Code No: R31023
R10
Set No: 1
III B.Tech. I Semester Regular and Supplementary Examinations, December-2013
POWER SYSYTEMS-II
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) What is the effect of unsymmetrical spacing of conductors in a 3-phase transmission
line.
b) Derive the formula for the internal inductance in H/m of a hollow conductor having
inside radius r1 and outside radius r2 and also determine the expression for the inductance in
H/m of a single - phase line consisting of the hollow conductors described above with
conductors spaced a distance D apart.
c) A three phase, 50 Hz, 110 kV, transmission line has flat horizontal spacing with 3.5m
between adjacent conductors. The conductor diameter is 1.05 cm. Find the capacitance to
neutral and the charging current per km of line. Assume full transposition.
2. a) What is the justification in neglecting line capacitance in short transmission lines?
b) What is the purpose of an overhead transmission line? How are these lines classified?
c) Find the A, B, C, D parameters of a 3-phase, 50 Hz transmission line with series
impedance (20+j40) Ω and a shunt admittance of 4 x10-6 mho based on nominal-T
method.
3. a) What do you understand by long transmission lines? How capacitance effects are taken
into account in such lines?
b) A 50 Hz transmission line 300 km long has a total series impedance of 40+j125 ohms
and a total shunt admittance of 10-3 mho. The receiving-end load is 50Mw at 220kV with
0.8 lagging power factor. Find the sending-end voltage, current, power and power factor
using approximation.
4. a) When the transmission line is terminated by the inducting load. How do you find the
reflected voltage and current wave?
b) A voltage having a crest value of 3000 kV is traveling on a 750 kV line. The protective
level is 1700 kV and the surge impedance of the line is 300Ω. Calculate (i) the current in
the line before reaching the arrester, (ii) current through the arrester, (iii) the value of
arrester resistance for this condition and (iv) reflect voltage. Verify the reflection and
refraction coefficient.
5. a) What is corona? What are the factors which affect corona?
b) A certain 3-phase equilaterally spaced transmission line has a total corona loss of
53kW at 106kV and a loss of 98kW at 110.9 kV. What is the disruptive critical voltage
between lines? What is the corona loss at 113 kV?
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Code No: R31023
R10
Set No: 1
6. a) How many types of suspension insulators are used in transmission systems? Explain
them.
b) Why String efficiency for a d.c. system is 100%? Discuss.
c) In a 33kV overhead line, there are three units in the string of insulators. If the
capacitance between each insulator pin and earth is 11% of self-capacitance of each
insulator, find (i) the distribution of voltage over 3 insulators and (ii) string efficiency.
7. a) How do elasticity and temperature affect the sag and tension of the conductor? Find out
the expression of tension when both of them are taken into account.
b) A transmission line has a span of 150 m between level supports. The conductor has a
cross-sectional area of 2cm2. The tension in the conductor is 2000kg. If the specific
gravity of the conductor material is 9.9 g/cm 3 and wind pressure is 1.5 kg/m length,
calculate the sag. What is the vertical sag?
8. a) Explain the causes of low power factor I the power systems.
b) A 3-phase overhead line has resistance and reactance per phase of 50Ω and 20 Ω
respectively. The load at the receiving end is 25mw and 33kV and a p.f of 0.8 lag. Find
the capacity of the synchronous condenser required for this load connection if it is
connected at the receiving end and the line voltage at both ends are maintained at 33kV.
**
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Code No: R31023
R10
Set No: 2
III B.Tech. I Semester Regular and Supplementary Examinations, December-2013
POWER SYSYTEMS-II
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) What is bundled conductor? Where and why is it used? What is the GMR of a tour
strand bundle.
b) Find the inductance of line consisting two ACSR conductors spaced between
conductors is 1.3m as shown in below Figure. The outer diameter of the single layer of
aluminum strand is 45.2mm and the radius of each strand is 7.3mm. Assuming neglect the
effect of the central strand of steel on inductance.
2. a) Why do we analyze a 3-phase transmission line on single phase basis?
b) What is the effect of load power factor on regulation and efficiency of a transmission
line?
c) A single phase overhead transmission line delivers 1100kW at 33 kV at 0.8 p.f.
lagging. The total resistance and inductive reactance of the line are 10Ω and 15Ω
respectively. Determine: (i) sending end voltage (ii) sending end power factor (iii)
transmission efficiency.
3. a) Using rigorous method, derive expressions for sending end voltage and current for a
long transmission line.
b) A three-phase, 50 Hz transmission line is 400km long. The voltage at the sending-end is
220kV. The line parameters are r =0.125 ohm/km, x = 0.4 ohm/km and y= 2.8x10 -6
mho/km. Find (i) the sending -end current and receiving-end voltage when there is
no-load on the line.(ii) The maximum permissible line length if the receiving end no-load
voltage is not to exceed 235kV.
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Code No: R31023
R10
Set No: 2
4. a) Develop the differential equation for the transient in the transmission systems. How
voltage and current expressions are establish from the above differential equation?
b) Two stations are connected together by an underground cable having a surge
impedance of 30 ohms joined to an overhead line with a surge impedance of 300 ohms. If a
surge having a maximum value of 115 kV travels along the cable towards the junction
with the overhead line, determine the value of the reflected and the transmitted wave of
voltage and current at the junction.
5. a) Explain skin effect and Ferranti effect of transmission lines?
b) A 3-phase, 220kV, 50 Hz transmission line consists of 1.5 cm radius conductor spaced
2 meters apart in equilateral triangular formation. If the temperature is 400c and
atmospheric pressure is 76 cm, calculate the corona loss per km of the line. Take
m0=0.85.
6. a) Explain various methods of improving string efficiency.
b) An insulator string consists of three units, each having a safe working voltage of 15kV.
The ratio of self-capacitance to shunt capacitance of each unit is 8:1. Find the maximum
safe working voltage of the string. Also find the string efficiency.
7. a) What do you mean by sag on transmission line? Find out the expression of the sag of
the transmission line.
b) A transmission line has a span of 200 m between level supports. The conductors has
across-sectional area of 1.29 cm 2,weighs 1170 kg/km and has a breaking stress of 4218
kg/cm2 .Calculate the sag for a safety factor of 5,allowing a wind pressure of 122kg per
square meter of projected area. What is the vertical sag?
8. a) Explain the importance of voltage control in the modern power systems.
b) A 3-phase, 50Hz, 400V motor develops 75 kw, the p.f being 0.75lag and efficiency
92%. A bank of capacitors is connected in delta across the supply terminals and p.f raised
to 0.95 lag. Each of the capacitance units is built of 4 similar 100V capacitors. Determine
the capacitance of each capacitor.
**
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Code No: R31023
2R10
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Set No: 3
III B.Tech. I Semester Regular and Supplementary Examinations, December-2013
POWER SYSYTEMS-II
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Calculate the capacitance of a two wire line. If the conductors are spaced equally, How
do you calculate the capacitance of a three -phase line?
b) A 220 kV, three-phase bundle conductor line with two sub conductor per phase has a
horizontal configuration as shown in below Figure. Find the inductance per phase, if the
radius of each sub conductor is 1.35cm
2. a) What are the drawbacks of localized capacitance methods?
b) Deduce an expression for voltage regulation of a short transmission line, giving the
vector diagram.
c) The following data refers to a 50 Hz, three-phase transmission line:
length 15 km; sending-end voltage =11 kV; load delivered at receiving end 75 kW at
0.85 p.f. lag; resistance of each conductor = 0.3 ohms/ km; reactance per phase = 0.5
ohms/km. Find (i) receiving-end voltage, (ii) line current and (iii) efficiency of
transmission.
3. a) Prove that the magnitude of the current in a transmission line increases exponentially
from receiving end when the receiving end impedance equal to the characteristic
impedance.
b) The per-unit-length parameters of a 220kV, 450km, 50Hz, three phase long
6
transmission line are y  j 3 2 10 mhos per km per phase and z = (0.2 + j 0.5)
ohm/km. The line supplies a 175 MW load at unity power factor. Determine (i) the
voltage regulation, (ii) the sending-end power and (iii) the efficiency of transmission by
using rigorous method.
4. a) Explain the causes for transients on power system.
b) A unit -step voltage surge is travelling on a long line of surge impedance Z1. It reaches the
junction with a cable of finite length whose far end is open. The cable has a surge
impedance of Z2 and time of one-way wave travel on it is T. Draw the Bewley lattice
diagram and find from it the value of voltage at the junction at time 4T after the surge
reaches the line-cable junction. Given Z1/Z2=9.
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Code No: R31023
1R10
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Set No: 3
5. a) Describe the various methods for reducing corona effect in an overhead transmission
line.
b) A 3-phase, 132kV line with 1.956 cm dia. conductor is built so that corona takes place,
if the line voltage exceeds 210kV (r.m.s.) . If the value of potential gradient at which
ionization occurs can be taken as 30kV per cm, find the spacing between the conductors.
6. a) What is strain insulator and where is it used? Give a sketch to show its location.
b) A string of 5 insulators is connected across a100kV line. If the capacitance of each disc
to earth is 0.1 of the capacitance of the insulator, calculate (i) the distribution of voltage
on the insulator discs and (ii) the string efficiency.
7. a) Deduce an approximate expression for sag in overhead lines when (i) supports are at
equal levels (ii) supports are at unequal levels.
b) A transmission line has a span of 214 meters between level supports. The conductors
have across-sectional area of 3.225 cm2. Calculate the factor of safety under the following
conditions:
Vertical sag= 2.35 m;
wind pressure = 1.5 kg/m run
2
Breaking stress = 2540 kg/cm ;
Weight of conductor = 1.125 kg/m run
8. a) Describe the synchronous condenser method of voltage control for the transmission
line. Illustrates your answer with a vector diagram.
b) A 3-phase motor connected to 400V, 50Hz supply takes 31.7A at a p.f of 0.7lag.
Calculates the capacitance required in parallel with the motor to raise the p.f 0.9 lag.
**
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Code No: R31023
R10
Set No: 4
III B.Tech. I Semester Regular and Supplementary Examinations, December-2013
POWER SYSYTEMS-II
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) What do you understand by the constants of an overhead transmission line?
b) An overhead line 50 km in length is to be constructed of conductors 2.56 cm in
diameter, for single-phase transmission. The line reactance must not exceed 31.4 ohms.
Find the maximum permissible spacing.
c) Find the capacitance of phase to neutral per km of a three-phase line having conductors
of 2 cm diameter placed at the corners of a triangle with sides 5 m, 6 m, and 7 m
respectively. Assume that the line is fully transposed and carries balanced load.
2. a) Why is leakage conductance negligible in overhead lines? What about underground
system?
b) Discuss the terms voltage regulation and efficiency as applied to transmission line.
c) A 3-phase, 50 Hz, overhead transmission line delivers 10MW at 0.8p.f lagging and at
66kV.The resistance and inductive reactance of the line per phase are 10Ω and 20Ω
respectively while capacitance admittance is 4x10-4 siemen. Calculate : (i) the sending
end current (ii) sending end voltage( line to line) (iii) sending end power factor (iv)
transmission efficiency. Assume nominal T method.
3. Deduce expression for the magnitudes of incident and reflected voltages at any point on a
transmission line in terms of voltage and current at receiving end characteristic
impedance and propagation constant of the line. Show how these expressions are useful in
determining voltage and current distribution along the line?
4. a) Explain about the surge impedance and give its significance.
b) A voltage pulse of magnitude E0 and length D travels on a transmission line of
characteristics impedance Z c at speed γ. In the middle of the line it meets a series lumped
inductance L. Find the expression for the voltage wave as a function of time that will be
injected into the line on the other side of the inductance. Also sketch the voltage wave
form.
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Code No: R31023
R10
Set No: 4
5. a) What is skin effect? Why is it absent in the D.C system?
b) A three phase 220KV 50Hz transmission line consists of 1.2cm radius conductor
spaced 2 meters apart in equilateral triangular formation. Find the disruptive critical
voltage between the lines, if the temperature is 200C and atmospheric pressure is 72.2cm.
Take mo=0.96. Dielectric strength of air=21.1KV (rms)/cm.
6. a) How many types of suspension insulators are used in transmission systems? Explain
them.
b) Each line of a three phase system is suspended by a string of 4 similar insulators. If the
voltage across the second unit is 15 kV and across the third unit is 27.0 KV, calculate the
voltage between conductors and string efficiency.
7. a) Explain effect of wind and ice on sag of the conductor.
b) A transmission line conductor at a river crossing is supported from two towers at
heights of 50 and 70 meters above water level. The horizontal distance between the
towers is 270m. If the tension in the conductor is 2000kg, find (i) the maximum clearance
between the conductor and water, (ii) the clearance between the conductor and water at a
point mid way between the towers. Weight of conductor is 0.75 kg/m. Assume that the
conductor takes the shape of a parabola.
8. a) Discuss the on load tap changing transformer method of voltage control. Lis out its
merits.
b) A load of 10,000kW at a p.f of 0.8 lag is supplied by a 3-phase line whose voltage has
to be maintained at 33kV at each end. If the line resistance and reactance per phase are 5Ω
and 10 Ω respectively. Calculate the capacities of the synchronous condenser to be
installed for the purpose comment on the results.
**
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