Power Systems MCQ Set 1
1. Transmission efficiency of a transmission line increases with the _______
a) decrease in power factor and voltage
b) increase in power factor and voltage
c) increase in power factor but the decrease in voltage
d) increase in voltage only power factor remains constant
Answer
Answer: b [Reason:] Increasing the power factor and voltage reduces the losses and hence improves the efficiency.
2. Surge impedance of a transmission line is the impedance at the time of breakdown of voltage.
a) True
b) False
Answer
Answer: b [Reason:] Surge impedance is the characteristic impedance of a TL.
3. When the transmission line is lousy, then its characteristic impedance will not depend on which of the following?
a) Length
b) Operating frequency
c) Sub synchronous frequency
d) Length and Sub synchronous frequency
Answer
Answer: d [Reason:] For a lousy line, the characteristic impedance will be independent of its sub synchronous frequency and length.
4. If the transmission line is lossless, then its characteristic will be ___
a) √(L/C)
b) √LC
c) √(L+C)
d) √(C/L)
Answer
Answer: a [Reason:] The characteristic impedance for the lossless lines is given by √(L/C) because the resistance is zero.
5. The surge impedance of multiple conductor lines as compared to single line is _______
a) higher
b) lower
c) same
d) length dependent
Answer
Answer: b [Reason:] Due to the reduced GMD, the impedance decreases and so the surge impedance.
6. When ______ then the transmission line will act as a distortion less line.
a) RC = GL
b) RG = LC
c) R = G
d) Cannot be determined
Answer
Answer: a [Reason:] When both the time constants become same, then the transmission will behave like distortion less line.
7. It is intended to increase the power despatch of the existing transmission line. Then what can be the most appropriate solution for improving it?
a) Installing series capacitors
b) Installing shunt capacitor
c) Installing shunt reactor
d) Installing series reactor
Answer
Answer: a [Reason:] Series capacitors reduce the power as 
8. Which of the following situations is very useless when the series capacitors to the transmission line are of little use?
a) When the VAR requirement is small
b) When the VAR requirement is large
c) When the VAR requirement is varying
d) Cannot be determined
Answer
Answer: a [Reason:] Because capacitance is usually neglected for a small transmission line.
9. Reactive power requirement of a power transmission system depends on __
a) Power angle δ
b) |Vs|-|Vr|
c) Vs
d) Vr
Answer
Answer: b
10. When do we use conduit pipe in the transmission of power?
a) Unsheathed cables
b) Armoured cables
c) PVC sheathed cables
d) All of the mentioned
Answer
Answer: a [Reason:] It is used in protection of wires and cables for underground transmission.
Power Systems MCQ Set 2
1. The synchronous machine is operating at a normal condition at the generating system, then the angle between rotor axis and the stator magnetic axis is _______
a) Load angle
b) Power factor angle
c) Impedance angle
d) None of the mentioned
Answer
Answer: a [Reason:]
2. The angle shown in the figure depends on which of the following given options?
a) Loading of the system
b) Power factor
c) Operating frequency
d) Power factor, operating frequency, loading of the system
Answer
Answer: a [Reason:] Load angle depends on the loading of the system, as the load in added or decreased, the synchronous machine will accelerate or decelerate with respect to stator field.
3. Swing equation is very famous for describing the relation between the _____
a) rotor motion and stator field
b) stator and rotor field
c) load angle and rotor field
d) moment of inertia and rotor motion
Answer
Answer: a [Reason:] Swing equation describes the relative motion between rotor load angle with respect to stator field as a function of time.
4. Swing equation which describes the relative motion of the rotor of the machine, is a ____
a) linear and second-order differential equation
b) non linear and second-order differential equation
c) non linear and first-order differential equation
d) non linear and hyperbolic equation
Answer
Answer: b
Hence it is a non linear second order differential equation.
5. A variation in the power transfer had been observed at the generating station while power exchange process due to ______ and is known as ______
a) swing of the machine, inertial response
b) sub synchronism, hunting
c) sub synchronism, inertial response
d) swing of the machine, falling out of the rotor
Answer
Answer: a [Reason:] The power exchange between the mechanical rotor and the electrical grid due to the swing of the rotor is called inertial response.
6. Why the angle δ, rotor angle is famously called as the load angle in the equation of rotor dynamics?
a) Due to power exchange is a function of δ
b) Because it dictates the reactive power compensation
c) Due to load variations
d) No dependency on the load dynamics
Answer
Answer: a [Reason:] Rotor angle is the emf angle and power is a function of it.
7. The transient stability and the steady state stability of a power transmission system ________
a) maintain synchronism between machine and tie lines at all the circumstances
b) maintains frequency exactly at 50 Hz
c) maintain voltage regulation precisely
d) all of the mentioned
Answer
Answer: a [Reason:] Transient stability and steady state stability maintain synchronism between the generating station and tie line.
8. We can say a system is stable at steady state is not affected by ______
a) line losses
b) line reactance
c) generator reactance
d) excitation of alternators
Answer
Answer: a [Reason:] Line losses do not affect the stability limit of the system.
9. The transient instabilities can occur in a system by _____
a) Sudden load changes
b) Switching operation
c) Faults
d) Suddenn load changes, switching operation and faults
Answer
Answer: d [Reason:] All the mentioned factors affect the transient stability of a system.
10. Post a three phase fault that occurs in a system, then its transient stability can be examined using _____
a) solution of swing equation
b) equal area criterion
c) solution of swing equation and equal area criterion
d) solution of swing equation or equal area criterion
Answer
Answer: a [Reason:] Swing equation provides the rotor dynamics at the transient conditions.
Power Systems MCQ Set 3
1. Which part of thermal power plant causes maximum energy losses?
a) Boiler
b) Alternator
c) Condenser
d) Ash and unburnt carbon
Answer
Answer: c [Reason:] About 54% of energy losses occurs in condenser. Losses in boiler and alternator are about 1% and 16% because of such high losses overall efficiency of thermal power plant reduces to 29% for normal old thermal power plant and 50% for modern super critical pressure steam power plant which employs many heat saving devices.
2. What is the effect of increasing steam pressure on efficiency of steam power plant?
a) Increases linearly
b) Increases nonlinearly
c) Decreases linearly
d) Does not changes
Answer
Answer: b [Reason:] The thermal efficiency of steam power plant depends upon the steam pressure. The efficiency increases with increasing steam pressure nonlinearly up to a level. After that level increase in efficiency becomes very low with respect to increasing pressure.
3. What is the effect of increasing steam temperature of thermal power plant on its thermal efficiency?
a) Increases linearly
b) Decreases
c) It does not depends on temperature
d) Increases nonlinearly
Answer
Answer: a [Reason:] Thermal efficiency of steam power plant increases with increase in steam temperature linearly. Increas in temperature of steam also causes its effect on cost. So temperature of steam is increased only upto a level at which it is economical.
4. Overall efficiency of thermal power plant is equal to _____
a) Thermal efficiency
b) Generation efficiency
c) Multiplication of thermal and generation efficiency
d) Ratio of thermal and generation efficiency
Answer
Answer: c [Reason:] Thermal efficiency of steam power plant is the ratio of the heat equivalent of mechanical energy transmitted to the turbine shaft and the heat of the combustion. Thermal efficiency is generally quite low about 30%. Efficiency of generator is called generation efficiency.
5. In supercritical pressure steam power plant temperature of steam is ____
a) 480°C
b) 240°C
c) 600°C
d) 150°C
Answer
Answer: c [Reason:] At a temperature about 600°C water enters in supercritical phase and has property between those of liquid and gas. advantage of such plant are that low grade fossil fuel (lignite) can be used. Efficiency of such plants may be reached up to 50%.
6. For increasing the thermal efficiency of thermal power plant pressure in condenser should be:
a) Reduced upto 0.04 kg/cm3
b) Kept above 2.45 kg/cm3
c) As high as boiler pressure
d) Same as atmospheric pressure
Answer
Answer: a [Reason:] The thermal efficiency can be effectively increased by decreasing the pressure in the condenser. Pressure in condenser is kept very low usually 0.04 kg/cm2 .
7. Economisers are necessary to use for pressure of 70 kg/cm2 or more.
a) True
b) False
Answer
Answer: a [Reason:] High cost of fuel consumption, high load factor and high pressure and temperature conditions, all justify the use of economiser. Installation of economiser involves extra cost of installation, maintenance and regular cleaning and additional requirement of space. By considering above points it has been found that use of economiser becomes necessary for pressure above 70 kg/cm2.
8. Supercritical pressure boiler scan produce steam at the rate of ___
a) less than 1,500 kg/hr
b) less than 340,000 kg/hr
c) less than 80,000 kg/hr
d) upto 1,000,000 kg/hr
Answer
Answer: d [Reason:] Modern supercritical pressure boiler with pressures as high as 350 atm temperature 600°C and capacity 1,000,000 kg/hour are available now days. Fire tube boiler can produce steam at the rate of 15,000 kg/hr. Water tube boiler can produce 340,000 kg of steam per hour.
Power Systems MCQ Set 4
1. Shunt admittance of a transmission line comprises of ______
a) capacitance with conductance
b) capacitance with inductance
c) resistance with inductance
d) capacitance with susceptance
Answer
Answer: a [Reason:] Shunt admittance is capacitance with conductance in the branch.
2. Conductance is the result of the leakage over the surface of insulators.
a) True
b) False
Answer
Answer: a [Reason:] Conductance comes into the picture due to the leakage current over the conductor’s surface.
3. Charging current in the transmission line is ___
a) drawn at open as well as closed end of the transmission line
b) drawn at closed end of the transmission line
c) drawn at open end of the transmission line
d) drawn at the shunt capacitance at closed end of transmission line.
Answer
Answer: a [Reason:] Charging current in transmission line is drawn even when the far end of the TL is open.
4. A three phase 50 Hz transmission line has flat horizontal spacing with 3.5 m between adjacent conductors. The conductors are No. 2/0 hard drawn copper (outside diameter = 1.05cm). The voltage f the line is 110 kV. The capacitance to neutral per kilometre of line is
a) 0.00826 μF/km
b) 0.0826 μF/km
c) 0.01652 μF/km
d) 0.1652 μF/km
Answer
Answer: a [Reason:]
5. A three phase 50 Hz transmission line has flat horizontal spacing with 3.5 m between adjacent conductors. The conductors are No. 2/0 hard drawn copper (outside diameter = 1.05cm). The voltage f the line is 110 kV. The capacitive reactance to neutral per kilometre of line is ______
a) 0.384 x 106 Ω/km to neutral
b) 0.768 x 106 Ω/km to neutral
c) 0.284 x 106 Ω/km to neutral
d) 0.642 x 106 Ω/km to neutral
Answer
Answer: a [Reason:]
6. A three phase 50 Hz transmission line has flat horizontal spacing with 3.5 m between adjacent conductors. The conductors are No. 2/0 hard drawn copper (outside diameter = 1.05cm). The voltage f the line is 110 kV. The line charging current per km of the line is ___
a) 0.17 A/km
b) 0.34 A/km
c) 0.24 A/km
d) 0.48 A/km
Answer
Answer: a [Reason:]
7. A three phase 50 Hz transmission line has flat horizontal spacing with 3.5 m between adjacent conductors. The conductors are No. 2/0 hard drawn copper (outside diameter = 1.05cm). The voltage f the line is 110 kV. The line charging current per conductor of the line is _______
a) 0.085 A/km
b) 0.17 A/km
c) 0.12 A/km
d) 0.24 A/km
Answer
Answer: a [Reason:]
Cn = 0.00826 μF/km
Xn = 1/ωCn
Xn = 1/(100π* .284 *10-6)
Xn = 0.384 x 106 Ω/km to neutral
Charging current = Vn/Xn
Charging current = (110/√3)*1000/(0.384 *106)
Charging current = 0.17 A/km
Charging current per conductor = I/2
Charging current per conductor = 0.17/2
Charging current per conductor = 0.085 A/km.
8. A three phase 50 Hz transmission line has flat horizontal spacing with 3.5 m between adjacent conductors. The conductors are No. 2/0 hard drawn copper (outside diameter = 1.05cm). The voltage f the line is 110 kV. The reactive volt amperes generated by the line is
a) 31.4 kVAR
b) 62.8 kVAR
c) 42 kVAR
d) 56 kVAR
Answer
Answer: a [Reason:]
Cn = 0.00826 μF/km
Xn = 1/ωCn
Xn = 1/(100π* .284 *10-6)
Xn = 0.384 x 106 Ω/km to neutral
Reactive volt amperes generated by the line = (VL)2/Xc
Reactive volt amperes generated by the line = 31.4 kVAR.
Power Systems MCQ Set 5
1. Which element of hydroelectric power plant prevents the penstock from water hammer phenomenon?
a) Valves and Gates
b) Draft tubes
c) Spillway
d) Surge Tank
Answer
Answer: d [Reason:] Sudden increase in water pressure in penstock due to closing of gates is called water hammer. Surge tank is a tank at sufficient height , connected to penstock through riser pipe. It takes the rejected water and relives the penstock from excessive water hammer pressure.
2. Dam having very wide base as compared to its height is called ___
a) buttress dam
b) arch dam
c) earth dam
d) solid gravity dam
Answer
Answer: c [Reason:] Buttress dams are the concrete dams supported on downstream side by buttresses. Arc dams are concrete dams curved from upstream side. Earth dam is a type of embankment dam and have rock filled inside the structure.
3. Spillway discharges the overflow water to the downstream side when the reservoir is full.
a) True
b) False
Answer
Answer: a [Reason:] A condition may arise during flood periods when water level increases beyond the capacity of reservoir. In such conditions spillway acts as safety valve.
4. Trash racks are built for _____
a) discharging the water freely from the turbine exit to tailrace
b) preventing the turbine from ingress of floating and other materials
c) creating artificial head to store sufficient potential energy of water
d) controlling the opening of valves
Answer
Answer: b [Reason:] Heavy solid materials flowing with water can damage the turbine blades if not stopped. Trash racks are closely spaced flat bars which provides narrow path from which such unwanted materials cannot pass.
5. Penstock in a hydroelectric power plant is _______
a) a pipe connected to runner outlet
b) nozzle that release high pressure water on turbine blades
c) a conduit connecting forebay to scroll case of turbine
d) a pipe connecting surge tank to dam
Answer
Answer: c [Reason:] Penstocks are the conduit built of steel or reinforced concrete. Penstock connects forebay or surge tank to scroll case of turbine. Their main function is to carry water from dam to the turbine.
6. The pressure at the inlet or exit of the draft tube should not be ___
a) less than one third of atmospheric pressure
b) greater than one third of atmospheric pressure
c) less than one atmospheric pressure
d) greater than one atmospheric pressure
Answer
Answer: a [Reason:] Decrease in pressure in any portion of turbine below one third of atmospheric pressure may cause vapour bubbles or cavities to form. This phenomenon is called cavitation. Also to maintain continuity of flow without vaporisation the pressure should not fall below vapour pressure of water.
7. Draft tube increases the operating head on the turbine.
a) True
b) False
Answer
Answer: a [Reason:] Draft tubes are the pipes of suitable diameter attached to the runner outlet. Draft tube converts the pressure developed by water leaving from turbine into kinetic energy. This in turn increases the operating head on turbine.
8. Which statement about surge tank is wrong?
a) Ideal location of surge tank is at the turbine inlet
b) A decrease in load demands cause a rise in water level in surge tank
c) Surge tanks are totally closed to avoid entry of unwanted objects to penstock
d) Surge tanks are installed to reduce harm effects of water hammer phenomenon
Answer
Answer: c [Reason:] Function of surge tank is to relieve the penstock from excessive water hammer pressure. It does so by receiving rejected flow of water into the tank. A totally closed tank can not release pressure so usually surge tanks are left open at the top.
9. Trash racks are located _______
a) near tailrace
b) at the entrance of turbine
c) inside penstock
d) intake
Answer
Answer: d [Reason:] Intake includes headworks at the entrance of conduit. Those headworks include different structures, trash racks are one of them. Trash racks are fitted directly at the intake to prevent the floating and other materials from going into the conduit.
10. What is the function of booms?
a) It supports the dam
b) It supports the penstock
c) It divert the Icebergs from flowing into the penstock
d) To hold the turbine structure
Answer
Answer: a [Reason:] Booms are the one of those structures made at the intake. They are constructed to prevent unwanted solid materials from flowing into the penstock. Solid materials like icebergs, wood logs or other heavy materials which may cause damage to the turbine blades if reached there.
