (iii) are true
B. Only (i) and (ii) are true
C. (ii) and (iii) are true
D. (i) and (iii) are true

Answer: A
Clarification: All the statements about the guy wire are true.