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Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE) PDF Download

Q1: The single line diagram of a lossless system is shown in the figure. The system is operating in steady-state at a stable equilibrium point with the power output of the generator being Pmaxsinδ. where δ is the load angle and the mechanical power input is  0.5Pmax. A fault occurs on line 2 such that the power output of the generator is less than 0.5. ⁡Pmax during the fault. After the fault is cleared by opening line 2. The power output of the generator is {Pmax/√2}sinδ. If the critical fault clearing angle is π/2 radians, the accelerating area on the power angle carve is ______ times Pmax (rounded off to 2 decimal places)      (2024)
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)(a) 0.25
(b) 0.11
(c) 0.36
(d) 0.42
Ans:
(b)
Sol: Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)A1 = A2
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q2: The two-bus power system shown in figure (i) has one alternator supplying a synchronous motor load through a Y − Δ transformer. The positive, negative and zero-sequence diagrams of the system are shown in figures (ii), (iii) and (iv), respectively. All reactances in the sequence diagrams are in p.u. For a bolted line-to-line fault (fault impedance = zero) between phases 'b' and 'c' at bus 1, neglecting all pre-fault currents, the magnitude of the fault current (from phase 'b' to 'c') in p.u. is _____ (Round off to 2 decimal places).       (2023)
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)(a) 7.22
(b) 5.47
(c) 6.98
(d) 9.82
Ans:
(a)
Sol: From positive sequence network :
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)From negative sequence network :
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)For L − L fault,
Fault current, Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)

Q3: The valid positive, negative and zero sequence impedances (in p.u.), respectively, for a 220 kV, fully transposed three-phase transmission line, from the given choices are      (2022)
(a) 1.1, 0.15 and 0.08
(b) 0.15, 0.15 and 0.35
(c) 0.2, 0.2 and 0.2
(d) 0.1, 0.3 and 0.1
Ans:
(b)
Sol: We have,
X> X1 = X2
(for 3 − ϕ transposed transmission line)  

Q4: A 30 kV, 50 Hz, 50 MVA generator has the positive, negative, and zero sequence reactancesof 0.25 pu, 0.15 pu, and 0.05 pu, respectively. The neutral of the generator is grounded with a reactance so that the fault current for a bolted LG fault and that of a bolted three-phase fault at the generator terminal are equal. The value of grounding reactance in ohms (round off to one decimal place) is ______     (2019)
(a) 2.2
(b) 1.8
(c) 3.6
(d) 4.2
Ans
: (b)
Sol: Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q5: Five alternators each rated 5 MVA, 13.2 kV with 25% of reactance on its own base are connected in parallel to a busbar. The short-circuit level in MVA at the busbar is_________        (2019)
(a) 50
(b) 75
(c) 100
(d) 150
Ans: 
(c)
Sol: Net reactance of parallel connection,
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)SC MVA = 20 x 5 = 100 MVA

Q6: In the circuit shown below, the switch is closed at t = 0. The value of  θ in degrees which will give the maximum value of DC offset of the current at the time of switching is      (2019)
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)(a) 60
(b) -45
(c) 90
(d) -30
Ans:
(b)
Sol: If the switch is closed at t = 0 in series R-L circuit. Then the circuit current i(t) expression is
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)The first term of expression indicates DC offset current.
For maximum value of DC offset current, the angle should be 90°
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q7: The positive, negative and zero sequence impedances of a three phase generator are Z1, Z2 and Z0 respectively. For a line-to-line fault with fault impedance Zf ,the fault current is  If1 = kIf , where If is the fault current with zero fault impedance. The relation between  Zf and k is      (2018)
(a) Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)

(b) Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
(c) Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
(d) Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Ans: (a)
Sol: For LL fault:
Without Zf
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q8: The positive, negative and zero sequence impedances of a 125 MVA, three-phase, 15.5 kV, star-grounded, 50 Hz generator are j0.1 pu, j0.05 pu and j0.01 pu respectively on the machine rating base. The machine is unloaded and working at the rated terminal voltage. If the grounding impedance of the generator is j0.01 pu, then the magnitude of fault current for a b-phase to ground fault (in kA) is __________ (up to 2 decimal places).        (2018)
(a) 35.82
(b) 73.52
(c) 87.23
(d) 97.66
Ans: 
(b)
Sol: For LG fault,
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q9: The series impedance matrix of a short three-phase transmission line in phase coordinates is
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)If the positive sequence impedance is (1 + j10)Ω, and the zero sequence is (4 + j31)Ω, then the imaginary part of Zm (in Ω) is ______(up to 2 decimal places).     (2018)
(a) 3
(b) 5
(c) 7
(d) 9
Ans:
(c)
Sol: Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)we know.
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)The imaginary part of Zm is 7.00.

Q10: The positive, negative and zero sequence reactances of a wye-connected synchronous generator are 0.2 pu, 0.2 pu, and 0.1 pu, respectively. The generator is on open circuit with a terminal voltage of 1 pu. The minimum value of the inductive reactance, in pu, required to be connected between neutral and ground so that the fault current does not exceed 3.75 pu if a single line to ground fault occurs at the terminals is _______ (assume fault impedance to be zero).      (SET-1  (2017))
(a) 0.033
(b) 0.05
(c) 0.1
(d) 0.2
Ans: 
(c)
Sol: Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)

For LG fault,

Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q11: Two identical unloaded generators are connected in parallel as shown in the figure. Both the generators are having positive, negative and zero sequence impedances of j0.4 p.u., j0.3 p.u. and j0.15 p.u., respectively. If the pre-fault voltage is 1 p.u., for a line-to-ground (L-G) fault at the terminals of the generators, the fault current, in p.u., is ___________.       (SET-2 (2016))
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)(a) 4.5
(b) 8.9
(c) 6.0
(d) 9.6
Ans: 
(c)
Sol: Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q12: The single line diagram of a balanced power system is shown in the figure. The voltage magnitude at the generator internal bus is constant and 1.0 p.u. The p.u. reactances of different components in the system are also shown in the figure. The infinite bus voltage magnitude is 1.0 p.u. A three phase fault occurs at the middle of line 2.
The ratio of the maximum real power that can be transferred during the pre-fault condition to the maximum real power that can be transferred under the faulted condition is _________.      (SET-2 (2016))
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)(a) 2.28
(b) 1.25
(c) 3.65
(d) 1.82
Ans:
(a)
Sol: Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q13: A 50 MVA, 10 kV, 50 Hz, star-connected, unloaded three-phase alternator has a synchronous reactance of 1 p.u. and a sub-transient reactance of 0.2 p.u. If a 3-phase short circuit occurs close to the generator terminals, the ratio of initial and final values of the sinusoidal component of the short circuit current is ________.       (SET-2  (2016))
(a) 3
(b) 4
(c) 5
(d) 6
Ans:
(c)
Sol: Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q14: If the star side of the star-delta transformer shown in the figure is excited by a negative sequence voltage, then     (SET-1(2016))
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)(a) VAB leads Vab by 60°
(b) VAB lags Vab by 60°
(c) VAB leads Vab by 30°
(d) VAB lags Vab by 30°
Ans: 
(d)
Sol: According to negative sequence phasors.
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q15: A 30 MVA, 3-phase, 50 Hz, 13.8 kV, star-connected synchronous generator has positive, negative and zero sequence reactances, 15%, 15% and 5% respectively. A reactance (Xn) is connected between the neutral of the generator and ground. A double line to ground fault takes place involving phases 'b' and 'c', with a fault impedance of j0.1 p.u. The value of Xn (in p.u.) that will limit the positive sequence generator current to 4270 A is _________.         (SET-1 (2016))
(a) 1.07
(b) 0.55
(c) 1.85
(d) 2.10
Ans: 
(a)
Sol: Base current,
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q16: The magnitude of three-phase fault currents at buses A and B of a power system are 10 pu and 8 pu, respectively. Neglect all resistances in the system and consider the pre-fault system to be unloaded. The pre-fault voltage at all buses in the system is 1.0 pu. The voltage magnitude at bus B during a three-phase fault at bus A is 0.8 pu. The voltage magnitude at bus A during a three-phase fault at bus B, in pu, is ________.     (SET-1(2016))
(a) 0.84
(b) 0.44
(c) 0.98
(d) 1.5
Ans: 
(a)
Sol: Voltage at bus B after 3-phase fault at A = 0.8 p.u.
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q17: A sustained three-phase fault occurs in the power system shown in the figure. The current and voltage phasors during the fault (on a common reference), after the natural transients have died down, are also shown. Where is the fault located?     (SET-1(2015)
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)(a) Location P
(b) Location Q
(c) Location R
(d) Location S
Ans:
(b)
Sol: From the phasor I2 and I4 are having 180° phase shift. ∣I1∣ is higher than all other. Hence fault occurred at point Q.

Q18: For a fully transposed transmission line      (SET-3 (2014))
(a) positive, negative and zero sequence impedances are equal
(b) positive and negative sequence impedances are equal
(c) zero and positive sequence impedances are equal
(d) negative and zero sequence impedances are equal
Ans: 
(b)
Sol: For a transmission line (static device),
 X1 = X2 and X0 >> X1

Q19: A three phase, 100 MVA, 25 kV generator has solidly grounded neutral. The positive, negative, and the zero sequence reactances of the generator are 0.2 pu, 0.2 pu and 0.05 pu, respectively, at the machine base quantities. If a bolted single phase to ground fault occurs at the terminal of the unloaded generator, the fault current in amperes immediately after the fault is_____.    (SET-2 (2014))
(a) 15393
(b) 25528
(c) 10547
(d) 35000
Ans:
(a)
Sol: Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Given, X1 = 0.2pu, X2 = 0.2pu, X3 = 0.05pu
For a line-to-ground fault on generator, fault current is given by
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Also, base MVA = 100
Base KVA = 25
Theerefore, Base current,
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)
Q20: In an unbalanced three phase system, phase current I= 1∠ −90° pu, negative sequence current Ib2 = 4∠(−150°) zero sequence current  Ic0 = 3∠ −90° pu. The magnitude of phase current Ib in pu is         (SET-1(2014))
(a) 1
(b) 7.81
(c) 11.53
(d) 13
Ans: 
(c)
Sol: Given,
Ia = 1∠(−90°)pu
Ib = 4∠(−150°)pu
Ic = 3∠(90°)pu
As zero sequence current in all the three phases of unbalanced 3 − ϕ system are equal, therefore,
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Also, Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE) = 4∠−150°
= Negative phase sequence current of phase b.
Therefore, referring to the negative phase sequence phasor,
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)= 4∠(−150° − 120°) = 4∠ − 270°pu
We know that, phase current vectors in matrix form are given by
Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE)

The document Previous Year Questions- Fault Analysis - 1 | Power Systems - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Power Systems.
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FAQs on Previous Year Questions- Fault Analysis - 1 - Power Systems - Electrical Engineering (EE)

1. What is fault analysis in electrical engineering?
Ans. Fault analysis in electrical engineering involves the study of faults or abnormalities in an electrical system to identify the cause of the fault and determine the necessary corrective actions.
2. Why is fault analysis important in electrical systems?
Ans. Fault analysis is important in electrical systems as it helps in maintaining the reliability and safety of the system by detecting and addressing faults promptly to prevent damage to equipment and ensure uninterrupted power supply.
3. What are the common types of faults in electrical systems?
Ans. Common types of faults in electrical systems include short circuits, ground faults, open circuits, overloads, and insulation failures. Each type of fault requires specific analysis and troubleshooting techniques.
4. How is fault analysis performed in electrical systems?
Ans. Fault analysis in electrical systems is typically performed by conducting tests such as insulation resistance tests, current and voltage measurements, and visual inspections to locate and diagnose the fault. Advanced techniques like power system analysis software can also be used for more complex systems.
5. What are the consequences of not performing fault analysis in electrical systems?
Ans. Not performing fault analysis in electrical systems can lead to equipment damage, power outages, safety hazards, and increased maintenance costs. It is essential to regularly conduct fault analysis to ensure the efficient operation of electrical systems.
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