Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech III POWER SYSTEMS II Previous Question Paper (October - 2020 - R16)
Answer any five questions. All questions carry equal marks.
Main
What is transposition and derive an expression for inductance of 3-phase double circuit line with transposition.
Answer
Transposition is the process of interchanging the positions of conductors at regular intervals along a transmission line to ensure that each conductor occupies each physical position for one-third of the total line length. This practice balances the line parameters, specifically inductance and capacitance, across the three phases, thereby preventing unbalanced voltage drops and reducing interference with neighboring communication lines.
For a 3-phase double-circuit line, the inductance per phase is derived by considering the Geometric Mean Radius (GMR) and Geometric Mean Distance (GMD). Let the conductors be arranged in two circuits (1, 2, 3 and 1', 2', 3'). The inductance per phase is given by:
Where the equivalent GMD is the cube root of the product of the mutual distances between phase groups, and the equivalent GMR is the cube root of the product of the GMRs of the three phases, accounting for the bundle or double-circuit configuration.
A 3-phase, 50 Hz, 132 kV overhead line has conductors placed in a horizontal plane 4 m apart. Conductor diameter is 2 cm. If the line length is 100 km, calculate the charging current per phase assuming complete transposition.
Answer
Given: V_L = 132 kV, f = 50 Hz, d = 2 cm (r = 0.01 m), spacing D_12 = 4 m, D_23 = 4 m, D_13 = 8 m, length l = 100 km.
First, calculate the equivalent spacing (GMD):
Calculate the capacitance per phase (C) using the formula for a transposed line:
For 100 km (10⁵ m), C_total = 9.65 × 10⁻⁷ F. The phase voltage V_ph = 132000 / √3 = 76210 V.
The charging current I_c is:
The charging current per phase is approximately 23.13 Amperes.
Determine the efficiency and regulation of a 3-phase, 50 Hz, 150 km long transmission line having three conductors spaced 3.5 meters delta formation when the receiving end delivers 25 MVA at 120 kV and p.f. 0.9 lagging. The resistance of the conductor is 0.25 ohm per km and the effective dia is 0.75 cm. Neglect leakage and use (a) nominal-T (b) nominal-π methods.
Answer
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Determine A, B, C, D parameters of a 3-phase, 50 Hz, 400 km long line having per unit impedance of (0.15 + j0.78) ohm per km and a shunt admittance of j5.0 × 10-6 ohm per km.
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An overhead line with inductance and capacitance per km of 1.24 mH and 0.087 μF respectively are connected in series with an underground cable having inductance and capacitance of 0.185 mH/km and 0.285 μF/km. Calculate the values of transmitted and reflected waves of voltage and current at the junction due to a voltage surge of 110 kV traveling to the junction (a) along the line towards the cable, and (b) along the cable towards the line.
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What is skin effect? Why is it absent in the d.c. system?
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What are the factors affecting corona?
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Discuss the advantages and disadvantages of (a) pin-type insulators (b) suspension type insulators.
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Derive the expressions for sag and tension when the supports are at equal heights.
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A transmission line conductor having a diameter of 19.5 mm weights 0.85 kg/m. The span is 275 metres. The wind pressure is 39 kg/m2 of projected area with ice coating of 13 mm. The ultimate strength of the conductor is 8000 kg. Calculate the maximum sag if the factor of safety is 2 and ice weighs 910 kg/m3.
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Derive an expression for the insulation resistance of a single-core cable.
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The capacitance per kilometre of a 3-phase belted cable is 0.3 μF between the two cores with the third core connected to the lead sheath. Calculate the charging current taken by five kilometres of this cable when connected to a 3-phase, 50 Hz, 11 kV supply.
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