EX-404 Power System-I - Nov 2023 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aDraw and explain the structure and components of modern power systems with typical voltage levels.
   How to answer

   • Define generation, transmission, distribution, load levels
   • List components: generating station, step-up substation, transmission, sub-transmission, distribution
   • Sketch single-line structure with typical voltages
2. 7 Marks
   bA $1000$ MW power station delivers $1000$ MW for $2$ hours, $500$ MW for $6$ hours and is shut down for rest of each day. It is also shut down for maintenance for $60$ days annually. Calculate its annual load factor.
   How to answer

   • Compute daily energy: 1000x2 + 500x6 kWh
   • Compute average load = daily energy / 24
   • Compute annual energy accounting 305 working days and load factor = average annual load / max demand
3. 7 Marks
   aExplain load curves and load duration curves. Discuss their utility in the economics of generation.
   How to answer

   • Define load curve and load duration curve with sketches
   • Procedure to derive LDC from chronological load curve
   • Uses: load factor, plant selection, economics of generation
4. 7 Marks
   bDerive an expression for capacitances of a single phase transmission system and discuss the effect of earth on capacitance with suitable equation.
   How to answer

   • Consider single-phase two-wire line with charges and potentials
   • Derive capacitance per km using potential difference
   • Include earth effect via method of images and modified equation
5. 7 Marks
   aCompare the cross-section area of conductors for DC two wire system and 1 phase AC system assuming equal length, equal power and equal losses.
   How to answer

   • Assume equal power, length, losses and voltage
   • Derive current and loss expressions for DC 2-wire and 1-phase AC
   • Obtain ratio of copper cross-sections
6. 7 Marks
   bDerive a formula for the electric stress in a single core cable. Where is maximum stress? Where is it minimum?
   How to answer

   • Consider single core cable geometry with core and sheath radii
   • Apply Gauss law for electric field / stress g = Q/(2 pi epsilon x)
   • Show maximum at conductor surface, minimum at sheath
7. 7 Marks
   aDeduce the expression for the sending end and receiving end power of a transmission line in terms of voltages and ABCD constants.
   How to answer

   • Write ABCD equations Vs = A Vr + B Ir, Is = C Vr + D Ir
   • Express complex power Sr and Ss in terms of voltages
   • Derive sending and receiving power expressions with |A|,|B| and angles
8. 7 Marks
   bA $50$ Hz, 3-phase transmission line is $280$ km long. It has a total series impedance of $(35 + j140)$ ohms and shunt admittance of $930 \times 10^{-6}$ siemen. It delivers $40,000$ kW at $220$ kV with $90$% p.f lagging. Find the Generalized Circuit Constants, Sending end voltage and current. By using

   1. i)Medium line nominal - T method
   2. ii)Medium line nominal - π method.
   How to answer

   • Compute ABCD for nominal-T and nominal-pi from Z and Y
   • Compute receiving current from P, V, pf
   • Compute Vs = A Vr + B Ir and Is for each method
9. 7 Marks
   aDescribe the concept and procedure to draw the power circle diagram.
   How to answer

   • Define power circle diagram concept with receiving voltage as reference
   • Steps to draw sending/receiving circles using ABCD / Z
   • Interpretation for power limit and operation
10. 7 Marks
    bA three phase overhead line is suspended by a suspension type insulator, which consists of three units. The potential across top unit and middle unit are $12$ kV and $18$ kV respectively. Calculate:

    1. i)The ratio of capacitance between pin and earth to the self-Capacitance of each unit
    2. ii)The line voltage
    3. iii)String efficiency
    How to answer

    • Relate unit voltages 12 kV, 18 kV to shunt to self capacitance ratio k
    • Solve for k and bottom unit voltage
    • Compute line voltage = sqrt3 x phase voltage and string efficiency
11. 7 Marks
    aExplain various types of insulators with neat diagrams and compare them.
    How to answer

    • List pin, suspension, strain, shackle insulators with sketches
    • Describe construction and application voltage range
    • Compare mechanical/electrical merits
12. 7 Marks
    bA transmission line has a span of $16$ m between level supports. The cross sectional area of the conductor is $2.25$ cm$^3$ and weighs $100$ kg/$100$ m. The breaking stress is $4420$ kg/cm$^2$. Calculate the factor of safety if the $50$g of the line is $3.5$m. Assume a maximum wind pressure of $100$ kg/m.
    How to answer

    • Compute conductor weight, wind load and resultant weight per metre
    • Compute allowable tension from breaking stress and area
    • Apply sag formula S = w L^2 / (8 T) and factor of safety
13. 7 Marks
    aDescribe the various methods to improve string efficiency.
    How to answer

    • Define string efficiency and voltage distribution problem
    • List methods: grading, guard ring, longer cross-arm
    • Explain how each equalizes voltage
14. 7 Marks
    bA transmission line conductor having a dia of $18.5$ mm weights $0.75$ kg/m. The span is $285$ metres. The wind pressure is $49$ kg/m$^2$ of projected area with ice coating of $12$ mm. The ultimate strength of the conductor is $9000$ kg. Calculate the maximum sag if the factor of safety is $2$ and ice weighs $810$ kg/m$^3$.
    How to answer

    • Compute ice weight and wind load on iced conductor
    • Compute total resultant weight per metre
    • Compute allowable tension with safety factor and sag = w L^2 / 8T
15. 14 Marks
    Write short notes on: (Any Two) $2\times7=14$

    1. a)Comparison of isolated versus interconnected power system.
    2. b)What is difference between string chart and Sag template?
    3. c)Explain the substation equipment's.
    4. d)Enumerate the different methods of grading of cables.
    How to answer

    • Select any two topics with definition and key points
    • Add diagrams/formulas where relevant
    • Conclude with applications/merits