Download GTU (Gujarat Technological University Ahmedabad) B.Tech/BE (Bachelor of Technology/ Bachelor of Engineering) 2020 Winter 6th Sem 2160908 Electrical Power System II Previous Question Paper
Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE- SEMESTER?VI (NEW) EXAMINATION ? WINTER 2020
Subject Code:2160908 Date:22/01/2021
Subject Name:Electrical Power system ? II
Time:02:00 PM TO 04:00 PM Total Marks: 56
Instructions:
1. Attempt any FOUR questions out of EIGHT questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
MARKS
Q.1 (a) Explain in brief, corona phenomenon in transmission
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lines.
(b) A 220 kV, 3- transmission line is 50 km long. The
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resistance is 0.15/km and inductance per phase is 1.33
mH/km. Use short line model to find voltage at the
sending end voltage when the line is supplying a load of
400 MVA, 220 kV at 0.8 pf lagging
(c) Using rigorous solution method derive the expression for
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A,B,C,D constants for long transmission line
Q.2 (a) What are symmetrical components? Explain how phasor
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voltages and currents can be expressed in terms of
symmetrical components
(b) An overhead line with inductance and capacitance per km
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of 1.24 mH and 0.087 F respectively is connected in
series with an underground cable having inductance and
capacitance per km of 0.185 mH and 0.285 F. Calculate
the values of transmitted and reflected waves of voltage at
the junction due to a voltage surge of 110 kV travelling to
the junction along the line towards the cable
(c) Discuss the nature of subtransient, transient and steady
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state current during fault. Under which condition dc
component is present in fault current?
Q.3 (a) The A & B parameters of 3-, 220 kV rated voltage
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medium length transmission line are given by A = 0.936
+ j0.16, B = 33.5 + j138 (/phase). If the load at the
receiving end is 50 MW, 220 kV at 0.9 pf lagging, find the
magnitude of line to line sending end voltage.
(b) Discuss the criteria for selection of circuit breakers
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(c) Two generators rated G1 and G2 are rated 15 MVA, 11
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kV and 10 MVA, 11 kV respectively. Two generators are
connected to a transformer as shown in figure 1. Calculate
the sub-transient current in each generator when a fault
occurs on the high voltage side of the transformer.
Q.4 (a) Prove that symmetrical component transformation is
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power invariant
(b) The phase voltages of a certain load are given as: Va =
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176-j132V, Vb = -128-j96V, Vc = -160 + j100V.
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Calculate positive, negative and zero sequence component
of voltage.
(c) Prove that zero sequence impedance of a transmission line
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is much greater than its positive or negative sequence
impedance
Q.5 (a) State the difference between symmetrical and
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unsymmetrical faults giving appropriate examples
(b) Explain how long transmission line can be represented by
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equivalent T-model.
(c) A 3-bus power system shown in figure 2 experiences a 3-
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phase solid fault on bus 3. Assuming pre-fault voltage to
be 1/00 pu and pre-fault current to be zero calculate the
magnitude of fault current. If the values of Z13 and Z33 are
j0.25pu and j0.35pu respectively, find the voltage at bus 1
under fault conditions
Q.6 (a) State the difference between disruptive critical voltage and
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visual critical voltage
(b) Determine the corona loss of a 3- 220 kV, 50 Hz, 200
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km long transmission line of three conductors each of
radius 1 cm and spaced 5m apart in equilateral triangle
formation. The air temperature is 300 C and the
atmospheric pressure is 760 mm of Hg. The irregularity
factor is 0.85
(c) Discuss factors affecting corona. State the methods to
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reduce corona
Q.7 (a) The following sequence currents were recorded in a power
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system under fault condition
Ia1 = -j1.65, Ia2 = j1.65, Ia0 = j0.0. Comment on the
nature of fault. Justify your answer
(b) Explain how voltage and current waves suffer reflection
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and refraction when line is short circuited at the receiving
end
(c) Derive the expression for attenuation of travelling surge
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wave on transmission line
Q.8 (a) Explain what is restriking voltage after removal of short
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circuit?
(b) Find the expression for fault current due to LLG fault
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(c) A 50 MVA, 11 kV, 3- alternator was subjected to
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different types of faults. The magnitude of fault currents
are as below:
3- fault = 1870 A, L-L fault = 2590A, L-G fault =
4130A. The neutral of the alternator is solidly grounded.
Calculate the positive and negative sequence reactance of
the alternator
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Figure 1
Figure 2
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This post was last modified on 04 March 2021