nov 2011

Code No: V3110/R07

III B.Tech I Semester Regular & Supplementary Examinations, November 2011

POWER SYSTEMS-II

(Electrical and Electronics Engineering)

Time: 3 Hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

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1. a) Briefly describe the VARIOUS types of conductors

b) Find the inductance of line consisting two ACSR conductors spaced between

conductors is 1.2m as shown in below Figure. The outer diameter of the single

layer of aluminum strand is 45mm and the radius of each strand is 7mm. Assuming

neglect the effect of the central strand of steel on inductance.

2. a) Define regulation, efficiency, losses and power factor at each end of the line and

explain how these characteristics are affected by the constants of the line

b) A short 3-phase transmission line with an impedance of (4+j7) ohm per phase has

sending end and receiving end voltages of 125kV and 112kV respectively for some

receiving end load at a power factor of 0.85 lagging. Determine (i) power output

and (ii) sending end power factor.

3. a) Derive the expressions for voltage and current distribution over a long line. Explain

the significance of characteristic impedance loading in connection with the long

lines. Deduce the above voltage and current relations in the hyperbolic form and

obtain the element values of an equivalent to represent the long lines.

b) A three-phase transmission line is 300km long and delivers a load of 150MVA,

0.85p.f lag at 220kV. The ABCD constants of the line are A=D=0.7∠1.80;

B=150∠800;

C =0.001∠900 mhos. Determine the following under full load and no load

conditions

i) Sending-end line to neutral voltage,

ii) The sending-end current, and

iii) The percent voltage regulation.

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Set No:1

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Code No: V3110/R07

4. a) Starting from first principles show that surges behave as travelling waves. Find

expressions for surge impedance and wave velocity

b) A 220 kV surge travels on a transmission line of 410 ohms surge impedance and

reaches a junction where two branch lines of surge impedances of 600 ohms and

350 ohms respectively are connected with the transmission line. Find the surge

voltage and current transmitted into each branch line. Also find the reflected

voltage and current.

5. a) Explain the Ferranti effect on EHV transmission lines with neat phasor diagram

b) Estimate the corona loss for a 3-phase, 130 kV, 50 Hz, 250 km long transmission

line consisting of three conductors, each of 12 mm diameter and spaced 2.8 meter

apart in an equilateral triangle formation. The temperature of air is 350C and the

atmospheric pressure is 750 mm of mercury. Taking the irregularity factor as 0.85.

6. a) Explain the strain and shackle insulators with the help of neat sketches

b) A string of four insulators has a self-capacitance of ‘C’ Farads. The shunting

capacitance of the connecting metal work of each insulator is 0.3 C to earth and 0.2

C to the line. A guard ring increases the capacitance to the line of the metal work of

the lowest insulator to 0.5 C. Determine the string efficiency of this arrangement

with the guard ring.

7. a) Explain the importance of conductor spacing and ground clearance of overhead

transmission lines

b) A copper conductor is suspended between two points at the same height and strung

to a tension of 1250 kg the load on the conductor is 1.8 kg/ m and its cross section

area 1.8 sq cm, the load on the conductor is increased to 2.8 kg/m. Calculate the

tension in the conductor and a sag under this move conditions the span is 200 m

and the modulus of the elasticity of the copper is 12.5 510× kg/sq cm

Use the approximate method, assuming the curve to be a parabola.

8. a)What are the factors determining the size of power cables? On which factors does

the current carrying condition depend

b) A single core cable has a conductor diameter of 2.5 cm and a sheath of inside

diameter 6 cm. Calculate the maximum stress. It is desired to reduce the maximum

stress by using two inter-sheaths. Determine their best positions, the maximum

stress and the voltage on each. System voltage is 66kV, 3-phase.

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Code No: V3110/R07


POWER SYSTEMS-II





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1. a) Derive an expression for the inductance per phase for a 3-phase overhead transmission

line when conductors are unsymmetrical placed but lines are un-transposed.

b) A three-phase, three-wire, 132 kV, 50 Hz overhead line conductor is placed in

horizontal plane as shown in the below Figure. The conductor diameter is 1.5 cm. If

the length of the line is 120 km, calculate (i) Capacitance per phase per meter,

(ii) Charging current per phase.

2. a)Explain how transmission lines are classified into short, medium and long lines and

explain their characteristics

b)The per-phase impedance of a short transmission line is (0.3 + j 0.6) ohm. The sending-

end line-to-line voltage is 3300 V, and the load at the receiving end is 320 kilowatts

per phase at 0.85 power factor lagging. Calculate (i) the receiving end voltage; (ii) the

line current; (iii) sending end power factor and (iv) power loss.

3. a) Explain the interpretation of the long line equations

b) A three phase, 50 Hz, 700 km long transmission line has the following line constants

per phase per km uniformly distributed r = 0.3 Ω ; x = 0.6 Ω ; g = 4 910× and b =

2.53 s6

10−

× . Find the auxiliary constants (i) by using convergent series of complex

angles ii) by using convergent series of real angles.

4. a) How can the analysis of a wave travelling on a line terminated by an inductance be

carried out? Derive the expression for the voltage across the terminating inductance

connected at the end of a line on which a step wave of magnitude ‘V’ is travelling.

b)Two stations are connected together by an underground cable having a surge

impedance of 50 ohms joined to an overhead line with a surge impedance of 300 ohms.

If a surge having a maximum valve of 120 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.

5. a)Explain briefly the factors which affect corona

b) An overload transmission line operates at 220 kV between phases at 50 Hz. The

conductors are arranged in a 4 meter delta formation. What is the maximum diameter

of conductor that can be used for no corona loss under fair weather conditions?

Assume an air density factor of 0.9 and irregularity factor of 0.82. The critical voltage

is 230 kV. Find also the power loss under storm conditions.

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Code No : V3110/R07

6. a) Explain different types of insulators used in overhead line? Give their field of

application.

b) A three phase over head transmission line is suspended by a suspension type insulator

which consists of three units. The potential across top unit and middle unit are 8 kV

and 12 kV respectively. Calculate (i) The ratio of capacitance between pin and earth to

the self capacitance of the each unit (ii) The line voltage and (iii) String efficiency

7. a) Show that an overhead line conductors strung between two level supports takes up the

shape of a parabola. Obtain expressions for the san and maximum tension when the

supports are at the same level.

b) An overhead line consists of 7 strands of copper, having a cross sectional area of 2.2 cm2

Weight of conductor =1.4kg/m, Ultimate strength =8000kg/cm2 ,Wind pressure =40 kg/m

2

of projected area. Calculate the vertical sag of the time for a span of 300 meters, assuming

a factor of Safety of 3.

8. a)Explain the need for grading of cables and what are the practical limitations of it

b) A single core 11 kV, 50Hz, 7.5 km long cable has a core diameter of 1.4 cm and

diameter of under sheath 3.0 cm. The relative permittivity of the insulating material is

5. The power factor on open circuit is 0.04. Determine (i) the capacitance of the cable

(ii) charging per conductor (iii) dielectric loss (iv)The equivalent insulation resistance.

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Set No:2


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POWER SYSTEMS-II





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1. a)What do you understand transposition in overhead lines? Explain why transposition is

done in such a line

b) Calculate the capacitance (phase-to-neutral) of a three-phase, 120 km long double

circuit line shown in below Figure with conductors of diameter 2 cm each arranged at

the corners of an hexagon with sides measuring 2.1 m.

2. a)What do you understand by generalized circuit constants of a transmission line? What

is their importance?

b) A three phase transmission line is 150 km long. The resistance per phase is 0.06 ohms

per km and the inductance per phase is 0.8 mH per km. The shunt capacitance is

0.0105 µF per km. The receiving end load is 90 MVA with 0.8 power factor lagging at

110 kV. Determine the voltage, powers at the sending end, voltage regulation and

efficiency by using nominal - π model.

3. a) Starting from first principle deduce expressions for ABCD constants of a long line

interms of its parameters? Define propagation constant and characteristic impedance.

b) A three-phase, 50 Hz and 200 km long line whose resistance per km is 0.02 Ω and

inductance per km is 0.6 mH and capacitance per km is 0.03 µF. Determine the

network constants of a long transmission line while neglecting the conductance of the

line.

4. a) Derive reflection and refraction coefficient of transmission line when terminated

through a resistance

b) An overhead line has a surge impedance of 500 Ω. A surge voltage V= 250(e0.05t

–e-t)

kV, where t is in µsec, travels along the line. The termination of the line is connected

to two parallel overhead line transformer feeders. The surge impedance of the feeder is

350Ω. These two transformers are protected by surge diverters each of surge

impedance being 40Ω. Determine the maximum voltage which would initially appear

across the feeder end windings of each transformer due to the surge. Assume the

transformer to have infinite surge impedance.

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Set No:3


Code No : V3110/R07

5. a) Describe the phenomenon of corona? Discuss the factors which affect corona loss

b)An overload transmission line operates at 210 kV between phases at 50 Hz. The

conductors are arranged in a 3.5 meter delta formation. What is the maximum diameter

of conductor that can be used for no corona loss under fair weather conditions?

Assume an air density factor of 0.9 and irregularity factor of 0.8. The critical voltage is

240 kV. Find also the power loss under storm conditions.

6. a)What are the advantages of suspension type insulators over pin type insulators

b) Find the potential difference across each unit of over head suspension insulators

connecting of four similar units. The potential between the line conductor and the earth

is 56 kV and the ratio of capacity of each insulator to the capacity 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.

7. a)What are disadvantages of providing too much or too small sag in a transmission line?

Name different types of line supports with their place of use

b)A transmission line conductor with diameter 14.5 mm, cross-sectional area of 125 mm2

weighing 1118 kg/km has a span of 200 meters. The supporting structures being level.

The conductor has an ultimate tensile stress of 42 kg/mm2 and allowable tension is not

to exceed 4

1 of ultimate strength. Determine the following

i) Sag in still air.

ii) Sag with a wind pressure of 60 kg/m2 and an ice coating of 10 mm.

Also calculate the vertical sag under this condition. Assume density

of ice as 0.915 gm/c.c.

8. a) Show that for the same dimensions of a cable with an intersheath can withstand 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-core, 3-phase metal sheathed cable has the following data. Capacitance between

all conductors bunched and sheath 0.8µF, Capacitance between two conductors

bunched with sheath and third conductor 0.6 µF.

Determine the capacitance when the sheath is insulated for the following conditions

(i) Between any two conductors

(ii) Between any two bunched conductors and the third conductor.

(iii) Calculate the capacitance to neutral and charging current taken

by the cable when connected to 66 kV, 3-phase, 50 Hz systems.

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Set No:3


Code No : V3110/R07


POWER SYSTEMS-II





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1. a) What is meant by the bundled conductors? Mention its merits and demerits?

b) Calculate the capacitance per phase of a three-phase double circuit line as shown in

below Figure. The diameter of the conductor is 2.2 cm. Assume that the line is

completely transposed.

2. a) Explain the physical significance of the generalized constants A, B, C and D.

b) A 50Hz, three phase transmission line is 250 km long. It has a total series impedance

of (20+j60) ohms and a shunt admittance of 914×10-6

mhos. It delivers 50 MW, 220

kV with 0.85 power factor lagging. Find the (i) sending end voltage, (ii) voltage

regulation and (iii) transmission efficiency by nominal-T method

3. a) Starting from first principles derive an expression for the sending end voltage and

current of a long transmission line interms of the line parameters and receiving end

voltage and current.

b) A three – phase overhead transmission line has series impedance per phase of 120

075∠ ohms and a total shunt admittance of 0.002 0

90∠ siemen per phase. The line

delivers a load of 125MW at 0.85 p.f lagging and 132kV between the

4. a)What is a travelling wave? Explain the development of such a wave along an overhead

transmission line

b) An over head line with inductance and capacitance per km length of 1.2 mH and 0.08

µF respectively is connected in series with an ungrounded cable having inductance and

capacitance of 0.16 mH / km and 0.28 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.

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Code No : V3110/R07

5. a) What is meant by the disruptive critical voltage and visual critical voltage? State the

effects of conductor size, spacing and condition of the surface of conductors on these

voltages

b) Find the critical disruptive voltage and the critical voltages for local and general

corona on a 3- phase overhead transmission line, consisting of 3-stranded copper

conductors spaced 2.2 m apart at the corners of an equilateral triangle. Air temperature

and pressure are 21o C and 73.6 cm of Hg respectively. Take conductor diameter 10

mm, irregularity factor 0.85, local and general surface factors 0.7 and 0.8 respectively.

6. a)Explain what is meant by a string efficiency of a suspension insulator consists of a

number of units. What causes the efficiency to be less than 100 percent? Describe any

one method of improving the same.

b) Each line of a three phase system is suspended by a string of 3 identical insulators of

self capacitance C farad. The shunt capacitance of the connecting metal work of each

insulator is 0.3C to earth and 0.2C to line. Calculate the string efficiency of the

system if the guard ring increases the capacitance to the line of the metal work of the

lowest insulator to 0.35C.

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

mm weighs 0.85 kg/m. The span length is 250 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. a) Explain the meaning of the term dielectric loss and power factor angle in cables with

the help of phasor diagram. How does dielectric loss vary with the change in voltage,

frequency of supply and capacitance of cable?

b) Determine the economical value of the diameter of a single core cable working on 66

kV, 3 phase system. Also calculate the overall diameter of the insulation if the

maximum permissible stress in the dielectric is not to exceed 50 kV/cm.

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Set No:4


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nov 2011