Download GTU (Gujarat Technological University) BE/BTech (Bachelor of Engineering / Bachelor of Technology) 2019 Summer 6th Sem New 2160908 Electrical Power System ? Ii Previous Question Paper
Seat No.: ________ Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER ?VI(NEW) ? EXAMINATION ? SUMMER 2019
Subject Code:2160908 Date:14/05/2019
Subject Name:Electrical Power system ? II
Time:10:30 AM TO 01:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
MARKS
Q.1 (a) Derive ABCD constant for medium transmission line using nominal ?
representation.
03
(b) A surge of 120 kv traveling in a line of natural impedance of 600 ?
arrives at a junction with two lines of impedances 900? and 300?
respectively. Find the surge voltages and currents transmitted into each
branch line.
04
(c) Determine the efficiency and regulation of a 3-phase, 120 km, 50Hz
transmission line delivering 30MW at 0.866 p.f. lagging,66 KV to a
balanced load. The line parameters are as follows: R = 0.1?/km/phase,
L =1mH/km/phase, C=0.99?F/phase. Use nominal T model of
transmission line.
07
Q.2 (a) Give classification of faults. What is the difference between steady state
reactance Xd, transient reactance Xd? and sub-transient reactance Xd??.
03
(b) A 3-phase 20 MVA, 11 KV genertor has internal reactance of 7.5% and
negligible resistance. Find the external reactance per phase to be
connected in series with the alternator so that steady current on short
circuit does not exceed 8 times full load current.
04
(c) Explain factors deciding selection of circuit breaker. 07
OR
(c) Explain the importance of bus impedance matrix in fault calculation. 07
Q.3 (a) Why 3-phase symmetrical fault is more severe than 3-phase
unsymmetrical fault?
03
(b) Prove that positive sequence impedance and negative sequence
impedance of fully transposed transmission line are always equal.
04
(c) One conductor of a 3-phase line is open. The current flowing to the delta
connected balanced load through the line R is 10A. With R phase as
reference and assuming phase B as open, find the symmetrical
components of the line currents.
07
OR
Q.3 (a) What is the reason for transient during short circuit? 03
(b) What are the symmetrical components and its need? 04
(c) Three impedances of 5-j10?, 6+j5? and 3+j15? are connected in star
to R,Y and B lines of a 3300V, 3-phase, 3-wire supply. The phase
sequence is RYB. Calculate the line current IR.
07
Q.4 (a) Differentiate symmetrical and unsymmetrical faults. List various
unsymmetrical faults.
03
(b) Describe analysis of line to line fault at a point of power system using
symmetrical components and sequence network. Calculate the value of
fault current.
04
(c) Figure 1 shows a synchronous generator whose neutral is grounded
through a reactance Xn. The generator has balanced emfs and sequence
reactance X1, X2 and X0 such that X1=X2 > X0. i) Draw a sequence
07
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Seat No.: ________ Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER ?VI(NEW) ? EXAMINATION ? SUMMER 2019
Subject Code:2160908 Date:14/05/2019
Subject Name:Electrical Power system ? II
Time:10:30 AM TO 01:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
MARKS
Q.1 (a) Derive ABCD constant for medium transmission line using nominal ?
representation.
03
(b) A surge of 120 kv traveling in a line of natural impedance of 600 ?
arrives at a junction with two lines of impedances 900? and 300?
respectively. Find the surge voltages and currents transmitted into each
branch line.
04
(c) Determine the efficiency and regulation of a 3-phase, 120 km, 50Hz
transmission line delivering 30MW at 0.866 p.f. lagging,66 KV to a
balanced load. The line parameters are as follows: R = 0.1?/km/phase,
L =1mH/km/phase, C=0.99?F/phase. Use nominal T model of
transmission line.
07
Q.2 (a) Give classification of faults. What is the difference between steady state
reactance Xd, transient reactance Xd? and sub-transient reactance Xd??.
03
(b) A 3-phase 20 MVA, 11 KV genertor has internal reactance of 7.5% and
negligible resistance. Find the external reactance per phase to be
connected in series with the alternator so that steady current on short
circuit does not exceed 8 times full load current.
04
(c) Explain factors deciding selection of circuit breaker. 07
OR
(c) Explain the importance of bus impedance matrix in fault calculation. 07
Q.3 (a) Why 3-phase symmetrical fault is more severe than 3-phase
unsymmetrical fault?
03
(b) Prove that positive sequence impedance and negative sequence
impedance of fully transposed transmission line are always equal.
04
(c) One conductor of a 3-phase line is open. The current flowing to the delta
connected balanced load through the line R is 10A. With R phase as
reference and assuming phase B as open, find the symmetrical
components of the line currents.
07
OR
Q.3 (a) What is the reason for transient during short circuit? 03
(b) What are the symmetrical components and its need? 04
(c) Three impedances of 5-j10?, 6+j5? and 3+j15? are connected in star
to R,Y and B lines of a 3300V, 3-phase, 3-wire supply. The phase
sequence is RYB. Calculate the line current IR.
07
Q.4 (a) Differentiate symmetrical and unsymmetrical faults. List various
unsymmetrical faults.
03
(b) Describe analysis of line to line fault at a point of power system using
symmetrical components and sequence network. Calculate the value of
fault current.
04
(c) Figure 1 shows a synchronous generator whose neutral is grounded
through a reactance Xn. The generator has balanced emfs and sequence
reactance X1, X2 and X0 such that X1=X2 > X0. i) Draw a sequence
07
2
network of the generator as seen from the terminals ii) derive expression
for fault current for solid line to ground fault on phase a. iii) prove that
if neutral is solidly grounded, the LG fault current is more than three
phase fault current.
OR
Q.4 (a) What is the reason for transient during short circuit? 03
(b) Describe analysis of line to ground fault at a point of power system using
symmetrical components and sequence network. Calculate the value of
fault current.
04
(c) Two 11KV, 30 MV, 3-phase star connected generators operates in
parallel as shown in figure 2. The positive, negative and zero sequence
reactance of each being j0.18, j0.15, j0.1 p.u. The star point of one of
the generator is isolated and that of other is grounded through 2?
resistor. A single line to ground fault occurs at the terminals of one of
the generators. Find i) fault current ii) current in grounding resistor iii)
voltage across grounding resistor.
07
Q.5 (a) Discuss disadvantages of corona. List methods to reduce corona losses. 03
(b) Explain capacitance switching with necessary waveforms. 04
(c) Define critical disruptive voltage. Derive necessary formula to calculate
critical disruptive voltage.
07
OR
Q.5 (a) Determine the critical disruptive voltage and corona loss for a 3-phase
line operating at 132 KV which has conductor of 1.25 cm dia arranged
ina 3.05 metre delta. Assume air density factor = 1.06 and the dielectric
strength of air to be 21KV/cm.
03
(b) Explain overvoltage due to arcing ground with necessary vector
diagram.
04
(c) List factors affecting corona loss. Discuss any two factors in detail. 07
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This post was last modified on 20 February 2020