Power Systems MCQ Set 1
1. Nominal-T and nominal-π are equivalent to each other.
a) True
b) False
Answer
Answer: b [Reason:] Nominal-T and nominal-π are not equivalent to each other. They can’t be interchanged.
2. For the circuit representation of a transmission line below, the transmission line T-matrix will be _____
Answer
Answer: a [Reason:] Applying KVL and KCL,
Vs = Vr
Is = Y * Vs + Ir.
3. For the circuit representation of a transmission line below, the transmission line T-matrix will be 
. Then AB – CD value will be ___
a) Y
b) 1
c) -Y
d) 1-Y
Answer
Answer: a [Reason:] Applying KVL and KCL,
Vs = Vr
Is = Y * Vs + Ir
AB-CD = 0-Y = -Y.
4. The transfer of the reactive power over a line mainly depends upon _____
a) Vr
b) Vs
c) |Vs| – |Vr|
d) Power angle
Answer
Answer: c [Reason:] Reactive power flow depends on the |Vs|-|Vr|.
5. For a medium transmission line system if the sending end line voltage is 143 kV with the line impedance as 101.24∠74° Ω and shunt admittance of 7.38*10-4∠90° Ω-1.Then the no load line voltage at the receiving end would be ___
a) 148.3 kV
b) 140.8 kV
c) 149.2 kV
d) 132 kV
Answer
Answer: a
6. For a medium transmission line system if the sending end line voltage is 143 kV with the line impedance as 101.24∠74° Ω and shunt admittance of 7.38*10-4∠90° Ω-1 delivering 25 MVA at 0.8 power factor lagging in nature to the load at 132 kV. The voltage regulation is _____
a) 12.3
b) 12.8
c) 14.2
d) -2.3
Answer
Answer: a
Voltage regulation = (148.3-132)/132 = 12.3%.
7. For the lines more than 250 km, the parameters of a line are considered not ______ but _____
a) lumped , distributed
b) distributed, lumped
c) lumped, non uniform lumped
d) uniform, lumped
Answer
Answer: a [Reason:] For long transmission lines, the parameters are considered uniformly distributed.
8. Which of the parameters of a long transmission line and medium transmission lines are same?
a) A and D
b) B, C
c) Only A
d) Only D
Answer
Answer: a [Reason:] A and D parameters of LTL and MTL are almost same.
9. With 100% series compensation of the transmission lines, which of the following effects are observed?
(i) Low transient voltage
(ii) High transient current
(iii) The current is series resonant at power frequency
a) (ii), (iii)
b) (i), (ii)
c) (i), (ii), (iii)
d) (iii)
Answer
Answer: a [Reason:] 100% compensation of the line results in high transient current at power frequency.
10. With 100% series compensation of the transmission lines, which of the following effects are not observed?
(i) Low transient voltage
(ii) High transient current
(iii) The current is series resonant at power frequency
a) (ii), (iii)
b) (i)
c) (i), (ii), (iii)
d) (iii)
Answer
Answer: b [Reason:] 100% compensation of the line results in high transient current at power frequency.
11. Estimate the transmission line parameters (ABCD), for the given distributed network.
Answer
Answer: a [Reason:] Vs= Z1*Is + Y-1*(Is+Ir)
Vr = Z2*Ir + Y-1*(Is+Ir)
VY = Is+Ir
Solving above equation, we get
12. Estimate the transmission line parameters (ABCD), for the given distributed network and find AB-CD.
a) 1
b) Y
c) Y-Z1Z2
d) 0
Answer
Answer: 1 [Reason:] Vs= Z1*Is + Y-1*(Is+Ir)
Vr = Z2*Ir + Y-1*(Is+Ir)
VY = Is+Ir
Solving above eq, we get
AD-BC = 1.
13. Estimate the transmission line parameters (ABCD), for the given distributed network.
Answer
Answer: a [Reason:] Applying KVL in the middle loop and KCL at the two nodes, we will get
14. Surge impedance for 400ohms implies for a transmission line that _____
a) line can be theoretically loaded up to 400ohms
b) line can be practically loaded up to 400ohms
c) open circuit impedance of 400ohms
d) short circuit impedance of 400ohms
Answer
Answer: a [Reason:] Surge impedance decides the maximum value for a transmission line loading.
Power Systems MCQ Set 2
1. A power system network is connected as shown in the figure.
Sd1=15+j5 pu
Sd2=25+j15 pu
Zcable = j0.05pu
|V1|=|V2|=1 pu.
The torue angle for the system will be ____
a) 14.4
b) 22.1
c) 16.2
d) 18.2
Answer
Answer: a [Reason:] As the resistance is zero, losses will be zero.
PG1=PD1+PD2=40 pu
For the equal sharing of load at the station
PG1=PG2=20pu
Real power flow from bus 1 to 2
2. A single phase distributor of 1 km long has resistance and reactance per conductor of 0.1Ω and 0.15 Ωrespectively. If the far end voltage Vb=200V and current is at 100A at 0.8 lag. At the midpoint a current of 100A is tapped at a pf of 0.6 pf with ref to voltage Vm at mid point. The voltage magnitude at M is ________
a) 218V
b) 200V
c) 232V
d) 220V
Answer
Answer: a [Reason:] Drop in MB= (100ʟ-36.67)(0.1+j0.15)= 18.027ʟ19.44
VM= 200+18.027 = 218.027 V.
3. A single phase motor is connected to 400V, 50Hz supply. The motor draws a current of 31.7A at a power factor 0.7 lag. The capacitance required in parallel with motor to raise the power factor of 0.9 lag (in micro farads) is ____
a) 94.62
b) 282.81
c) 108.24
d) 46.87
Answer
Answer: a [Reason:] Active power drawn by the motor=VIcosФ = 400*31.7*0.7 = 8876 W
Reactive power = VIsinФ=400*31.7*sin(45.57.29) = 9055.3 VAR
New power factor=cosθ2 = 0.9
θ2=cos-1(0.9)
Q2=8876*tan(25.84) = 4298.855 VAR
Change in reactive power=9055.3-4298.855 = 4756.4 VAR
Qc = V2/Xc = V2*2πfC
C=4756.4/(4002*2π*50) = 94.62μF.
4. A single phase motor is connected to 400V, 50Hz supply. The motor draws a current of 31.7A at a power factor 0.7 lag. The additional reactive power (in VAR) to be supplied by the capacitor bank will be _____
a) 4756
b) 4873
c) 4299
d) 9055.3
Answer
Answer: a [Reason:] Active power drawn by the motor=VIcosФ = 400*31.7*0.7 = 8876 W
Reactive power = VIsinФ=400*31.7*sin(45.57.29) = 9055.3 VAR
New power factor=cosθ2 = 0.9
θ2=cos-1(0.9)
Q2=8876*tan(25.84) = 4298.855 VAR
Change in reactive power=9055.3-4298.855 = 4756.4 VAR.
5. A 275 kV TL has following line constants A=0.85ʟ5o, B=200ʟ75o. The active power received if the voltage to be maintained is 275kV will be ____
a) 117.63
b) 220
c) 120
d) 115.25
Answer
Answer: a [Reason:] |Vs|=|Vr|=275 kV
6. A 275 kV TL has following line constants A=0.85ʟ5o, B=200ʟ75o. The active power angle such that the voltage to be maintained at the other end will be 275 kV ______
a) 22
b) 16
c) 18
d) 24
Answer
Answer: a [Reason:] |Vs|=|Vr|=275 kV
α= 5o , β= 75o
7. A power system has a maximum load of 15 MW. Annual load factor is 50%. The reserve capacity of plant is _____ if the plant capacity factor is 40%.
a) 3.75 MW
b) 4.75 MW
c) 18.75 MW
d) 5.75 MW
Answer
Answer: a [Reason:] LF = (Average Demand)/(Maximum Demand)=0.5
Plant capacity factor =(Average Load)/(Plant Capacity)= 0.5/0.4
Plant capacity= (0.5/0.4)*15 = 18.75 MW
Reserve Capacity = 18.75-15 = 3.75 MW.
8. A 100 MVA synchronous generator operates on full load at a frequency of 50 Hz. The load is suddenly reduced to 50 MW. Due to time lag in governor system, the steam valve begins to close after 0.4s. The change in the frequency is ________(H=5 kW-s/KVA).
a) 1
b) 0.5
c) -1.5
d) 0.8
Answer
Answer: a [Reason:] Energy transferred in 0.4 sec = 50*0.4= 20 J
9. A 50 Hz four pole turbo-generator rated 100 MVA, 11 kV has an inertia constant of 8 MJ/MVA. If the mechanical input is suddenly raised to 80 MW for an electrical load of 50MW, then the rotor acceleration is ______
a) 337.5
b) 3.375
c) 457.5
d) 4.57
Answer
Answer: a [Reason:] Energy stored = 100*8 =800MJ
Accelerating power = Md2δ/dt2
M=GH/180f = 800/(180*50) = 4/45 MJs/elect. Deg
10. A single phase TL has copper conductor of 0.775 cm2 cross section through which 200 kW at UPF at 330 V is to be maintained. If the efficiency of transmission line is 90%, then the minimum length of TL is _____(in km and take specific resistance to be 1.785 μΩ/cm).
a) 13.6 km
b) 14 km
c) 136 km
d) 16.4 km
Answer
Answer: a [Reason:] Pr=200 kW, efficiency=0.9
Ps= 200/0.9=222.22 kW
Losses=22.22 kW
Current, I=200000/3300 = 60.60 A
Line losses=2I2R ( for a 2 wire line)
R=22.22/(2*60.602)=3.02 Ω
R=ρl/a
Length, l = (3.025*0.775)/(1.785*10-6) = 13.6 km.
11. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the reactance of transformer in per units is ____
a) 0.1
b) 0.3
c) 0.03
d) 1.5
Answer
Answer: a [Reason:] The pu value of a transformer does not change.
12. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the high voltage side impedance ______
a) 1763.3 Ω
b) 158.7 Ω
c) 15.87 Ω
d) 176.3 Ω
Answer
Answer: a [Reason:] On the high voltage side, Zb=kVb2/MVAb(3-ph) = 2302/30 = 1763.33 Ω.
13. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the low voltage side impedance is _____
a) 158.7 Ω
b) 176.3 Ω
c) 1763.3 Ω
d) 15.87 Ω
Answer
Answer: a [Reason:] On the low voltage side, Zb=kVb2/MVAb(3-ph) =692/30 = 158.7 Ω.
14. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the transformer reactance referred to the high voltage side will be ___(in ohms).
a) 176.33 Ω
b) 17.67 Ω
c) 158.7 Ω
d) 15.87 Ω
Answer
Answer: a [Reason:] Zb=kVb2/MVAb(3-ph) = 2302/30 = 1763.33 Ω
X Ω = Xpu*Xb(HV) = 0.1*1763.33 = 176.33 Ω.
Power Systems MCQ Set 3
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
Answer: a [Reason:] 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
Answer: b [Reason:] 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
Answer: d [Reason:] 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
Answer: c [Reason:] 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
Answer: a [Reason:] 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
Answer: c [Reason:] 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
Answer: a [Reason:] 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
Answer: d [Reason:] 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
Answer: a [Reason:] 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
Answer: a [Reason:] All the statements about the guy wire are true.
Power Systems MCQ Set 4
1. What happens to the tension in a conductor hanged between two poles, when temperature varies?
a) Tension increases with increase in temperature
b) Tension decreases with increase in temperature
c) Tension first increases and decreases with decrease in temperature
d) Tension in conductor is independent of temperature variation
Answer
Answer: b [Reason:] The relationship between tension and sag is dependent on the loading conditions and temperature variations. For instance, the tension increases when temperature decreases.
2. What is the effect of rise in temperature on sag when Ice and wind effect are eliminated?
a) Sag decreases
b) Sag increases
c) Sag remains constant
d) Sag becomes zero
Answer
Answer: b [Reason:] All metallic body expand with rise in temperature and therefore the length of the conductor increases with the rise in temperature and so does Sag.
3. What is the relation between length of span and sag?
a) sag ∝ √span
b) sag ∝ (1/span)
c) sac ∝ span2
d) Sagar ∝ span3
Answer
Answer: c [Reason:] Ultimate stress is directly proportional to the square of span length. Other conditions such as type of conductor working tension temperature it is it remains the same affection with longest and we’ll have much greater Sag.
4. What should be the value of sag for proper operation of overhead transmission line?
a) High
b) Low
c) Nither too low nor too high
d) Anything
Answer
Answer: c [Reason:] If the sag is too high more conductor material is required resulting more weight on the supports. If the sag is too low, there is more tension in the conductor and thus it is liable to break if any additional stress such as due to vibrations or due to fall in temperature occurs.
5. An overhead transmission line has a span of 220 metres the conductor waiting 0.604 kg/m. What will be the maximum sag if the working tension is 2879 kg.
a) 8.96 m
b) 8.86 m
c) 8.85 m
d) 1.27 m
Answer
Answer: d [Reason:]
(Maximum sag ) S = ωL2/8T
Where ,
ω – weight of conductor per meter
L – span
T – Working tension
Therefore, S = (0.604×2202)/(8×2879)
= 1.269 m ~ 1.27 m.
6. What is the value of working stress in overhead conductors?
a) Less than ultimate stress
b) More than ultimate stress
c) Always equal to ultimate stress
d) Should be zero
Answer
Answer: a [Reason:] Working tensile strength of the conductor is determined by multiplying the ultimate stress and area of cross section and dividing by a factor of safety. Due to division of ultimate stress by factor of safety the value of working stress is always less than value of ultimate stress.
7. When the sag exceeds 10% of the span length, the shape made by the conductor is similar to which of the following shape?
a) Hyperbola
b) Parabola
c) Catenary
d) Straight line
Answer
Answer: c [Reason:] A flexible wire of uniform cross section when string between two suppose at the same level will form a category however if the sag is very small as well as less than 10% of the span length then its shape approximates a parabola.
8. What will be the resultant weight on per meter of length of conductor if weight of conductor is 150 kg/m, weight of ice in per meter of length is 60 kg/m and wind force is 200 Kg/m.?
a) 300 kg/m
b) 468 kg/m
c) 290 kg/m
d) 390 Kg/m
Answer : c
Answer
Explanation :
Resultant stress = √[{(wc + wi )2 + ww2}]
= √[{(150 + 60 )2 + 2002}]
= 290 kg.
9. What are aeoline vibrations in overhead transmission line conductors?
a) High frequency and low amplitude vibrations
b) High frequency and high amplitude vibrations
c) Low frequency and low amplitude vibrations
d) Low frequency and high amplitude vibrations
Answer
Answer: a [Reason:] Aeoline vibrations are high frequency (as high as 500 Hz) and low amplitude (20 mm to 50 mm ) vibrations. They are caused by vortex phenomenon in light winds (5 to 20 Km/hr).
10. What are galloping vibrations in overhead transmission line conductors?
a) High frequency and low amplitude vibrations
b) High frequency and high amplitude vibrations
c) Low frequency and low amplitude vibrations
d) Low frequency and high amplitude vibrations
Answer
Answer: d [Reason:] Galloping vibrations in overhead transmission lines are low friquency ( 1 or 2 Hz) and high amplitude ( about 6 m ) vibrations. They occurs during sleet storms with a strong winds. In such situations conductors are said to ‘dance’.
11. Which of the following vibrations causes different conductors to touch due to high swing?
a) Aeoline vibrations
b) Galloping vibrations
c) Aeoline and Galloping
d) Amplitude vibrations
Answer
Answer: b [Reason:] Galloping vibrations in overhead transmission lines are high amplitude ( about 6 m ) vibrations. During such vibrations conductors ‘dance’ in horizontal and vertical directions with high amplitude which makes the conductors to touch each other.
12. ‘Dancing’ of overhead conductors occurs during which of the following types of vibrations?
a) Aeoline vibrations
b) Galloping vibrations
c) Aeoline and Galloping
d) Amplitude vibrations
Answer
Answer: b [Reason:] During galloping vibrations in overhead transmission lines conductors vibrates with high amplitude ( about 6 m ) and low friquency (about 1 or 2 Hz). Swinging of conductors with high amplitudes in horizontal and vertical directions are called as they are ‘dancing’.
13. What are the method for prevention of low frequency high amplitude vibrations?
a) Horizontal conductor configuration
b) Vertical conductor configuration
c) Horizontal and Vertical conductor configuration
d) There is no method for prevention of such vibrations
Answer
Answer: d [Reason:] Horizontal conductor configuration can be used to reduce the danger of low frequency vibrations. But no method can prevent such vibrations.
14. What are the methods used to protect conductors against high frequency resonant vibrations?
a) Horizontal conductor configuration
b) Vertical conductor configuration
c) By using dampers
d) There is no method for prevention of such vibrations
Answer
Answer: c [Reason:] The conductors are protected by dampers. Dampers prevents the resonant vibrations from reaching the conductors at the clamps or supports.
Power Systems MCQ Set 5
1. Transient in synchronous generator is similar to which of the following circuit?
a) Parallel RLC circuit
b) Series RLC circuit
c) Series RL circuit
d) Parallel RL circuit
Answer
Answer: c [Reason:] The current flowing in armature of synchronous generator when its terminals are short circuited is similar to that flowing when sinusoidal voltage is suddenly applied to an RL series circuit.
2. When all three phases of a synchronous generator on no load are suddenly short circuited then symmetry of the short circuit current depends on which of the following?
a) Position of fault
b) Symmetry of fault
c) Instantaneous Voltage at which fault occurs
d) Resistance of armature winding
Answer
Answer: c [Reason:] Symmetry of the short circuit current about time axis and DC offset depends upon the value of phase voltage at which short circuit occurs.
3. In a synchronous generator for how much time subtransient period of symmetrical short circuit current lasts?
a) For 200 Cycles
b) For 500 Cycles
c) For 30 Cycles
d) For 2 Cycles
Answer
Answer: d [Reason:] Subtransient period lasts for only about 2 cycles. During this period the subtransient current decay is very rapidly.
4. In a synchronous generator for how much time transient period of symmetrical short circuit current lasts?
a) For 200 Cycles
b) For 500 Cycles
c) For 30 Cycles
d) For 2 Cycles
Answer
Answer: c [Reason:] In a synchronous generator transient period of symmetrical short circuit current lasts for about 30 cycles. During this period current decreases somewhat slower than the current in subtransient period.
5. After how many cycles in a synchronous generator symmetrical short circuit current reaches to its steady state value?
a) After 200 Cycles
b) After 500 Cycles
c) After 30 Cycles
d) After 2 Cycles
Answer
Answer: c [Reason:] In a synchronous generator subtransient period of short circuit current lasts for up to 2 cycles the transient period of that lies for about 30 cycles. After that the current reaches to its steady state value.
6. What is the phase current in phase Y and in phase B out of R-Y-B when sudden short circuit to all three phases occurs at no load? It is given that after fault current in phase R is equal (Vm Sin ωt)
a) Y = Vm Sin(ωt +60°) & B = VmSin(ωt +120°)
b) Y = Vm Sin(ωt +120°) & B = VmSin(ωt+240°)
c) Y = Vm Sin(ωt +240°) & B = Vm Sin(ωt +120°)
d) Y = Vm Sin(ωt +90°) & B = Vm Sin(ωt -90°)
Answer
Answer: b [Reason:] The given phase sequence is R-Y-B . When all three phases are short circuited then the fault is symmetrical that means fault current flowing through each phases will be 120° apart from each other.
7. In case of synchronous generator the reactants is constant.
a) True
b) False
Answer
Answer: b [Reason:] In case of synchronous generator the reactance is not a constant one but is a function of time. The reactance of synchronous generator is different for subtransient state, transient state and steady state.
8. If Xl is leakage reactance, Xf is reactance due to field winding, Xdw is reactance due to damper winding and Xa is armature winding reactance then reactance of synchronous generator in subtransient state is due to which of the following?
a) Xl ,Xdw and Xa
b) Xl and Xa
c) Xl , Xf ,Xdw and Xa
d) Xl and Xdw
Answer
Answer: c [Reason:] During the initial part of short circuit or during subtransient period equivalent circuit of alternator contains the field winding reactance damper winding reactance armature winding reactance in parallel and leakage reactance in series.
9. Which of the following reactance is eliminated first in synchronous generator just after the symmetrical fault?
a) Leakage reactance
b) Damper winding reactance
c) Armature winding reactance
d) Field winding reactance
Answer
Answer: b [Reason:] As the damper winding currents are first to die out, damper winding reactance effectively becomes open circuited. After that Approximate circuit model of synchronous generator contains only leakage reactance, field winding reactance and armature winding reactance.
10. Steady state direct axis reactance is greater than subtransient direct axis reactance and transient direct axis reactance.
a) True
b) False
Answer
Answer: a [Reason:] During transient and subtransient period the armature winding reactance contains other reactances like damper winding reactance and field winding reactances in parallel with it. So, the net reactance during subtransient and transient period are less than the net reactance during steady state period.
