r05310203 Power Systems II
r05310203 Power Systems II
r05310203 Power Systems II
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III B.Tech I Semester Supplimentary Examinations, February 2008
POWER SYSTEMS-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
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2. (a) Show how regulation and transmission efficiency are determined for medium
lines using nominal - π method and illustrate your answer with suitable vector
diagram.
(b) An overhead 1-phase delivers a load of 1.5kW at 33kV at 0.9 p.f. lagging. The
total resistance and inductance of the over head transmission line is 8Ω and
15Ω respectively. Determine the following:
i. Percentage of voltage regulation
ii. Sending end power factor
iii. Transmission efficiency. [6+10]
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Code No: R05310203 Set No. 1
irregularity factor m= 0.85 (Roughness factor) mv for local corona= 0.72 and
mv for general corona=0.82. Find
i. Disruptive voltage
ii. Visual corona voltage for local corona
iii. Visual corona voltage for general corona and
iv. Power loss due to corona using Peek’s formula under fair weather and wet
conditions. [8+8]
6. (a) What is guard ring which is being used in the suspension string type insulator?
Deduce the relation for determining the capacitance formed by the ring.
(b) A three phase over head line is being supported by tree discs suspension in-
sulators, the potential across the first and second insulators are 12 and 18 kV
respectively. Calculate
i. the line voltage,
ii. the ratio of capacitance between pin and earth to self-capacitance of each
unit,
iii. the string efficiency. [8+8]
8. (a) Show that for the same dimensions of a cable with an intersheath can with-
stand a working voltage of 33% higher than a non-intersheath cable. Assume
same homogeneous dielectric and most economical designs for both cables.
(b) A 3-phase, single core 66 kV cable has a conductor diameter of 3 cm and a
sheath of inside diameter 6 cm. If two intersheaths are introduced in such a
way that the stress varies between the same maximum and minimum in the
three layers. Find
i. Positions of intersheaths
ii. voltage on the intersheaths
iii. Maximum and minimum stress. [8+8]
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Code No: R05310203 Set No. 2
III B.Tech I Semester Supplimentary Examinations, February 2008
POWER SYSTEMS-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆⋆⋆⋆⋆
1. (a) Determine the capacitance of a three-phase double circuit line when conduc-
tors are placed flat vertical unsymmetrical spacing.
(b) Three conductors of a 3-phase line are arranged at the corners of a triangle
of sides 2m, 3.2m and 4m.The diameter of each conductor is 2.5cm, Calculate
the inductance per km of the line. [8+8]
2. (a) Define regulation of a short 3-phase transmission system and develop an ex-
pression for approximate voltage regulation.
(b) A balanced 3-phase load of 30MW is supplied at 132kV, 50Hz and 0.85 p.f.
lagging by means of a transmission line. The series impedance of a single
conductor is (20 + j52) ohms and the total phase-neutral admittance is 315
× 10−6 mho. Using nominal-T method, determine:
i. The A, B, C and D constants of the line,
ii. Sending end voltage,
iii. Regulation of the line. [16]
4. (a) Show that a travelling wave moves along an overhead line with a velocity of
light and its speed is proportional to √1εr in case of a cable with dielectric
material of relative permittivity εr .
(b) Two stations are connected together by an underground cable having a surge
impedance of 60 ohms joined to an overhead line with a surge impedance of
400 ohms. If a surge having a maximum valve of 100 kV travels along the
cable towards the junction with the overhead line, determine the value of the
reflected and transmitted wave of voltage and current at the junction. [8+8]
5. (a) Derive an equation for calculating the maximum electric intensity on the con-
ductor surface of a 3-phase single circuit horizontal configuration line with two
sub-conductors per phase.
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Code No: R05310203 Set No. 2
(b) In a 3-phase overhead line, the conductors have an overall diameter of 3.0
cm each and are arranged in delta formation. Assuming a critical disruptive
voltage of 250 kV between lines and an air density factor of 0.90 and m0 =
0.95, find the minimum spacing between conductors allowable, assume fair
weather conditions. [8+8]
6. (a) Write a short notes on different types of insulators used for overhead lines and
their applications.
(b) Find the potential difference across each unit of over head suspension insula-
tors connecting of four similar units. The potential between the line conductor
and the earth is 58 kV and the ratio of capacity of each insulator to the ca-
pacity relative to earth, of each intermediate section of connecting work is 6:1.
it is assumed that no leakage takes place. Also find the string efficiency.[8+8]
8. (a) Compare the merits and demerits of underground system and overhead system.
(b) Determine the thickness of insulation and operating voltage of a single core
cable if the maximum and minimum stress in the dielectric is 38 kV/cm (r.m.s)
and 12 kV/cm (r.m.s) respectively and the diameter of core is 3 cm. [8+8]
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Code No: R05310203 Set No. 3
III B.Tech I Semester Supplimentary Examinations, February 2008
POWER SYSTEMS-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆⋆⋆⋆⋆
2. (a) Define regulation of a short 3-phase transmission system and develop an ex-
pression for approximate voltage regulation.
(b) A balanced 3-phase load of 30MW is supplied at 132kV, 50Hz and 0.85 p.f.
lagging by means of a transmission line. The series impedance of a single
conductor is (20 + j52) ohms and the total phase-neutral admittance is 315
× 10−6 mho. Using nominal-T method, determine:
i. The A, B, C and D constants of the line,
ii. Sending end voltage,
iii. Regulation of the line. [16]
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Code No: R05310203 Set No. 3
of an equilateral triangle. Air temperature and pressure are 210 C and 73.5
cm of Hg respectively. Conductor diameter is 1.8 cm, irregularity factor (m0 )
0.85, and surface factors (mv ) 0.7 for local and general corona 0.7 and 0.8
respectively. Breakdown strength of air is 21.1 kV (r.m.s) / cm. [6+10]
6. (a) Give reasons for unequal potential distribution over a string of suspension
insulators.
(b) Each line of a 3-phase system is suspended by a string of 3 similar insulators.
The voltage across the unit nearer to the line conductor is 12 kV. Calculate
the line to neutral voltage. Assume the shunt capacitance between each insu-
lator and earth is 1/6 capacitance of the insulator itself. Also find the sting
efficiency. [6+10]
8. (a) Discuss the methods of grading of cables. Why are they not used generally?
(b) A three-phase, single core, lead covered cable has radius of core 0.5 cm and
internal diameter of sheath 6 cm. Its 3 insulating materials A, B, and C have
relative permittivity of 4, 4, and 2.5 with maximum permissible stress of 50,
40, and 30 kV/cm respectively. Find the operating voltage of the cable. [8+8]
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Code No: R05310203 Set No. 4
III B.Tech I Semester Supplimentary Examinations, February 2008
POWER SYSTEMS-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆⋆⋆⋆⋆
3. (a) Using rigorous method, derive expressions for sending end voltage and current
for a long transmission line.
(b) A 3- phase transmission line is 480km long and serves a load of 400MVA,
0.8p.f lag at 345kV. The ABCD constants of the line are A=D=0.8186 1.30 ;
B=172.26 84.20 ; C=0.0019336 90.40 mhos.
i. Determine the sending end line to neutral voltage, the sending end current
and the percent voltage drop at full load.
ii. Determine the receiving end line to neutral voltage at no load, the sending
end current at load and the voltage regulation. [8+8]
4. (a) Starting from first principles show that surges behave as travelling wave.
(b) An over head line with inductance and capacitance per km length of 1.24 mH
and 0.087 µF respectively is connected in series with an ungrounded cable
having inductance and capacitance of 0.185 mH / km and 0.285 µF / km
respectively. Calculate the values of reflected and refracted waves of voltage
and current at the junction due to a voltage surge of 110 kV traveling to the
junction
i. along the line towards the cable, and
ii. along the cable towards the line. [8+8]
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Code No: R05310203 Set No. 4
(b) Determine the disruptive critical voltage and the visual critical voltages for
local and general corona on a 3-phase overhead transmission line consisting of
three stranded copper conductors spaced at 2.5 meters apart at the corners
of an equilateral triangle. Air temperature and pressure are 210 C and 73.5
cm of Hg respectively. Conductor diameter is 1.8 cm, irregularity factor (m0 )
0.85, and surface factors (mv ) 0.7 for local and general corona 0.7 and 0.8
respectively. Breakdown strength of air is 21.1 kV (r.m.s) / cm. [6+10]
6. (a) Explain why suspension type of insulators are preferred for high voltage over-
head lines. Sketch a sectional view of one unit of the suspension type insulator
and describe the construction.
(b) An insulator string containing five units has equal voltage across each unit by
using disc of different capacitances. If the top unit has a capacitance of C and
pin to tower capacitance of all units is 20 percent of the mutual capacitance
of top unit. Calculate mutual capacitance of each disc in a string. [8+8]
7. (a) What are the various types of line supports? Discuss the suitability of each
with reference to system voltage and span.
(b) Determine the maximum sag of an overhead line conductor having a diameter
of 19.5 mm weighs 0.85 kg/m. The span length is 275 meters, wind pressure is
40 kg/m2 of projected area with ice coating of 13 mm. The ultimate strength
of the conductor is 8000 kg, the factor of safety is 2 and ice weighs 910 kg/m3 .
[8+8]
8. (a) What do you understand by grading of cable? Explain why grading is more
of theoretical interest than practical? What is the modern practice adopted
to avoid grading?
(b) Determine the maximum and minimum stress in the insulation of a 33 kV
single core cable which has a core diameter of 1.5 cm and a sheath of inside
diameter 5 cm. [8+8]
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