Transmission and Distribution Objective Questions with Easy Explanation Part-5

 41. RCC poles usually have the spans of

(a) 250-400 m.

(b) 80-150 m.

(c) 50-80 m.   

(d) 25-50 m.

 

Answer: (b) 80-150 m.

 

Explanation:

Concrete poles:

  • Reinforced concrete poles have become very popular as line supports in recent years
  • They have greater mechanical strength, longer life and permit longer spans than steel poles
  • Moreover, they give a good outlook, require little maintenance and have good insulating properties.
  • The maximum permissible span for RCC poles is 80 - 150 meters.
  • The main difficulty with the use of these types of electric poles is the high cost of transport owing to their heavyweight
  • Therefore, such poles are often manufactured at the site in order to avoid the heavy cost of transportation

 

42. In India for the distribution of electric power we usually use

(a) wooden poles.      

(b) steel poles.

(c) RCC poles.

(d) both (b) and (c).

 

Answer: (d) both (b) and (c).

 

Explanation:

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

 

Concrete poles:

  • Reinforced concrete poles have become very popular as line supports in recent years
  • They have greater mechanical strength, longer life and permit longer spans than steel poles
  • Moreover, they give a good outlook, require little maintenance and have good insulating properties.
  • The maximum permissible span for RCC poles is 80 - 150 meters.
  • The main difficulty with the use of these types of electric poles is the high cost of transport owing to their heavyweight
  • Therefore, such poles are often manufactured at the site in order to avoid the heavy cost of transportation


43. Conductors used in ht transmission lines are stranded because of 

(a) increased tensile strength. 

(b) ease in handling.

(c) cheaper in cost.    

(d) reduced resistivity.

 

Answer: (b) ease in handling.

 

Explanation:

  • A stranded conductor is consisting of several thin wires of small cross-sectional area called strands.
  • At the center of stranded conductor, we are using steel conductor which provided the high tensile strength to conductor. In the outer layers of stranded conductor, we use aluminum conductors, which provide the conductivity to stranded conductor.

Characteristics of stranded conductors:

  • The stranded conductor is having sufficient flexibility, which makes stranded conductor suitable to be coiled easily to transport it over long distance.
  • For a stranded conductor of same cross-sectional area, the flexibility of conductor increases with increase of number of strands in conductor.
  • Stranded wire tends to be a better conductor than solid wire because the individual wires collectively comprise a greater surface area.
  • It is used when higher resistance to metal fatigue is required
  • The stranded conductor is formed by twisting the strands together in layers.
  • The strands of each layer are laid in helical fashion over the preceding layer. This process is called stranding.

 

Important Points:

Generally, in successive layer, the stranding is done in opposite direction to preceding layer. This mean, if the strands of one layer are twisted in clockwise direction, the strands of next layer will be twisted in anticlockwise direction and so on ‘x’ is number of layers in conductor.

Generally, the total number of strands in any conductor is given by the formulae of,

N = 3x2 – 3x + 1

Where, N is total number of strands in stranded conductor.

Generally the diameter of conductor can be calculated by using the formula of,

D = (2x – 1)d

Where, D is the diameter of conductor,

‘d’ is the diameter of each strand.

 

44. Which of the following properties has got higher value for aluminum in comparison to that of copper?

(a) Electrical resistivity.        

(b) Melting point.

(c) Thermal conductivity. 

(d) Specific gravity.

 

Answer: (a) Electrical resistivity.

 

Explanation:

  • Copper has much greater radius than aluminium, so the outer electrons are less strongly attracted as compared to aluminium
  • Aluminium wires expand in high temperatures and contract in low temperatures when compared to copper, which can withstand thermal changes
  • The conductivity of aluminium is about 60 percent of the conductivity of copper
  • Electrical resistivity of aluminium is higher than copper.

 

45. In a 7/30 ACSR conductor why is grease put between steel and aluminium conductors?

(a) To reduce corrosion by electrolytic action between zinc (galvanizing agent on steel) and aluminium

(b) To reduce friction between the strands.

(c) To reduce leakage of current from aluminium strands to steel strands.

(d) To eliminate air pockets. 

 

Answer: (a) To reduce corrosion by electrolytic action between zinc (galvanising agent on steel) and aluminium

 

Explanation:

  • This conductive grease prevents the formation of oxides, sulfides and other corrosion deposits on copper and aluminum surfaces and conductors can be prevented with its use.
  • The purpose of a electrical contact lubricant is to prevent corrosion and lubricate the connection for easier maintenance.
  • Conductive greases for electronics efficiently lubricate moving parts, offer superior protection against corrosion and help ensure electrical continuity between irregular surfaces.

 

46. ACSR is used in place of copper in overhead lines because of

(a) higher current carrying capacity.

(b) being lighter in weight.

(c) economy.  

(d) higher tensile strength.

 

Answer: (a) higher current carrying capacity.

 

Explanation:

Aluminum conductor steel reinforced (ACSR):

  • It is one of the stranded conductors composed of one or more layers of hard-drawn 1350-H19 aluminum wire that is on galvanized coated steel core.
  • The ACSR conductors are mainly used in distribution and transmission lines due to their high tensile strength, economical design, less weight, suitable for medium and long periods with fewer supports, and good properties of sag and high voltage overhead lines.

 

Advantages of ACSR conductors over copper conductors:

  • The conductivity of the ACSR is higher than the copper conductor.
  • ACSR conductor can be used for medium and long spans and needs less support.
  • The mechanical performance and Tensile strength of the ACSR conductor are High.
  • It provides lesser Sag than copper conductors.

 

47. ACSR conductors have

(a) all conductors made of aluminium.

(b) outer conductors made of aluminium.

(c) inner conductors made of aluminium.

(d) no conductors made of aluminium. 

 

Answer: (b) outer conductors made of aluminium.


Explanation:

  • Aluminium conductor steel-reinforced cable (ACSR) is a type of high-capacity, high-strength stranded conductor typically used in overhead power lines.
  • The outer strands are high-purity aluminium, chosen for its good conductivity, low weight and low cost.
  • The center strand is galvanised steel for additional strength to help support the weight of the conductor and provide additional mechanical strength.
  • Steel has higher strength than aluminium which allows for increased mechanical tension to be applied to the conductor.

 

48. "Expanded ACSR" are conductors composed of

(a) larger diameter individual strands for a given cross-section of the aluminium strands.

(b) the larger diameter of the central steel strands for a given overall diameter of the conductor.

(c) larger diameter of the aluminium strands only for a given overall diameter of the conductor.

(d) A filter between the inner steel and the outer aluminium strands to increase the overall diameter of the conductor.

 

Answer: (b) the larger diameter of the central steel strands for a given overall diameter of the conductor.

 

Explanation:

  • For reducing the corona loss and radio interference at a high voltage a fibrous or plastic material is filled between the strands.
  • The diameter of the conductor expands due to the filling material and hence, it is called an expanded conductor.
  • Expanded ACSR conductors are used in extra high voltage (EHV) transmission line.
  • By the use of a filler such as paper or hessian between various layers of strands so as to increase the overall conductor diameter to reduce the corona loss and electrical stress at conductor surface.

 

49. The function of steel wire in an ACSR conductor is to

(a) compensate for the skin effect.

(b) take a cape of surges.

(c) provide additional mechanical strength.

(d) reduce inductance.

 

Answer: (c) provide additional mechanical strength.

 

Explanation:

  • Aluminium conductor steel-reinforced cable (ACSR) is a type of high-capacity, high-strength stranded conductor typically used in overhead power lines.
  • The outer strands are high-purity aluminium, chosen for its good conductivity, low weight and low cost.
  • The center strand is galvanised steel for additional strength to help support the weight of the conductor and provide additional mechanical strength.
  • Steel has higher strength than aluminium which allows for increased mechanical tension to be applied on the conductor.
  • Steel also has lower elastic and inelastic deformation (permanent elongation) due to mechanical loading (e.g. wind and ice) as well as a lower coefficient of thermal expansion under current loading.

 

50. The material used for the manufacture of the ground wire is

(a) aluminium.

(b) galvanised steel.

(e) cast iron.   

(d) stainless steel.

 

Answer: (b) galvanised steel.

 

Explanation:

  • Earthing is used to protect from an electric shock.
  • It helps by providing a path for a fault current to flow to earth.
  • Earth wire is made of galvanized steel because of good mechanical properties.
  • The purpose of a ground wire is to give excess electrical charges a safe place to go.
  • The solid mass of earth below our feet has a negative electrical charge, which means positive electrical charges are naturally attracted to it.


 


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