JNTU
JNTU B.Tech III POWER SYSTEMS II Previous Question Paper (July/August - 2023 - R16)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
What is meant by geometric mean distance?
Answer
The Geometric Mean Distance (GMD), also known as the mutual geometric mean distance, is the equivalent distance between two groups of conductors. It is defined as the n-th root of the product of all possible distances between the conductors of one group and the conductors of the other group.
where m and n are the number of conductors in the two groups, and D represents the distance between individual conductors.
What factors must be taken into account while calculating the resistance of overhead line conductors?
Answer
The resistance of an overhead line conductor is determined by the following primary factors:
- Material of the conductor: The resistivity of the material (e.g., copper or aluminum) directly influences the resistance, as defined by the formula R = ρ(L/A).
- Temperature: Resistance increases with temperature according to the linear relationship R₂ = R₁[1 + α(T₂ - T₁)], where α is the temperature coefficient of resistance.
- Skin Effect: At higher frequencies or for large cross-sectional areas, current tends to flow near the surface of the conductor, effectively reducing the cross-sectional area used and increasing the AC resistance compared to the DC resistance.
- Stranding and Spiraling: Overhead lines are typically stranded to provide flexibility. The spiraling of strands increases the actual length of the conductor compared to the straight-line distance, which slightly increases the effective resistance.
What is the purpose of an overhead transmission line?
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What is mean by surge impedance loading?
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What are the types of system transients?
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What is meant by the disruptive critical voltage and visual critical voltage?
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Define the sag.
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What are the methods for improvement of string efficiency?
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What are the types of cables?
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What do you understand be the term grading of cable?
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Part B
Show that the inductance of each conductor of a single phase line using composite conductor is given by
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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.
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State the factors which governs the capacitance of a transmission line.
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Calculate the capacitance charging current of a 100 km long, 3 phase, 66kV, 60Hz, overhead transmission line consisting of 3-conductors each of diameter 2.2 cm and spaced 3.5 meters at the corners of an equilateral triangle.
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Draw the phasor diagram of a short transmission line and derive an expression for voltage regulation.
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A 100km long 3-phase, 50Hz overhead transmission line has the following line constants: Resistance/km/phase = 0.1 ohm, Inductive reactance/km/phase = 0.5 ohm, Capacitive susceptance/km/phase = 10^-6 mho. If the line supplies a load of 20MW at 0.9 p.f. lagging at 66kV at the receiving end, Calculate by nominal-π method (i) the sending end power factor (ii) regulation (iii) transmission efficiency.
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Starting from first principles 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.
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Given A=D=(0.936+j 0.016); B=(33.5+j138) ohms; C=(-5.18+j914) 10^-6 mhos. The load at the receiving end is 50 MW at 220 kV with a power factor of 0.9 lagging. Find the sending end voltage and regulation of line.
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Develop an expression for the voltage across a capacitance C connected at the end of a line of surge impedance Zc when a step wave V travels on the line and reaches the capacitance.
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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 value 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.
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Explain the phenomena of corona and derive an expression for disruptive critical voltage.
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Describe the i) ferranti effect ii) skin effect of transmission lines.
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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.
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A three phase overhead line is being supported by three disc suspension insulators, the potential across the first and second insulators, the potential across the first and second insulators are 7 and 10 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.
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What is a stringing chart? Explain clearly the procedure adopted for stringing the power conductors on the supports?
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An overhead conductor consists of seven strands of silicon-bronze having cross-sectional area of 2.0 cm^2 and ultimate strength of 10000 kg/cm^2. When connected between supports 650 m apart and having a 20 m difference in level, determine the vertical sag, which must be allowed such that the factor of safety shall be 5. Assume the wire weight 2 kg/m, ice loading is 1 kg/m and wind loading is 1.75 kg/m.
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Show that the insulation resistance of a cable is inversely proportional to its length.
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A 3 km long, 11kV, 3-phase, 3-core, belted cable have the following data: Capacitance between two conductors joined to sheath and the third conductor is 1.8 μF. Capacitance between all the three conductors joined and the sheath is 2.1 μF. Determine Effective capacitance of each core to neutral and Capacitance between any two cores.
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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?
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In a 3-phase, 3-core metal-sheathed cable the measured capacitance between any two cores is 2.1 μF. Determine the kVA taken by the cables when it is connected to 50 Hz, 13.2 kV bus bars.
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