EX-404 Power System-I - Jun 2024 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
   aDescribe the method of controlling:

   1. i)The active power (kW)
   2. ii)The reactive power (kVAR) between two interconnected power stations.
   How to answer

   • Define active power control via load-frequency / prime-mover input and power-angle control
   • Define reactive power control via excitation / voltage magnitude control
   • Explain interconnection power flow relations P vs delta and Q vs V
   • Role of governors, AVRs and tap changers
2. 7 Marks
   bThe yearly duration curve of a certain plant can be considered as a straight line from 140 MW to 30 MW. Power is supplied with one generating unit of 95 MW capacity and two units of 45 MW capacity each. Determine:

   1. i)Installed capacity
   2. ii)Load factor
   3. iii)Plant capacity factor
   4. iv)Maximum Demand
   5. v)Utilization factor
   How to answer

   • Installed capacity = sum of unit ratings
   • Average load from area of trapezoidal duration curve (140 MW to 30 MW)
   • Load factor = average load / maximum demand
   • Plant capacity factor = average load / installed capacity and utilization factor = max demand / installed capacity
3. 7 Marks
   a

   The load on a power plant on a typical day is a under:

   Time	12-5 AM	05-9 AM	09-6 PM	6-10 AM	10-12 AM
   Load (MW)	20	40	80	100	20

   Plot the chronological load curve and load duration curve. Find the load factor of the plant and the energy supplied by the plant in 24 hours.

   How to answer

   • Tabulate time vs load and draw chronological curve over 24 h
   • Rearrange in descending order to draw load duration curve
   • Compute energy = sum(Load x hours) in MWh
   • Load factor = average load / peak load
4. 7 Marks
   bDetermine the inductance per phase per km of a double circuit 3-phase line. The radius of each conductor is 20 mm and the conductors are placed on the circumference of an imaginary circle of radius 7 m forming a regular hexagonal figure.
   How to answer

   • Identify double-circuit 6-conductor hexagonal arrangement with radius 7 m
   • Compute GMR of conductor and GMD between phases including both circuits
   • Apply inductance formula $L = 2\times10^{-7} \ln(GMD/GMR)$ per metre
   • Convert to per km value
5. 7 Marks
   aWhat do you understand by transposition in overhead lines? Explain why transposition is done in such lines.
   How to answer

   • Define transposition as cyclic interchange of conductor positions
   • Explain need to balance inductance and capacitance per phase
   • Explain elimination of unbalanced voltages and interference
   • Sketch transposition cycle over line length
6. 7 Marks
   bFind the inductive reactance of a 3-phase bundled conductor line with 2 conductors per phase with spacing of 40 cm. Phase to phase separation is 7 m in horizontal configuration. All conductors are ACSR with dia of 3.5 cm. Compare the above value with that of an equivalent single-conductor line.
   How to answer

   • Compute GMR of 2-conductor bundle with 40 cm spacing and ACSR dia 3.5 cm
   • Compute GMD for 7 m horizontal phase separation
   • Calculate $X_L = 2\pi f L$ for bundled line
   • Repeat with single conductor and compare reduction
7. 7 Marks
   aDerive the A, B, C, D constants of a medium length transmission line and draw the phasor diagram assuming a $\Pi$-configuration.
   How to answer

   • Write nominal-Pi shunt halves at each end with series Z
   • Derive $V_S = A V_R + B I_R$ and $I_S = C V_R + D I_R$ expressions
   • Show $A=D=1+ZY/2$, $B=Z$, $C=Y(1+ZY/4)$
   • Draw phasor diagram for lagging load with $V_R$ reference
8. 7 Marks
   bA single-phase line is transmitting 1,100 kW power to factory at 11 kV and at 0.8 pf lagging. It has a total resistance of $2 \, \Omega$ and a loop reactance of $3 \, \Omega$. Determine

   1. i)the voltage at the sending end
   2. ii)percentage regulation
   3. iii)transmission efficiency
   How to answer

   • Compute load current from $P = V_R I \cos\phi$
   • Compute sending-end voltage $V_S = V_R + I(R+jX)$ phasor magnitude
   • Compute % regulation = $(|V_S|-|V_R|)/|V_R|\times100$
   • Compute efficiency = output / (output + $I^2R$ losses)
9. 7 Marks
   aA 3-phase transmission line delivers a load of 5 MW at 0.8 p.f (lagging). Resistance of each conductor is $0.5 \Omega/\mathrm{km}$. Receiving-end voltage is 33 kV. If the line loss is not to exceed 10%, determine the length of the line.
   How to answer

   • Express line current in terms of 5 MW, 33 kV, 0.8 pf
   • Express copper loss $3I^2R = 3I^2 r_0 l$ and set ≤10% of load
   • Solve for length $l$ in km
   • State assumption of receiving-end voltage for current
10. 7 Marks
    bA transmission line conductor having a diameter of 19.5 mm weighs 0.85 kg/m. The Span is 275 meters. The wind pressure is $39 \, \mathrm{kg/m^2}$ of projected area with ice coating of 13 mm. The ultimate strength of the conductor is 8,000 kg. Calculate the maximum sag if the factor of safety is 2 and ice weighs $910 \, \mathrm{kg/m^3}$.
    How to answer

    • Compute total weight with ice coating and wind load per metre
    • Compute resultant weight and allowable tension (ultimate/FOS)
    • Apply sag formula $S = w L^2 / 8T$ for maximum sag
    • Include ice density and projected wind area
11. 7 Marks
    aExplain, with neat sketches, the constructional features of pin type and suspension type insulators. List the advantages of the latter type over the former for high voltage transmission lines.
    How to answer

    • Sketch pin insulator and suspension string with parts labelled
    • Describe construction: pin, shed, spindle and cap-pin units
    • List advantages of suspension for high voltage: flexibility, grading, replacement
    • Mention voltage scalability by adding discs
12. 7 Marks
    bExplain 'string efficiency' and methods to improve this for 'insulators'. How are these tested to determine 'flash-over voltage'.
    How to answer

    • Define string efficiency = voltage across string / (n x voltage across nearest disc)
    • Explain causes of uneven distribution: shunt capacitance to earth
    • Describe improvement methods: grading, guard ring, longer cross-arm
    • Outline flash-over test procedure and voltage measurement
13. 7 Marks
    aAn existing single-phase AC system comprising of two overhead conductors is to converted into a 3-phase, 3-wire system by providing an additional conductor of same size. Calculate the percentage of additional load that can be transmitted by the three-phase system if the operating line voltage and percentage line losses remain the same in both the systems.
    How to answer

    • Write power expressions for 1-phase 2-wire vs 3-phase 3-wire at same V and loss
    • Equate $I^2R$ losses to find current ratio
    • Derive transmitted power ratio and % additional load
    • State assumptions of same conductor size and pf
14. 7 Marks
    bCompare the copper efficiencies of a 3-phase, 3-wire and 3-phase, 4-wire systems for the same maximum potential difference between conductors and the earth for both the systems.
    How to answer

    • Assume same max voltage to earth and same power, pf and loss
    • Derive copper volume for 3-phase 3-wire vs 3-phase 4-wire
    • Express ratio in terms of line vs phase voltage
    • Conclude which system is more copper-efficient
15. 14 Marks
    Write a short note on (any two)

    1. i)Conventional, non-conventional and distributed generation.
    2. ii)Bundle conductors.
    3. iii)Tuned power lines.
    4. iv)Voltage distribution of suspension insulator.
    How to answer

    • For each chosen topic give definition and need
    • Explain construction / principle with key points
    • Mention advantages, limitations and applications in power system
    • Add neat sketch / example where relevant