R10 Code No: R31023 Answer any FIVE Questions JNTU World€¦ · respectively. The supply voltage is 132 KV while the load end voltage is maintained 125 KV for all loads by an automatically

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Code No: R31023

III B.Tech. I Semester Supplementary Examinations, June/July -2014

POWER SYSTEMS - II

(Electrical and Electronics Engineering)

Time: 3 Hours Max Marks: 75

Answer any FIVE Questions

All Questions carry equal marks

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1. (a) Discuss how transposition helps in equalizing the capacitance in an unsymmetrically

spaced 3-phase overhead transmission line.

(b) Find the inductive reactance in ohms per km at 50Hz of a 3-phase bundled conductor line

with two conductors per phase as show in below fig. All the conductors are ACSR with radii

of 1.725m.

2. (a) Obtain the relation between the sending end and receiving end voltages and currents of a

medium line using nominal-II representation. Draw its equivalent circuit and phasor diagram.

(b) Input to a single phase short line shown in below fig. is 2000KW at 0.8 p.f lag. The line

has a series impedance of (0.4+j0.4) ohms. If the load voltage is 3KV, find the load and

receiving and p.f. Also find the supply voltage.

3. Draw the equivalent circuit of a long transmission line. Derive from fundamentals the

following relationship between sending and receiving end voltages and currents.

4. (a) A unit step voltage wave is travelling along a line of characteristic impedance Zc. The

line terminates is short inductance L. Derive the expression for the voltage at the line and the

inductance junction as a function of time.

(b) An overhead line with surge impedance Ω500 bifurcates into two lines of surge

impedance Ω500 and Ω50 .If a surge of 25KV is incident on the overhead line determine the

magnitudes of voltage and current which enter the bifurcated line.

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0.4m 0.4m 0.4m

7m 7m

3KV VS 2,000 KW

0.8 P.f lag

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Code No: R31023

5. (a) Explain the effect of shunt compensation on the performance of transmission lines.

(b) A certain 3-phase equilateral transmission line has a total corona loss of 53KW at 106KV

and a loss of 98 KW at 110.9 KV what are the disruptive critical voltages. What is the corona

loss at 113 KV.

6. (a) What is the sag templates? Explain.

(b) A transmission line over a hill side, where the gradient is 1:25, is supported by two 25m

high towers with a distance of 300m between them. The lowest conductor is fixed 2m below

the top of each tower. If the conductor weight 1.2 Kg/m and allowable tension is 1650 Kg,

find the clearance of the conductor from the ground.

7. (a) Explain the static shielding method of improving string efficiency.

(b) A3-phase transmission line is supported by a 3-unit suspension insulator string. The

voltage across the line unit is 20 KV and that across the adjacent unit is 15 KV. Determine (i)

ratio of ground to material capacitance (ii) system line voltage and (iii) string efficiency.

8. (a) Explain the effect of uncompensated line under no-load and load conditions.

(b) A 35 KW induction motor has p.f 0.9 lag and efficiency 0.9 at full load and p.f 0.75 lag

and efficiency 0.7 at half load. At no load the current is 25% of the full load current and p.f

0.2 lag. capacitors are supplied to make the line p.f 0.85 lag at halt load. With these

capacitors in the circuit, find the line p.f at (i) full load and (ii) no load.

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1. (a) What are the factors which govern the inductance of a transmission line.

(b) A 3-phase line with equilateral spacing of 3m is to be rebuilt with horizontal spacing

)( 221213 DDD == . The conductors are to be fully transposed. Find the spacing between

adjacent conductors such that the new line has the same inductance as the original line.

2. (a) Give the classification of transmission lines. Explain the influence of P.f on the

performance of a transmission lines.

(b) A 3-plase voltage of 11KV is applied to a line having Ω=10R and Ω= 12X per

conduction. At the end of the line is a balanced load of P KW at a leading P.f. At what value

of ‘P’ is the voltage regulation zero when the P.f of the load is (i) 0.707 and (ii) 0.85.

3. (a) Derive equations which represents the performance of a long transmission line with its

electrical parameters uniformly distributed along its length.

(b) The line constants of a 3-phase long line are oA 3.285.0 ∠= ,oB 75180 ∠= and

oC 900014.0 ∠= . Determine the sending end quantities, when the open circuit voltage at the

receiving end of the line is 220KV.

4. An overhead line with inductance and capacitance per km of 1.2 mH and 0.9 µF is connected

in series with an underground cable having inductance and capacitance of 0.16 mH/km and

0.28 µF/km. Calculate the values of transmitted and reflected waves of voltages and currents

at the junction due to a voltage surge of 110KV travelling to the junction. Along the line

towards the cable.

5. (a) Explain the effect of charging current on the performance of transmission line.

(b) A 3-Phase, 220 KV, 50Hz transmission line consists of 30 mm diameters conductor

spaced 2.5m apart in the form of an equilateral triangle. In the temperature is 38oC and

atmospheric pressure is 76 cm, Calculate the corona loss per km of the line. Assume the

irregular factor as 0.83.

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6. (a) Why it is necessary to provide sag while stringing the transmission line conductors.

(b) A transmission line conductor crossing a river is supported form two towers of heights of

30m and 80m above the water level. The horizontal distance between the towers is 450m. If

the tension of the conductors is 1500Kg and weight of the conductor is 1.4 Kg/m length. Find

the minimum clearance of the conductor and water and clearance mid way between the

supports.

7. (a) Explain how the string efficiency can be improved in practice.

(b) An insulator string has three units each having a safe working voltage of 15KV. The ratio

of unit self capacitance to stray capacitance of earth is10:1. Calculate the maximum safe

working voltage and the string efficiency.

8. (a) Explain the booster transformer with neat diagram? Where is it used.

(b) A star connected 400 HP, 2000V, 50Hz motor works at a P.f the 0.75 lag. A bank of mesh

connected capacitors is used to rise the P.f of 0.95 lag. Determine the capacitance of each

unit and total number of units required, if each is rated 500V, 50Hz . The motor efficiencies

is 85%.

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1. (a) Distinguish between AC and DC resistance of conductors. Why the two differ. Explain

fully.

(b) An overhead line 50km 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.

2. (a) Define regulation, efficiencies, losses and P.f at each end of the line and explain. how

these characteristic are affected by the constants of the lines.

(b) A 40MVA generating station is connected to a three phase line having Ω∠=oZ 75300 ,

mhoYo

900025.0 ∠=

The power at the generating station is 40MVA at units P.f at a voltages of 120KV. There is a

load of 10MW at UPF at the midpoint of the line. Calculate the voltage and load at the

distant end of the line use nominal –T circuit for the line.

3. Explain the physical significance of the generalized ABCD constants of a transmission line.

State the units of these constants. Determine these constants for a long transmission line.

4. (a) Define surge impedance of a line. Obtain the expressions for voltage and current waves at

a junctions or at a transitions point.

(b) An inductance of 700µH connects two sections of a transmission line each having a surge

impedance of Ω350 , A 400KV, 1µ sec rectangular surge travels along the line towards the

inductance. Find the maximum value of the transmitted wave.

5. (a) Explain the effect of skin and proximities effects on the performance of AC transmission

line.

(b) A 3-Phase,220KV,50Hz transmission line consists of 15mm a radius conductor spaced

2m apart in the form of an equilateral triangle. If the temperature is 40oC and atmospheric

pressure is 76cm. Calculates the corona loss per km of the line. Assume irregular factor is

0.85.

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6. (a) Discuss the importance of stringing chart and mention its applications.

(b) An overhead has the following data:

Span length = 200m, cross sectional area of the conductor =2.9 cm2, vertical sag =2.28 cm,

wind force =1.4 Kg/m run, breaking stress = 2480 Kg/cm2, weight of the conductor = 1.12

Kg/m run, calculate the safety factor.

7. (a) What do for understand by grading of insulators? Explain.

(b) Each conductor of a 3-Phase over head transmission line suspended form a cross arm of a

steel tower by a string of four suspension insulators. The voltage across the second unit is

15KV and across the third 20 KV. Find the voltage between conductors and string efficiency.

8. (a) What are the specifications of load compensation?

(b) A 3-phase transmission line has resistance and reactance per phase of Ω10 and Ω15

respectively. The supply voltage is 132 KV while the load end voltage is maintained 125 KV

for all loads by an automatically controlled synchronous phase modifier. If the KVAr rating

of a modifies has a same value for zero loads as for a load of 40MW, find the rating of phase

modifies.

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1. (a) Name and explain briefly the four parameters on which the performance of a transmission

line as an element of power systems depends?

(b) Calculate the 50Hz inductive reactance at 1m spacing in ohms/km of a cable consisting of

12 equal stands around a non conducting core. The diameter of each strand is 0.25cm and the

outer diameter of the cable is 1.25 cm.

2. (a) How are transmission lines classified.

(b) Using the nominal П method. Find the sending end voltage and voltage regulation of a

250 km 3-phase,50Hz transmission line delivering 25MVA at 0.8 lag P.f to a balanced load

at 132 KV. The line conductors are spaced equilaterally 3m apart. The conductor resistance is

0.11 ohm/km and its effective diameter is 1.6 cm. neglect leakage.

3. Starting form first principle derive an expression for the sending end voltage and current of a

long transmission line in terms of the line parameters and receiving end voltage and current.

4. An overhead line is connected to terminal apparatus through a length of single phase cable,

the characteristic impedance being 500 and Ω25 A travelling wave of vertical front and

infinites tail of 230 KV magnitude originates in the line and travels towards the junction with

the cable. Calculate the energy transmitted into the cable during a period 5 µ sec after the

arrival of the wave at the junction, what voltage is reflected back into the line.

5. (a) Explain the effect of Ferranti effect on the performance of transmission lines with neat

phasor diagram.

(b) Find the disruptive critical and visual corona voltage of a grid line operating at 132 KV.

The following data is given:

Conductor diameter = 1.9cm, conductor spacing = 3.81 cm

Temperature = 44oC, barometric pressure = 73.7cm

Conductor surface factor: Fine weather = 0.8, rough weather = 0.66.

6. (a) What is the effect of wind and ice on sag? Explain with diagram.

(b) An overhead line has a span of 180m. The conductor has cross sectional area of 1.22 cm2

and weight 1.12 Kg/m and has a breaking stress of 4150Kg/cm2. Allowing a wind pressure of

110 Kg/cm2, Calculate the sag of a factor of safety of 4.

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7. (a) What are the reasons of insulation failure? Why arcing horn is provided across the

insulators.

(b) For a string insulator with four discs, the capacitance of the disc is 12 times the

capacitance between the pin and earth. Calculates the voltage across each disc when used

on132KV. Also calculate the string efficiencies.

8. (a) Explain the objectives of load compensation.

(b) A 3-phase transmission line have an impedance of Ω+ )83( j supplies a load of 2.5 MW

at 0.8 P.f lag. The receiving and voltage is maintained at 11KV by means of static condenser

drawing 2MVAr form the line. Calculate the sending end voltage and P.f. What is the

regulation and efficiency of feeder.

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R10 Code No: R31023 Answer any FIVE Questions JNTU World€¦ · respectively. The supply voltage is 132 KV while the load end voltage is maintained 125 KV for all loads by an automatically