31. With the same maximum voltage to earth, which of the following ac systems with 0.8 pf will need more copper in comparison to dc 2-wire system?
(a) Single-phase, 2-wire (midpoint earthed).
(b) Single-phase. 3-wire (neutral half of outer).
(c) Three-phase, 3-wire.
(d) Three-phase, 4-wire (neutral = outer).
Answer: (d) Three-phase, 4-wire (neutral = outer).
Explanation:
- With the same maximum voltage to earth, Three-phase, 4-wire (neutral = outer) ac systems with 0.8 pf will need more copper in comparison to dc 2-wire system.
- Volume of conductor material required is 0.3125 times of that required in two-wire dc system with one conductor earthed.
32. The main reason for using high voltage for long-distance power transmission is
(a) reduction in transmission losses.
(b) reduction in time of transmission.
(c) increase in system reliability.
(d) none of the above.
Answer: (a) reduction in transmission losses.
Explanation:
- The primary reason that power is transmitted at high voltages is to increase efficiency.
- As electricity is transmitted over long distances, there are inherent energy losses along the way.
- High voltage transmission minimizes the amount of power lost as electricity flows from one location to the next.
- if voltage increased then line loss will decrease and vice versa.
33. 66 kV is suitable for transmission of power over
(a) 30 km
(b) 60 km
(c) 120 km
(d) 200 km
Answer: (b) 60 km
Explanation:
Line-to-line voltage (kV) | Length of line in km | |
Minimum | Maximum | |
11 | 15 | 30 |
33 | 30 | 60 |
66 | 40 | 120 |
110 | 50 | 140 |
132 | 50 | 160 |
166 | 80 | 180 |
230 | 100 | 300 |
400 | 300 | 400 |
66 kV is suitable for transmission of power over 60 km.
34. If 3 MW power is to be transmitted over a distance of 30 km, the desirable transmission voltage will be
(a) 11 kV
(b) 33 kV
(c) 66 kV
(d) 3.3 kV
Answer: (b) 33 kV
Explanation:
Desirable transmission Voltage V=5.5√(km/1.6+load in kvA/150)
Here total power is 3MW and 30km.
V = 5.5 √(30/1.6 + 3000/150)
V = 5.5 √(18.75 + 20)
V = 5.5 * √38.75
V = 5.5 * 6.2249
V = 34.54 kV
So approximate transmission voltage should be 33KV.
35. The highest transmission voltage used in India is
(a) 400 kV
(b) 220 kV
(c) 132 kV
(d) 765 kV
Answer:
Explanation:
- Transmission voltage in India (highest) is 765 kV AC and these lines are erected by Power Grid Corporation for interstate connections throughout India.
- DC transmission voltage (highest) in India is 800 kV .
- This has restrictions due to conversion i.e. AC/DC & inversion i.e. DC/AC for bulk power transmission.
36. If the variable part of annual cost on account of interest and depreciation on the capital outlay is equal to the annual cost of electrical energy wasted in the conductors, the total annual cost will be minimum, and the corresponding size of the conductor will be the most economical. This statement is known as
(a) Lenz's law.
(b) Faraday's law.
Answer: (a) Lenz's law.
Explanation:
If the variable part of annual cost on account of interest and depreciation on the capital outlay is equal to the annual cost of electrical energy wasted in the conductors, the total annual cost will be minimum, and the corresponding size of the conductor will be the most economical. This statement is known as Lenz's law.
37. For high-voltage transmission lines, conductors are suspended from towers so as to
(a) increase clearance from ground.
(b) reduce clearance from ground.
(c) take care of increase in length.
(d) reduce wind and snow effects.
Answer: (a) increase clearance from ground.
Explanation:
- In high voltage transmission, the conductor lines are suspended from towers in order to increase clearance from the ground.
- This is done due to:
- Since conductors are loaded with heavy voltage & current so from the security point of view we suspend them from towers.
- It helps to make suitable clearance height from the earth so that the insulation breakdown does not occur.
38. The supports used for transmission lines should have the characteristic(s) of
(a) high mechanical strength and longer life.
(b) good looking, light in weight and easily accessible for painting and erection of line conductors.
(c) cheap in initial as well in maintenance cost.
Answer: (d) all of the above.
Explanation:
- The line supports should have the high mechanical strength to withstand the load of the conductors, insulators, and wind load on the conductors and supports themselves.
- The line supports should have lightweight without losing mechanical strength.
- The line supports should be cheaper in cost.
- The line supports should have a low maintenance cost.
- The line supports should have a longer life,
- The line supports should be good-looking.
- The light supports should be easily accessible for painting and the erection of the conductors on it.
39. The wooden poles well impregnated with creosite oil or any preservative compound have life of
(a) 25-30 years.
(b) 20-25 years.
(c) 10-15 years.
(d) 5-10 years.
Answer:
Explanation:
Wooden poles:
- These are made of seasoned wood (sal or chir) and are suitable for lines of the moderate X-sectional area and of relatively shorter spans, say up to 50 meters
- Such supports are cheap, easily available, provide insulating properties and, therefore, are widely used for distribution purposes in rural areas as an economic proposition
- The wooden poles generally tend to rot below the ground level, causing foundation failure
- In order to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil
- They have a comparatively smaller life (20-25 years) and cannot be used for voltages higher than 20 kV
- They have less mechanical strength and require periodical inspection
Steel Poles:
- They are used for system voltages up to 33 kV in low and high-voltage distribution systems
- When compared to wooden poles steel poles have advantages like lightweight, long life, and greater strength
- These are used for a longer span, i.e., from 50 to 80 m
- These are costlier than wooden and RCC poles
- All steel supports should be well-galvanized and have a life of at least 30 years
Steel Towers:
- These towers are robust in construction
- They can be used for spans 300 m or above
- They are used for the transmission of power above 66 kV and are more useful for valleys, railway lines, rivers, etc
- They are mechanically very strong and have a longer life than steel poles
- They are capable of withstanding the most severe climatic conditions and cannot be destroyed by forest fires
40. Steel poles for transmission lines need protection against
(a) borer.
(b) termites.
(c) corrosion.
(d) all of these.
Answer: (c) corrosion.
Explanation:
- Steel poles possess greater mechanical strength and thus permit the use of longer spans (60 to 80 metres).
- These poles have the longer life (more than 40 years) which can further be increased by regular painting.
- These poles need protection against corrosion. Hence at the bottom (the portion which is buried underground), these poles are set in concrete muffs in order to protect them from chemical reactions.
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