71. Which type of insulators are used on 132 kV transmission lines?
(a) Pin type.
(b) Disc type.
(c) Shackle type.
(d) Pin and Shackle type.
Answer: (b) Disc type.
Explanation:
Disc type insulators:
Pin Insulator:
- 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 the 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
Suspension type insulator:
- It consists of a number of porcelain discs connected in series by metal links in the form of a string.
- These are the string of insulators in vertical position.
- The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower.
- Used above 132 kV
Strain insulators:
- When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension.
- These are the string of insulators in horizontal position.
- In order to relieve the line of excessive tension, strain insulators are used.
Shackle insulators:
- They are frequently used for low voltage distribution lines.
- Such insulators can be used either in a horizontal position or in a vertical position.
- Used at low voltage.
72. Whenever the conductors are dead-ended or there is a change in the direction of the transmission line, the insulators used are of the
(a) Pin type
(b) Suspension type
(c) Strain type.
(d) Shackle type.
Answer: (c) Strain type.
Explanation:
- When suspension string is used to sustain the extraordinary tensile load of conductor it is referred as string insulator.
- When there is a dead-end or there is a sharp corner in the transmission line, the line has to sustain a great tensile load of conductor or strain.
- A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.
- Strain insulators are generally used up to 33 kV line. These insulators should not be fixed below three meters from the ground level.
Rated System Voltage VS Number of disc insulator used in strain type tension insulator string
- 33KV----3
- 66KV----5
- 132KV---9
- 220KV---15
Rated System Voltage VS Number of disc insulators used in suspension insulator string
- 33KV-----3
- 66KV-----4
- 132KV----8
- 220KV----14
73. post-type insulators are generally used in lines operating
(a) above 100 kV.
(b) below 33 kV.
(c) at any voltage level, hv or ehv.
Answer: (c) at any voltage level, hv or ehv.
Explanation:
- Pin insulators are used for holding the line conductors on the straight running of poles. These are commonly used in power networks up to 33 kV system.
- Suspension insulators consist of a number of porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross- arm of the tower. For high voltage (>33KV), it is a usual practice to use suspension type insulators.
- When there is a dead-end of the line or there is a corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used.
- For low voltage lines (<11 kV) shackle insulators are used as strain insulators.
- Stay insulators are also known as strain insulators and are generally used up to 33 kV line. These insulators should not be fixed below three meters from the ground level. These insulators are also used where the lines are strained.
74. The number of discs in a string of insulators for 400 kV ac overhead transmission line lies in the range of
(a) 32 to 33
(b) 22 to 23
(c) 15 to 16
(d) 9 to 10
Answer (b) 22 to 23
Explanation:
Number of insulator discs n = (Operating voltage/phase)/ Maximum voltage of each disc
Operating voltage/phase = 220*10³/√3 = 127 kV
Maximum voltage of each disc = 11 kV
Number of insulator discs n = 127/11 =11.547 ≈ 12
For safety operation one extra disc is added to the string.
Therefore, total number of insulator discs = 13
The number of discs in a string of insulators for 400 kV ac overhead transmission line lies in the range of 22-23 Nos.
Number of Insulator per String
- For 400KV Line ---- 22 Disc
- For 220KV Line ---- 13 Disc
- For 132KV Line ---- 8 Disc
- For 66KV Line ----- 5-6 Disc
- For 33KV Line ----- 3 Disc
75. The non-uniform distribution of voltage across the units in a string of suspension-type insulators is due to
(a) unequal self-capacitance of the units.
(b). non-uniform distance of separation of the units of the tower body.
(c) the existence of stray capacitance between the metallic junctions of the units and the tower body.
(d) non-uniform distance between the cross-arms and the units.
Answer: (c) the existence of stray capacitance between the metallic junctions of the units and the tower body.
Explanation:
Important points regarding the voltage distribution over a string of suspension insulators:
1) Due to the presence of shunt capacitor, the voltage across the suspension insulators does not distribute itself uniformly across each disc.
2) The voltage across the nearest disc to the conductor is maximum than others disc.
3) The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured.
4) In the case of D.C voltage, the voltage across each unit would be the same. It is because insulator capacitance are ineffective for D.C.
- The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance.
- The disc nearest to the conductor has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
- The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.
76. The voltages across the various discs of a string of suspension insulators having identical discs are different due to
(a) surface leakage currents.
(b) series capacitance.
(c) shunt capacitance to ground.
(d) series and shunt capacitances.
Answer: (c) shunt capacitance to ground.
Explanation:
- Due to the presence of a shunt capacitor, the voltage across the suspension insulators does not distribute itself uniformly across each disc.
- The voltage across the nearest disc to the conductor is maximum than the other discs.
- The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured.
- In the case of D.C voltage, the voltage across each unit would be the same. It is because insulator capacitance is ineffective for D.C
- The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance.
- The disc nearest to the conductor has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
- The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.
77. The string efficiency of a string of suspension insulators is dependent on
(a) the size of the insulators.
(b) a number of discs in the string.
(c) size of tower.
Answer: (b) a number of discs in the string.
Explanation:
- The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.
- String efficiency depends upon the value of shunt capacitance. Lesser the value of capacitance, the greater is the string efficiency.
- As the value of shunt capacitance approaches zero, the string efficiency approaches 100%.
- The greater the string efficiency, the more uniform is the voltage distribution in each disc insulator. 100% string efficiency implies that the potential across each disc is the same.
- In order to decrease the shunt capacitance, the distance between the insulator string and the tower should be increased, i.e. longer cross-arms should be used.
78. 100 percent string efficiency means
(a) one of the insulator discs shorted.
(b) zero potential across each disc.
(c) equal potential across each insulator disc.
(d) none of the above.
Answer: (c) equal potential across each insulator disc.
Explanation:
String efficiency:
- The voltage applied across the suspension insulator string is unequally distributed across the individual unit.
- The disc near the line conductor is extremely stressed and takes the maximum voltage.
- The voltage distribution on the insulator string determines the flashover voltage and the voltage at which the localized corona and radio interference is started.
- The string efficiency is defined as the ratio of conductor voltage to the voltage across the disc nearest to the conductor multiplied by number of discs.
- String efficiency = (conductor voltage)/(number of discs × voltage across the disc nearest to the conductor)
- String efficiency depends upon the value of shunt capacitance. Lesser the value of capacitance, the greater is the string efficiency.
- As the value of shunt capacitance approaches to zero, the string efficiency approaches to 100%.
- In order to decrease the shunt capacitance, the distance between the insulator string and the tower should be increased, i.e. longer cross-arms should be used.
- The greater the string efficiency, the more uniform is the voltage distribution in each disc insulator. 100% string efficiency implies that the potential across each disc is same.
79. In a suspension-type insulator the potential drop is
(a) maximum across the lowest disc.
(b) maximum across the topmost disc.
(c) uniformly distributed over the discs.
Answer: (a) maximum across the lowest disc.
Explanation:
- The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance.
- The disc nearest to the conductor has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
- The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.
- The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance.
- The disc nearest to the conductor has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
- The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.
- If the voltage impressed across the string was d.c., then the voltage across each unit would be the same. It is because insulator capacitances are ineffective for d.c.
- Hence, in a suspension-type insulator, the potential drop is maximum across the disc nearest to the conductor, which is nothing but the lowest disc in the string of discs.
80. If the frequency of a transmission system is changed from 50 Hz to 100 Hz, the string efficiency
(a) will increase.
(b) will decrease.
(c) remain unchanged.
(d) may increase or decrease depending on the line parameters.
Answer: (c) remain unchanged.
Explanation:
- The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.
- String efficiency = Operating phase voltage (Vph)n x voltage across the disc nearest to the conductor)
- Hence string efficiency does not depend upon the frequency so if the frequency is increased string efficiency remains unchanged.
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