EX-404 Power System-I - Jun 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 the Single line diagram showing a typical arrangement of A.C. distribution system. State the requirements of ideal distribution system.
   How to answer

   • Single line diagram of AC distribution: generating station, primary transmission, secondary transmission, primary distribution, secondary distribution
   • Components: feeders, distributors, service mains
   • Requirements of ideal system: continuity, efficiency, low voltage drop, safety, economy
2. 7 Marks
   bA generating station supplied the following loads. 150 MW, 120 MW, 85 MW, 60 MW and 5 MW. The station has a maximum demand of 220 MW. The annual load factor of the station is 48%. Calculate

   1. i)The number of units supplied annually
   2. ii)The diversity factor and
   3. iii)Demand factor
   How to answer

   • Compute sum of individual maximum demands
   • Annual energy = max demand x load factor x 8760
   • Diversity factor = sum of individual demands / station max demand
   • Demand factor = station max demand / total connected load
3. 7 Marks
   aDefine the term diversity factor and prove that load factor of supply system is improved by an increase in diversity of load.
   How to answer

   • Definition of diversity factor
   • Relation between load factor, diversity and coincidence
   • Proof that higher diversity reduces max demand and improves load factor
   • Effect on plant capacity and economy
4. 7 Marks
   bThe energy cost of a 100 MW steam station working at 40% load factor comes out to be 12 paise/kWh of energy generated. What will be the cost of energy generated if the load factor is improved to 60%? The fuel cost of the power station due to increased generation increase the annual generation cost by 5%.
   How to answer

   • Annual energy at 40% load factor
   • Fixed cost component from 12 paise/kWh
   • New energy at 60% load factor with 5% increase in annual cost
   • Cost per kWh at improved load factor
5. 7 Marks
   aDiscuss the concept of GMR and GMD in the calculation of transmission line inductance.
   How to answer

   • Definition of GMR and GMD
   • Role in inductance of single and stranded conductors
   • Formula $L = 2\times10^{-7} \ln(GMD/GMR)$
   • GMR for bundle and GMD for three-phase spacing
6. 7 Marks
   bA single core cable has a conductor diameter of 1 cm and internal sheath diameter of 1.9 cm. If impregnated paper of relative permittivity 4 is used as the insulation, calculate the capacitance for 1 km length of the cable.
   How to answer

   • Capacitance formula $C = 2\pi\varepsilon_0\varepsilon_r / \ln(D/d)$ per metre
   • Convert diameters to radii, length 1 km
   • Substitute $\varepsilon_r=4$
   • Result in $\mu F$ for 1 km
7. 7 Marks
   aDraw the phasor diagram of a short transmission line and derive an expression for voltage regulation and transmission efficiency.
   How to answer

   • Phasor diagram of short line with $V_s, V_r, I, R, X$
   • Voltage drop components and regulation formula
   • Power at sending and receiving ends
   • Efficiency = power delivered / power sent
8. 7 Marks
   bA 15 km long 3-phase overhead line delivers 5 MW at 11 Kv at 0.8 lagging power factor. Line loss is 12% of power delivered. Line inductance is 1.1 MH per km per phase. Find sending end voltage and voltage regulation.
   How to answer

   • Receiving end current from $P=\sqrt{3}V I \cos\phi$
   • Line loss and sending end power, find $I$ and $R$
   • Reactance from $L=1.1$ mH/km
   • Sending voltage via $V_s$ phasor and % regulation
9. 7 Marks
   aMention the various methods of voltage control in transmission line. Also explain any one method in details.
   How to answer

   • List methods: tap changing, auto transformer, booster, synchronous condenser, shunt capacitors, SVC
   • Explain one method in detail with diagram
   • Location and control principle
   • Advantages and limitations
10. 7 Marks
    bDetermine the efficiency and regulation of a 3-phase, 100 km, 50 Hz transmission line delivering 25MW at a.p.f. of 0.9 lagging and 66 kV to a balanced load. The conductors are of copper, each having resistance 0.1 ohm per km, inductance 0.1118 H per km and capacitance 0.9944 $\mu$F per km. Neglect leakage and use nominal-$\pi$ method.
    How to answer

    • Nominal-pi parameters: total Z, Y
    • ABCD constants and receiving end phasor
    • Sending end voltage/current calculation
    • Efficiency and regulation percentages
11. 7 Marks
    aWhat do you mean by string efficiency? Explain different method of improving string efficiency.
    How to answer

    • Definition of string efficiency
    • Methods: grading, guard ring, longer cross-arm
    • Capacitance grading principle
    • Guard ring equalises voltage
12. 7 Marks
    bIn a 33 kV overhead line, three are there units in the string of insulators. If the capacitance between each insulator pin and earth is 11% of self-capacitance of each insulator, find the distribution of voltage over 3 insulators and string efficiency.
    How to answer

    • Ratio K = shunt/ self capacitance = 0.11
    • Voltage distribution $V_1:V_2:V_3$ formula
    • Total voltage 33 kV / phase voltage
    • String efficiency = total / (n x top unit)
13. 7 Marks
    aDerive the expression for voltage distribution in insulator string and string efficiency.
    How to answer

    • Shunt and series currents in string
    • Voltage of each unit derivation with K
    • Total voltage expression
    • String efficiency formula
14. 7 Marks
    bDescribe the main limitation of Kelvin's law. The cost of 3 phase overhead transmission line having cross-sectional area A $cm^2$ is Rs [600+2800 A] per kW. Calculate the most economical current density for the conductor if the rate of interest and depreciation is 11% per annum. The cost of energy wasted is Rs 0.06/kwh. The resistance of each conductor is 0.16/A ohm/kM. Take load factor for loss = 12%.
    How to answer

    • Statement of Kelvin's law and its assumption
    • Limitations: interest, corona, stability, growth ignored
    • Cost equation Rs [600+2800 A] and loss equation
    • Most economical density calculation with 11% and Rs 0.06/kWh
15. 14 Marks
    Write short notes on: (Any Two) $2\times7=14$

    1. a)Problems associated with modern large interconnected power system.
    2. b)Derive the expression for dielectric stress of a single core cable.
    3. c)Comparison between AIS & GIS Substation.
    4. d)Interpretation of long line equation and its equivalent equation.
    How to answer

    • Two topics with definition, formula/diagram and comparison
    • Dielectric stress derivation or AIS vs GIS points
    • Long line ABCD and equivalent pi
    • Interconnected system problems