Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech III POWER SYSTEM - II Previous Question Paper (January/February - 2023 - R18)
Note: Part A is compulsory (25 marks). Answer all questions in Part A. In Part B, Answer any one question from each unit (10 marks each).
Part A
Answer all questions:
Give the formulas for transmission parameters of long lines.
Answer
For a long transmission line of length l, with propagation constant γ and characteristic impedance Zc, the ABCD parameters are:
Give reasons for choosing π over T representation of lines.
Answer
- The π-representation is preferred for long transmission lines because it allows for the cascading of multiple sections to represent a long line more accurately.
- In a π-network, the shunt capacitance is divided into two halves, one at each end, which aligns better with the distributed nature of line parameters.
- It simplifies the calculation of voltage and current at intermediate points and is more convenient for power flow studies compared to the T-model.
Give comparison between uncompensated and compensated lines.
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Explain about OLTC transformer as a voltage controller.
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What are the advantages of per unit system representation of system?
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Define reflection and refraction coefficients.
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Distinguish between surge diverters and rod gaps.
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What are volt-time curves? How are they useful?
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Give the classification of faults.
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What are symmetrical components? Explain.
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Part B
A long symmetrical line with A = D = 0.9 ∠ 0° and B = 150 ∠ 65° Ω has at the load end a transformer having a series impedance ZT = 100 ∠ 67° Ω. The load voltage and current are VL and IL. Obtain expressions for VS and IS.
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A 40 MVA generating station is connected to a three-phase line. There is a load of 10 MW at unity power factor at the mid-point of the line. Calculate the voltage and load at the distant end of the line. Use nominal-T circuit for the line.
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What is Ferranti effect in transmission lines? Explain.
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Explain the transmission line voltage control using shunt and series Capacitance methods. Discuss their merits and demerits.
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What is load compensation? Discuss its objectives in power system.
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A short 230 kV transmission line with a reactance of 18 Ω/phase supplies a load at 0.85 lagging power factor. For a line current of 1,000A the receiving- and sending-end voltages are to be maintained at 230kV. Calculate (a) rating of synchronous capacitor required, (b) the load current, (c) the load MVA. Power drawn by the synchronous capacitor may be neglected.
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Draw the per unit impedance diagram on a common base for the system shown. All per unit impedances shown are with respect to their own base. Take the system base MVA same as generator MVA and system base KV as generator side voltage.
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A 500 kV, 2μs rectangular wave travels on a line having a surge impedance of 350 Ω and approaches a termination with a capacitance C equal to 300 pF. Determine the magnitudes of the reflected and transmitted waves.
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From fundamentals obtain the expressions for reflection and transmission co-efficient on a line terminated with load impedance equal to the surge impedance of the line.
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Briefly discuss about various causes of over-voltages in the power system network.
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Explain the various methods of transmission line protection against over voltages due to lightning strokes.
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Explain, with a neat sketch, the working principle and constructional details of expulsion type lightning arrester.
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The line currents in a three phase system are given. Calculate the symmetrical components of current.
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An 11.2 kV bus-bar is fed from three synchronous generators. Calculate the fault current and MVA if 3-phase symmetrical fault occurs on the bus-bar.
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A double line to ground fault occurs on phases b and c at point F. Find the fault current in phase c of G1. Both the machines are rated 1.2MVA, 0.6kV with X1 = X2 = 0.1 p.u. and X0 = 0.05 p.u. Transformers are 1.2 MVA each with leakage reactance of 0.05 p.u. Transmission line reactances are XL1 = XL2 = 0.2 p.u. and XL0 = 0.4 p.u. on the MVA base of the machines.
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