EX-404 Power System-I - Jun 2025 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
   aWhat are the advantages and disadvantages of interconnection of power stations? Explain the methods of affecting a power transfer between two stations.
   How to answer

   • Define interconnection of power stations
   • List advantages e.g. reliability, economy, diversity, reserve
   • List disadvantages e.g. fault level, control complexity, cost
   • Methods of controlling power transfer between two stations e.g. phase angle, voltage, reactance control
2. 7 Marks
   bA power station is to supply four regions of loads whose peak loads are 15,000 kW, 10,000 kW, 9,000 kW and 11,000 kW respectively. The diversity factor of the loads at the station is 1.5 and the average annual load factor is 50%, Calculate the maximum demand on the stations and the annual energy from the station. Suggest the installed capacity and the number of units taking all aspects into account.
   How to answer

   • Compute sum of peak loads and maximum demand = sum / diversity factor
   • Compute average load = load factor x maximum demand
   • Compute annual energy = average load x 8760 hours
   • Suggest installed capacity with reserve and number of units
3. 7 Marks
   aExplain clearly how a good load factor and a good diversity factor help to keep overall cost of generation.
   How to answer

   • Define load factor and diversity factor
   • Explain effect of high load factor on fixed cost per unit
   • Explain effect of high diversity factor on reduced installed capacity
   • Conclude how both lower overall cost of generation
4. 7 Marks
   bCalculate the loop inductance per km of a single-phase line comprising of 2 parallel conductors 1 meter apart and 1 cm in diameter, when the material of conductor is (i) copper and (ii) steel of relative permeability 50.
   How to answer

   • Write loop inductance formula for single-phase line
   • Calculate radius from diameter and internal inductance with permeability
   • Calculate external inductance due to spacing 1 m
   • Compute total loop inductance per km for copper and for steel mu_r=50
5. 7 Marks
   aDistinguish between bundled conductor 3-phase line with two conductors per bundle and double circuit 3-phase line. What differences in electrical characteristics do you expect between the two lines?
   How to answer

   • Define bundled conductor and double circuit line
   • Compare GMR, GMD, inductance, capacitance
   • Compare corona, surge impedance, current capacity
   • Summarize electrical characteristic differences
6. 7 Marks
   bExplain the terms Geometrical Mean Distance (GMD) and 'self GMD' in the inductance calculation of single-phase transmission lines with composite conductors. Find the self GMD of a conductor consisting of seven identical strands each having a radius $r$.
   How to answer

   • Define GMD and self-GMD for composite conductors
   • Explain use in inductance calculation
   • Derive self-GMD for 7-strand conductor radius r
   • Give final expression in terms of r
7. 7 Marks
   aA 15 km long 3-phase overhead line delivers 5 MW at 11 kV at 0.8 lagging pf. Line loss is 12% of power delivered. Line inductance is 1.1mH per km per phase. Find the sending end voltage and voltage regulation.
   How to answer

   • Calculate receiving end current from 5 MW, 11 kV, 0.8 pf
   • Calculate line resistance from 12% loss and reactance from 1.1 mH/km
   • Calculate sending end voltage using phasor sum
   • Calculate voltage regulation = (Vs-Vr)/Vr
8. 7 Marks
   bExplain the Ferranti effect and derive an expression for power loss in an open-circuited line.
   How to answer

   • Define Ferranti effect on open-circuited line
   • Explain capacitance charging current and voltage rise
   • Derive expression for power loss / voltage rise
   • State conditions and effects
9. 7 Marks
   aShow a diagrammatic representation of a medium line in terms of its nominal-$\Pi$ model. Indicate also the values of series impedance and shunt admittance of the model in terms of the resistance, inductance and capacitance per unit length of the line.
   How to answer

   • Draw nominal-Pi model with lumped parameters
   • Show series impedance Z and two shunt admittances Y/2
   • Express Z = (R+jwL)l and Y = jwC l
   • Label sending and receiving ends
10. 7 Marks
    bA 132-kV transmission line has the following data: Wt of conductor = 680 kg/km, Length of span = 260 m. Ultimate strength = 3,100 kg; Safety factor = 2 Calculate the height about ground at which conductor should be supported. Ground clearance required is 10 m.
    How to answer

    • Compute working tension = ultimate strength / safety factor
    • Compute sag = w*l^2/(8*T)
    • Add required ground clearance 10 m for support height
    • Give height above ground
11. 7 Marks
    aShow that overhead line conductor strung between two level supports takes up the shape of a parabola. Obtain expression for the sag and maximum tension when the support is at the same level.
    How to answer

    • Consider conductor equilibrium between level supports
    • Show catenary approximates to parabola for small sag
    • Derive sag S = wL^2/8T
    • Derive maximum tension at supports
12. 7 Marks
    bIn string of three identical suspension insulator units supporting a transmission line conductor, if the self-capacitance of each unit is denoted as C farads, the capacitance of each connector pin to ground can be taken as 0.1 C farads. Determine the voltage distribution across the string if the maximum permissible voltage per unit is given as 20 kV. Also determine the string efficiency.
    How to answer

    • Calculate voltage across each unit using shunt capacitance ratio k=0.1
    • Apply max 20 kV on top unit nearest conductor to find total string voltage
    • Find voltage distribution across three units
    • Calculate string efficiency = total voltage / (3 x max unit voltage)
13. 7 Marks
    aState Kelvin's law for obtaining the size of conductors for transmission. Discuss the limitations.
    How to answer

    • State Kelvin's law: annual copper loss = annual interest on capital
    • Explain derivation for most economical conductor size
    • Discuss limitations e.g. load growth, safety, corona, practical sizes
    • Conclude applicability
14. 7 Marks
    bGive reasons why is power transmitted by 3-phase, 3-wire system whereas the distribution is done by 3-phase, 4-wire system.
    How to answer

    • Explain 3-phase 3-wire for balanced high voltage transmission, no neutral needed
    • Explain need of neutral for unbalanced single-phase distribution loads
    • Compare copper economy and safety
    • Give reasons for 3-phase 4-wire distribution
15. 14 Marks
    Write a short note on (any two)

    1. i)Effect of transmission voltage on power system economy.
    2. ii)Skin and proximity effect.
    3. iii)Sag template.
    4. iv)Sectionalized double bus bar system and ring mains.
    How to answer

    • Select any two topics from i-iv
    • Define principle and key formulas/factors for each
    • Explain significance with diagram/example where relevant
    • Summarize advantages/limitations