61. The effect of wind pressure is more predominant on
(a) insulators.
(b) transmission lines.
(c) supporting towers.
(d) none of the above.
Answer: (c) supporting towers.
Explanation:
- The effect of wind pressure is more predominant on supporting towers than transmission lines.
- The towers do have a reasonable surface area that would receive the force generated by any constant wind.
- It would also have the forces at the suspension point generated by the wind on the transmission lines.
- The transmission towers have flat surfaces whereas the transmission lines are circular.
- Then there is the possibility of the wind being just at the right speed to cause the towers to oscillate.
62. Galloping in transmission line conductors arises due to
(a) asymmetrical layers of ice formation.
(b) vortex phenomenon in light winds.
(c) heavy weight of the line conductors.
(d) adoption of horizontal conductor configuration.
Answer: (a) asymmetrical layers of ice formation.
Explanation:
- High winds and freezing rain can cause transmission lines to gallop.
- Galloping transmission or power lines can occur when freezing rain creates icicles and odd-shaped ice on transmission towers and conductors.
- High winds push on the icicles and conductors and lifts them up, creating a galloping, or jumping, motion.
- Galloping is a low frequency, large amplitude, wind-induced vibration of both single and bundle overhead conductors, with a single or a few loops of standing waves per span.
- It is caused by moderately strong, steady crosswind acting upon an asymmetrically- iced conductor surface.
63. Which one of the following is reduced by using stock bridge dampers on power overhead transmission lines?
(a) Sag.
(b) conductor vibration.
(c) Line losses.
(d) Mechanical tension.
Answer: (b) conductor vibration.
Explanation:
- Stockbridge damper is used to control or suppress the wind induced vibration of overhead conductors.
- The Stockbridge damper targets oscillations due to aeolian vibration; it is less effective outside this amplitude and frequency range.
- Aeolian vibration is nothing but winding induced oscillation which has amplitude of millimeters to centimeters and a frequency of 3 to 150 Hz.
- Aeolian vibration causes damaging stress fatigue to the cable.
- It causes failure of conductor strands.
- It is connected at two end of the transmission power line span. The vibration damper has a length of steel messenger cable.
- Two metallic weights are attached to the ends of the messenger cable.
- The center clamp, which is attached to the messenger cable, is used to install the vibration damper onto the overhead conductor.
64. The sag of the conductors of a transmission line is 2.5 m when the span is 250 m. Now if the height of the supporting tower is increased by 25%, the sag will
(a) reduce by 25%
(b) increase by 25%
(d) remain unchanged.
(e) reduce by 12.5%
Answer: (d) remain unchanged.
Explanation:
The distance between the highest point of electric poles or towers and the lowest point of a conductor connected between two poles or towers.
Span length: It is the shortest distance between two towers or poles.
S=Wl2/ 8T
S is the sag of the conductor
W is the weight of the conductor
l is the span length of the conductor
T is the working tension on the conductor
Here in the question height of the supporting towers is increased by 25 % which is independent of sag. Hence there will be no change in sag.
65. For a 400 kV line, the spacing between phase conductors is around
(a) 8 m.
(b) 11 m.
(c) 14 m.
(d) 17 m
Answer: (a) 8 m.
Explanation:
- As per Indian Electricity Rule 1956, Clause No 77, the minimum distance between bottom conductor and ground of a 400KV transmission line is 8.84 meter.
- the ground clearance of 220KV bottom conductor would be 7 meter.
- the ground clearance of 132KV bottom conductor would be 6.1 meter.
- The minimum clearance of 66KV transmission line is also taken as 6.1 meters.
An empirical formula for Found Clearance between conductors
Ground clearance=5.182+(0.305∗K)
Where:
k=V−33/33 , V in kV .
According to this, if for example the Transmission line voltage is 220 kV then first calculate K :
K=220–33/33=5.66666667
Now calculating:
Ground clearance=5.182+(0.305∗5.66666667)=6.91033333 m=7 m
66. Stranded conductors usually have a central wire around which there are successive layers of 6,12,18,24 wires. For n-layers, the total number of individual wires is
(a) 3 n (n + 1)
(b) 2 n (n + 1)
(c) 3 n (n + 1) + 1
(d) 2 n (n + 1) + 1
Answer: (c) 3 n (n + 1) + 1
Explanation:
- The most commonly used conductor materials for overhead lines are copper, aluminum, steel-cored aluminum, steel-cored copper, galvanized steel, and cadmium copper.
- The choice of a particular material will depend upon the cost, the required electrical and mechanical properties, and the local conditions.
- All conductors used for overhead lines are preferably stranded* in order to increase flexibility.
- In stranded conductors, there is generally one central wire and round this, successive layers of wires containing 6, 12, 18, 24 ...... wires.
- If there are n layers, the total number of individual wires is [3n(n + 1) + 1].
- In the manufacture of stranded conductors, the consecutive layers of wires are twisted or spiraled in opposite directions so that layers are bound together.
67. The diameter of each strand is d then the diameter of n-layer stranded conductor will be
(a) (2n + 1) d
(b) 3 (n + 1) d
(c) (2 n - 1) d
(d) 3 (n - 1) d
Answer: (c) (2 n - 1) d
Explanation:
Generally, the total number of strands in any conductor is given by,
Total number of strands in a stranded conductor = 3n(n – 1) + 1
Generally the diameter of conductor can be calculated by,
D = (2n – 1)d
Where, D is the diameter of the conductor,
‘d’ is the diameter of each strand.
68. Strain-type insulators are used
(a) at dead ends.
(b) at intermediate anchor towers.
(c) on straight runs.
(d) any of (a) or (b).
Answer: (d) any of (a) or (b).
Explanation:
- Pin, Suspension, and Strain insulators are used in medium to high voltage systems. While Stay and Shackle Insulators are mainly used in low voltage applications.
- Strain type insulator used at dead ends and on a straight line as suspension type for voltage 3.3 kV and above.
- A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.
69. Wavy structure of pin insulator increases its
(a) mechanical strength.
(b) puncture strength.
(c) flash-over voltage.
(d) thermal strength.
Answer: (c) flash-over voltage.
Explanation:
- Wavy structure of pin insulator increases its flashover voltage. Wavy structure of pin insulator depends on the variation of voltage. So, when we increase the voltage, the structure of pin insulators also increases.
- Flashover voltage: The voltage at which an electric discharge occurs between two electrodes that are separated by an insulator; the value depends on whether the insulator surface is dry or wet. Also known as sparkover voltage.
70. The voltage rating of a multiple shells (petticoat or rain shed) pin type insulator unit cannot be increased beyond a limiting value by increasing the number of shells, because
(a) the internal voltage distribution between shells becomes unequal.
(b) the leakage path resistance starts diminishing.
(c) the disruptive critical voltage for the material of the insulator is reached.
(d) the puncture voltage of the material of the insulator is reached.
Explanation:
- The pin insulator is used in power distribution for the voltage up to 33kV.
- It is placed on the cross arm of the supporting tower.
- The pin insulator has grooves on the upper end for keeping the conductor.
- The conductor is tied to the insulator on the top groove on straight line positions and side groove in angle positions by annealed binding wire of the same material as that of the conductor.
- A lead thimble is cemented into the insulator body to receive the pin.
- The voltage rating of a multiple shells (petticoat or rain shed) pin type insulator unit cannot be increased beyond a limiting value by increasing the number of shells, because the internal voltage distribution between shells becomes unequal.
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