Previous Year Questions- Performance of Transmission Lines, Line Parameters and Corona - 2 | Power Systems - Electrical Engineering (EE) PDF Download

Q19: For a fixed value of complex power flow in a transmission line having a sending end voltage V, the real loss will be proportional to      (2009)
(a) V
(b) V²
(c) 1/V²
(d) 1/V
Ans: (b)
Sol: S = Complex power
S is defined as, S = VI
I = S/V
If R is the resistance of transmission line
Real power loss
Since, S and R are fixed
Real power loss ∝ 1/V²

Q20: An extra high voltage transmission line of length 300 km can be approximate by a lossless line having propagation constant β = 0.00127 radians per km. Then the percentage ratio of line length to wavelength will be given by      (2008)
(a) 24.24 %
(b) 12.12 %
(c) 19.05%
(d) 6.06%
Ans: (d)
Sol: Wave length = λ,
Propogation constant = β

Q21: The total reactance and total suspectance of a lossless overhead EHV line, operating at 50 Hz, are given by 0.045 pu and 1.2 pu respectively. If the velocity of wave propagation is 3 × 10⁵ km/s, then the approximate length of the line is     (2007)
(a) 122km
(b) 172km
(c) 222km
(d) 272km
Ans: (c)
Sol: For loss less line,

Q22: Considered a bundled conductor of an overhead line consisting of three identical sub-conductors placed at the corners of an equilateral triangle as shown in the figure. If we neglect the charges on the other phase conductor and ground, and assume that spacing between sub-conductors is much larger than their radius, the maximum electric field intensity is experienced at
(a) Point X
(b) Point Y
(c) Point Z
(d) Point W
Ans: (b)
Sol: Electric field intensity at various points are shown as follow.
It is clear from the above diagrams that minimumcancellation of vector occurs at the point Y. Hence maximum electric field intensity.

Q23: A single phase transmission line and a telephone line are both symmetrically strung one below the other, in horizontal configurations, on a common tower, The shortest and longest distances between the phase and telephone conductors are 2.5 m and 3 m respectively. The voltage (volt/km) induced in the telephone circuit, due to 50 Hz current of 100 amps in the power circuit is      (2006)
(a) 4.81
(b) 3.56
(c) 2.29
(d) 1.27
Ans: (c)
Sol: where, P₁ and P₂ from power line and T₁ and T₂ from telephone line
Current in power circuit = I = 100 A
Mutual inductance between power line and telephone line
Voltage induced in the telephone line

Q24: The A, B, C , D constants of a 220 kV line are :
 A = D = 0.94∠1°,  B = 130∠73°,  C = 0.001∠90°.
If the sending end voltage of the line for a given load delivered at nominal voltage is 240 kV, the % voltage regulation of the line is      (2006)
(a) 5
(b) 9
(c) 16
(d) 21
Ans: (c)
Sol: Sending end volatge V_s = 240kV
Full-load receiving end voltage
No-load receiving end voltage (per phase)
No-load receiving end voltage ( line to line)

Q25: The concept of an electrically short, medium and long line is primarily based on the      (2006)
(a) nominal voltage of the line
(b) physical length of the line
(c) wavelength of the line
(d) power transmitted over the line
Ans: (b)
Sol: L = length of line
If L ≤ 80 km
The line id short line.
If 80km < L ≤ 250km
The line is medium line.
If L > 250km
The line is long line.

Q26: The generalized circuit constants of a 3-phase, 220 kV rated voltage, medium length transmission line are
If the load at the receiving end is 50 MW at 220 kV with a power factor of 0.9 lagging, then magnitude of line to line sending end voltage should be      (2004)
(a) 133.23 kV
(b) 220.00 kV
(c) 230.78 kV
(d) 246.30 kV
Ans: (c)
Sol: 
Q27: The ABCD parameters of a 3-phase overhead transmission line are A = D = 0.9∠0°, B = 200 ∠90°Ω and  C = 0.95 × 10⁻³ ∠90°S. At no-load condition a shunt inductive, reactor is connected at the receiving end of the line to limit the receiving-end voltage to be equal to the sending-end voltage. The ohmic value of the reactor is      (2003)
(a) ∞ Ω
(b) 2000 Ω
(c) 105.26 Ω
(d) 1052.6  Ω
Ans: (b)
Sol: 
Q28: Bundled conductors are mainly used in high voltage overhead transmission lines to      (2003)
(a) reduces transmission line losses
(b) increase mechanical strength of the line
(c) reduce corona
(d) reduce sag
Ans: (c)
Sol: By bonding of conductor, the self GMD of the conductors is increased.
Corona loss ∝ (V − V₀)²
V₀ is approximately directly proportional to the size of conductor, hence larger the size of the conductor lrger will be the critical disuptive voltage and smaller will be the factor (V − V₀)² and hence, smaller will be the corona loss.

Q29: A long wire composed of a smooth round conductor runs above and parallel to the ground (assumed to be a large conducting plane). A high voltage exists between the conductor and the ground. The maximum electric stress occurs at      (2002)
(a) the upper surface of the conductor
(b) the lower surface of the conductor
(c) the ground surface
(d) midway between the conductor and ground
Ans: (b)

Q30: Consider a long, two-wire line composed of solid round conductors. The radius of both conductors is 0.25 cm and the distance between their centers is 1m. If this distance is doubled, then the inductance per unit length       (2002)
(a) doubles
(b) halves
(c) increases but does not double
(d) decreases but does not halve
Ans: (c)
Sol: In(2d) > In d.

Q31: The conductors of a 10 km long, single phase, two wire line are separated by a distance of 1.5m. The diameter of each conductor is 1 cm. If the conductors are of copper, the inductance of the circuit is      (2001)
(a) 50 mH
(b) 45.3 mH
(c) 23.8 mH
(d) 19.6 mH
Ans: (c)
Sol: