21. Overhead ground wires are used to protect a transmission line against
(a) line-to-ground faults.
(b) arcing earths.
(c) voltage surges due to direct lightning stroke.
(d) high voltage oscillations due to switching.
Answer: (c) voltage surges due to direct lightning stroke.
22. An overhead transmission line is provided with earth wire for protection against
(a) switching surge.
(b) lightning surge.
(c) power frequency over-voltage.
(d) none of these.
Answer: (b) lightning surge.
23. In a thyrite lightning arrester the resistance
(a) varies linearly with the applied voltage.
(b) increases with the applied voltage.
(c) decreases linearly with the applied voltage.
(d) is high at low current and low at high current.
Answer: (d) is high at low current and low at high current.
24. An ideal surge diverter should have the characteristics
(a) its power frequency breakdown or spark-over must be above normal or abnormal fundamental frequency.
(b) when the value of voltage transient peak exceeds the spark-over value of the surge diverter, a conducting path to earth must be provided.
(c) after occurrence of breakdown, it must carry the resulting discharge current without any damage to it.
(d) after breakdown the power frequency current must be interrupted as soon as the transient voltage has dropped below the breakdown value.
(e) all of the above.
Answer: (e) all of the above.
25. Consider the following statements:
- Present-day surge diverters use nonlinear resistance elements.
- A travelling wave is usually represented as a step wave in the analysis.
- A travelling wave suffers reflection when it reaches a discontinuity.
- The function f(vx ± t) represents a travelling wave.
Which of the above statements is/are correct? .
(a) 1 only.
(b) 1 and 2 only.
(c) 1, 2 and 3.
(d) 3 and 4 only.
Answer: (c) 1, 2 and 3.
26. A lightning arrester provides
(a) low impedance path.
(b) high impedance path.
(c) low resistance path.
(d) high resistance path between line and earth during operation.
Answer: (a) low impedance path.
27. Surge absorbers are used for protection against
(a) high voltage low frequency oscillations.
(b) high voltage high frequency oscillations.
(c) low voltage high frequency oscillations.
(d) low voltage low frequency oscillations.
Answer: (c) low voltage high frequency oscillations.
28. Surge modifiers are employed for
(a) reducing the steepness of wavefront.
(b) reducing the current of wavefront.
(c) reducing the voltage of wavefront.
(d) modify the shape of the wavefront.
Answer: (a) reducing the steepness of wavefront.
29. Which of the following are the important limitations of rod gap surge arresters ?
- They are not capable of sealing off power frequency follow up current.
- After a discharge, the rods are destroyed completely.
- Performance is affected by climatic conditions.
Use the following codes for selecting the correct answer.
(a) 1, 2 and 3
(b) 1 and 2
(c) 2 and 3
(d) 1 and 3
Answer: (d) 1 and 3
30. A thyrite type lightning arrester
(a) blocks the surge voltage appearing in a line.
(b) absorbs the surge voltage appearing in a line.
(c) offers a low resistance path to the surge appearing in a line. (d) returns the surge back to the source.
Answer: (c) offers a low resistance path to the surge appearing in a line.
31. A lightning arrester connected between the line and earth in a power system
(a) protects the terminal equipment against travelling surges. (b) protects the terminal equipment against direct lightning stroke.
(c) suppresses high frequency oscillations in the line.
(d) reflects back the travelling waves approaching it.
Answer: (a) protects the terminal equipment against travelling surges.
32. Which of the following is a non-linear diverter?
(a) Expulsion type arrester.
(b) Valve type arrester.
(c) Electrolytic type arrester.
(d) Rod gap arrester.
Answer: (b) Valve type arrester.
33. A valve type lightning arrestor in a substation should be placed
(a) close to the circuit breaker.
(b) close to the transformer.
(c) away from the transformer.
(d) none of these.
Answer: (b) close to the transformer.
34. Surge absorber
(a) absorbs
(b) reflects
(c) diverts the energy of travelling waves
(d) partly absorbs and partly diverts
Answer: (d) partly absorbs and partly diverts
35. Coupling factor of a ground wire can be increased by
(a) reducing the footing impedance.
(b) increasing the ground wire size.
(c) using cantilever rods on the crossing along with the areas of ground wire.
(d) all of the above.
Answer: (d) all of the above.
36. In the presence of corona, electrostatic coupling…….and electro-magnetic coupling……..
(a) decrease, increases.
(b) increases, decreases.
(c) increases, remains the same.
(d) remains the same, decreases.
Answer: (c) increases, remains the same.
37. When a transmission line is energized,………propagate on it.
(a) voltage wave only
(b) current wave only
(c) both voltage and current waves
(d) none of the above.
Answer: (c) both voltage and current waves
38. When a wave propagates on a transmission line, it suffers reflection several times at
(a) load end.
(b) sending end.
(c) sending end and other end.
(d) tapping point.
Answer: (c) sending end and other end.
39. Travelling voltage wave and current wave have the same waveforms and travel together along the transmission line at a velocity
(a) of sound.
(b) of light.
(c) slightly lesser than that of light.
(d) slightly lesser than that of sound.
Answer: (c) slightly lesser than that of light.
40. The relation between travelling voltage wave and current wave is given as
(a) ei = √(L/C)
(b) e/i = √(L/C)
(c) ei = √(LC)
(d) e/i = √(LC)
Answer: (b) e/i = √(L/C)