Code No: 45008
R07
Set No - 1
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III B.Tech I Semester Regular Examinations,Nov/Dec 2009 POWER SYSTEM-II Electrical And Electronics Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) What do you understand by the constants of an over head transmission line?
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(b) A single phase transmission line has two parallel conductors 1.5 meters apart, the diameter of each conductor being 0.5 cm. Calculate line to neutral capacitance for a line of 150 km long [8+8] 2. Explain the following in terms with reference to corona? (a) Critical disruptive voltage (c) Power loss due to corona.
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(b) Visual critical voltage
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3. (a) What do you understand by long transmission lines? How capacitance effects are taken into account in such lines?
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(b) What is the justification in neglecting line capacitance in short transmission lines? [8+8] 4. A transmission line has a span of 250m between the level supports. The cross sectional area of the conductor is 1.29 cm2 . The ultimate strength is 4220 kg/cm2 and the factor of safety is 2. If the wind pressure is 40 Kg/cm2 . Calculate the height of the conductor above ground level at which it should be supported if a minimum clearance of 7m is to be kept between the ground and the conductor.[16]
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5. Starting from first principles show that surges behave as traveling waves. Find expressions for surge impedance and wave velocity. [16]
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6. Each of the three insulators forming a string has a self capacitance of C farad. The shunt capacitance of each insulator is 0.2C to earth and 0.1C to line. A guard ring increases the capacitance of line of metal work of the lowest insulator to 0.3C.calculate the string efficiency of the arrangement with and without guard ring. [16] 7. (a) State the classification of cables (according to voltage) and discuss their general construction (b) Explain carefully the constructional difference between and application of i. deleted ii. screened (H type) iii. S.L and 1
Code No: 45008
R07
Set No - 1
iv. H.S.L types of cables. (c) write a short note on pressure cables.
[16]
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8. A system consists of long line of surge impedance 400 Ω, a cable of length 300 m, surge impedance 50 Ω, a line of length 300 m, surge impedance 400 Ω, a cable of length 300 m, surge impedance 50 Ω and a long line of surge impedance 400 Ω. The velocity of propagation of wave is 300 m/µsec in line and 150 m/µsec in cable. A step wave of 100 KV travels along one of the long lines. Draw the Bewley lattice diagram and plot voltage versus time at the junction of long line and cable for 10 µsec. [16]
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Code No: 45008
R07
Set No - 2
1. (a) What is corona?
[16]
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(b) Explain the theory of corona formation in detail?
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III B.Tech I Semester Regular Examinations,Nov/Dec 2009 POWER SYSTEM-II Electrical And Electronics Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ?????
2. The generalized circuit constants of a transmission line are A = 0.93 + j0.016 B = 20 + j140 The load at the receiving end is 60 MVA, 50 HZ, 0.8 p.f. lagging. The voltage at the supply end is 220 KV. Calculate the load voltage [16]
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3. A transmission line has a span of 200m between level supports. The cross sectional area of the conductor is1.29 cm2 weighs 1170Kg/Km and has breaking stress of 4218Kg/cm2 . Calculate the sag for a factor of safety of 5, allowing wind pressure of 122 Kg per square meter of projected area. What is the vertical sag? [16]
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4. Derive an expression for the inductance per phase for a 3-phase over head transmission line (a) Conductors are symmetrically placed (b) Conductors are unsymmetrically placed but the lines is completely transposed. [16]
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5. An overhead line of surge impedance 400Ω is connected in series with a cable surge impedance 40 Ω. The cable is 5 km in length and is open at its far end. A surge of peak value 100 KV travels along the overhead line towards the junction of the line and the cable. Find the voltage distribution at the instants 1 and 50 µs after the surge reaches the junction. The velocities of propagation in the overhead line and the cable are 3 x 105 km/s and 1.4 x 105 km/s respectively. [16]
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6. A 1-core cable has a core diameter of 8 mm and a diameter under the sheath of 2 cm. The relative permittivity of the dielectric is 4. The power factor on open circuit is 0.03. Calculate for 1 km length of the cable (a) the capacitance
(b) its equivalent insulation resistance (c) the charging current (d) the dielectric loss when the cable is connected to 10 kv, 1-phase 50 Hz supply system. [16]
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Code No: 45008
R07
Set No - 2
7. A string of 5 insulators is connected across 100kV line. If the self capacitance of each unit is equal to 5 times pin to earth capacitance. Calculate (a) the potential difference across each unit and string efficiency?
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8. Calculate the velocity of propagation for waves
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(a) in an over head line of capacitance 0.147 x 10−10 F/m and inductance 0.75 x 10−6 H/m (b) in a cable of inductance 0.75 x 10−6 H/m and capacitance 13.3 x 10−10 F/m
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(c) estimate the relative permittivity of insulating material in case (ii)
Code No: 45008
R07
Set No - 3
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1. (a) What is corona? (b) What are the factors which effect corona? (c) Discuss the advantages and disadvantages of corona?
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III B.Tech I Semester Regular Examinations,Nov/Dec 2009 POWER SYSTEM-II Electrical And Electronics Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ?????
2. Calculate A, B, C, D constants of a 3-phase, 50 Hz transmission line 160 km long having the following distributed parameters: R = 0.15Ω/km; L = 1.20 x 10−3 [16] H/km; C = 8 x 10−9 F/km; G = 0.
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3. (a) Name the important components of an overhead transmission line? (b) Discuss the various conductor materials used for overhead lines. What are their relative advantages and disadvantages? [8+8] 4. Deduce an expression for line to neutral capacitance for a 3-phase over head transmission line when the conductors are
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(a) symmetrically placed (b) unsymmetrically placed but transposed.
[16]
5. (a) Write a short note on pressure cables (b) Calculate the insulation resistance for a 5 km length of a 1-core cable. Resistance of insulation (impregnated paper) is 5*1014 ohm-cm, insulation thickness is 1 cm and radius of conductor is 1.25 cm. [8+8]
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6. A transmission line has a span of 275m between level supports. The conductor has an effective diameter of 1.96 cm and weighs 0.865 kg/m. The ultimate strength is 8060kg. If the conductor has ice coating of radial thickness 1.27 cm and is subjected to a wind pressure of 3.9 gm/ cm2 of projected area, calculate sag for a safety factor of 2.Weight of 1c.c of ice is 0.91 gm. [16]
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7. An overhead transmission line with surge impedance 400Ω is 300 km long. One end of this line is short circuited and at the other end a source of 11KV is suddenly switched on. Calculate the current at source end 0.005 sec after the voltage is applied. [16] 8. The ends of two long transmission lines A and C are connected by a cable B km long. The surge impedances of A, B and C are 600, 75 and 500 Ω respectively. A rectangular voltage wave of 20 KV magnitude and of infinite length is initiated in A and travels to C. Determine the first and second voltages impressed on C. [16] ????? 5
Code No: 45008
R07
Set No - 4
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III B.Tech I Semester Regular Examinations,Nov/Dec 2009 POWER SYSTEM-II Electrical And Electronics Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) What are the factors which effect corona?
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(b) Describe the various methods for reducing corona effect in an overhead transmission line? [8+8] 2. A 3-core 3-phase metal sheated cable on testing for capacitance gave the following results : (a) Capacitance between all conductors bunched and sheath = 0.90µ F
(a) between any two conductors
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(b) Capacitance between two conductors bunched with sheath and third conductor=0.4 µ F. With the sheath insulated find the capacitance
(b) between any two bunched conductors and the third conductor.
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(c) Calculate the capacitance to neutral and charging current taken by the cable when connected to 11kv, 3-phase, 50 Hz system. [16] 3. A string of 5 insulators has self-capacitance equal to 5 times the pin to earth capacitance. calculate (a) the voltage distribution across various units as a percentage of total voltage across the string and [16]
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(b) string efficiency.
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4. A 3-phase transmission line, 160 km long has the following constants: Resistance/phase/km = 0.2Ω Reactance/phase/km = 0.3127 Ω Shunt admittance/phase/km = 1.875 x 10−6 S Determine the sending end voltage and current by rigorous method when the line is delivering a load of 25 MVA at 0.8 p.f. lagging. The receiving end voltage is kept constant at 110 KV. [16] 5. A transmission line over a hill side where the gradient is 1:20 , is supported by two 22m high towers with a distance of 300m between them. The lowest conductor is fixed 2m below the top of each tower. Find the clearance of the conductor from the ground. Given that conductor weighs 1Kg/m and the allowable tension is 1500 kg. [16]
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Code No: 45008
R07
Set No - 4
6. A 3-phase, 50 Hz over head line has regularly transposed conductors equilaterally spaced 4 m apart. The capacitance of such a line is 0.01µF/km. Recalculate the capacitance per km to neutral when the conductors in the same horizontal plane with successive spacing of 4 m and are regularly transposed. [16]
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7. What is an equivalent π and equivalent T circuit of a long line? Derive expressions for parameters of these circuits in terms of line parameters. [16]
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8. A cable with a surge impedance of 100 Ω is terminated in two parallel connected, open wire lines having a surge impedance of 600 and 1000 Ω respectively. If a steep fronted voltage wave of 1000 V travels along the cable, find from the first principles the voltage and current in the cable and the open wire lines immediately after the travelling wave has reached the transition point. The line may be assumed to be of infinite length. [16]
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