81. The string efficiency of a high-voltage line is around
(a) 100%
(b) 80%
(c) 40%
(d) 10%
Answer: (b) 80%
Explanation:
- 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 a 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 the string efficiency.
- As the value of shunt capacitance approaches 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 string efficiency of a high-voltage line is around 80 %.
82. In three-unit insulator string, voltage across the lowest unit is 17.5 kV and string efficiency is 84.28%. The total voltage across the string will be equal to
(a) 8.285 kV
(b) 44.25 kV
(c) 88.25 kV
(d) 442.5 kV
Answer: (b) 44.25 kV
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.
η=V/n×Vn
Where,
V is the total voltage across the string.
Vn is the voltage across the bottom disc near to conductor
n is the number of the disc or unit insulator in a string
Calculation:
Given that,
Let the total voltage = V
Voltage across the bottom-most unit, Vn = 17.5 kV
Number of insulators units (n) = 3
String efficiency = (conductor total voltage)/(number of discs × voltage across the disc nearest to the conductor)
String efficiency,
η=V/n×Vn
84.28%=V3×17.5×103×100
∴ Total voltage, V = 44.25 kV
83. Two-insulator discs of identical capacitance value C makes up a string for a 22 kV, 50 Hz, single-phase overhead line insulation system. If the pin to earth capacitance is also C, then the string efficiency is
(a) 50%
(b) 75%
(c) 90%
(d) 86%
Answer: (b) 75%
Explanation:
String efficiency=Voltage across the string / n× Voltage across the lower most unit
η=V1+V2/n×V2×100
V2=V1+KV1
V2=2V1
η=V1+2V1/2×2V1×100
=3V1/4V1×100=0.75×100=75%
(a) decreases by about 6%.
(b) increases by about 6%.
(c) increases by about 15%.
(d) decreases by about 15%
Answer: (b) increases by about 6%.
Explanation:
- The electrostatic stress in a cable isn’t uniformly distributed. The potential gradient is inversely proportional to the distance from the centre of the cable.
- Hence it will be maximum (gmax) at the surface of the conductor and goes on decreasing until it becomes minimum (gmin) at the surface of the sheath.
- That means electrostatic stress in the dielectric of a cable is maximum at the surface of the conductor and minimum at the surface of the sheath.
- In a cable the sheath radius is R and the conductor radius is r. As r changes from 0.5 R to 0.25 R the maximum voltage gradient in the dielectric.
85. In the case of suspension-type insulators, the string efficiency can be improved by
(a). using a longer cross arm.
(b). using a guard ring,
(c). grading the insulator discs.
(d). all of above
Answer: (d). all of above
Explanation:
The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency.
String efficiency of a string of disc insulators can be improved by using following methods.
- By using longer cross-arms: The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution. The value of K can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used. However, limitations of cost and strength of tower do not allow the use of very long cross-arms. In practice, K = 0·1 is the limit that can be achieved by this method.
- By grading the insulators: In this method, insulators of different dimensions are so chosen that each has a different capacitance. The insulators are capacitance graded i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is reached. Since voltage is inversely proportional to capacitance, this method tends to equalise the potential distribution across the units in the string. This method has the disadvantage that a large number of different-sized insulators are required. However, good results can be obtained by using standard insulators for most of the string and larger units for that near to the line conductor.
- By using a guard ring (Static Shielding): The potential across each unit in a string can be equalised by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator. The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is contoured in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2 etc. The result is that same charging current I flows through each unit of string. Consequently, there will be uniform potential distribution across the units.
86. The ratio of puncture voltage to the flash-over voltage of a line insulator is
(a) equal to 1
(b) lower than 1
(c) much greater than 1
Answer: (c) much greater than 1
Explanation:
The electrical breakdown of an insulator due to excessive voltage can occur in one of two ways:
- A puncture arc is a breakdown and conduction of the material of the insulator, causing an electric arc through the interior of the insulator.
- Flashover voltage is the voltage that causes a flash-over arc.
- The ratio of the puncture voltage to the flashover voltage of an insulator is always greater than one.
87. The insulators may fail due to
(a) flash over.
(b) short-circuits.
(c) deposition of dust.
(d) any of the above
Answer: (d) any of the above
Explanation:
Causes of Insulator Failure: There are different causes due to which failure of insulation in the electrical power systems may occur:
Cracking of Insulator:
- The porcelain insulator mainly consists of three different materials.
- The main porcelain body, steel fitting arrangement, and cement to fix the steel part with porcelain.
- Due to changing climate conditions, these different materials in the insulator expand and contract at different rates.
- These unequal expansion and contraction of porcelain, steel, and cement are the chief cause of the cracking of insulators.
Porosity in The Insulation Materials: If the porcelain insulator is manufactured at low temperatures, it will make it porous, and due to this reason it will absorb moisture from air thus its insulation will decrease and leakage current will start to flow through the insulator which will lead to insulator failure.
Flash Over Across Insulator: If a flashover occurs, the insulator may be overheated which may ultimately result in shuttering of it.
Mechanical Stresses on Insulator:
- If an insulator has any weak portion due to a manufacturing defect, it may break from that weak portion when mechanical stress is applied to it by its conductor.
- These are the main causes of insulator failure. Now we will discuss the different insulator test procedures to ensure the minimum chance of failure of insulation.
- Glazing on ceramic insulators is done to make it smooth and non-absorbent.
89. The purpose of guard ring in transmission lines is to
(a) reduce the earth capacitance of the lowest unit.
(b) increase the earth capacitance of the lowest unit.
(c) reduce the transmission line losses.
(d) none of the above.
Answer: (a) reduce the earth capacitance of the lowest unit.
Explanation:
Guard Ring or Grading Ring:
- It is a metal ring electrically connected to the conductor and surrounding the bottom insulator
- A guard ring is used to protect high impedance nodes in a circuit from surface leakage current and The guard ring is a ring of copper
- Grading Ring or Guard Ring equalizes the potential distribution across each disc in Suspension Insulator
- Guard Ring nullifies the effect of shunt capacitance of string insulator
- It reduces the earth capacitance of the lowest unit and provides uniform or equal distribution of voltage at each disk
- When it is used with an arcing horn it protects the hole insulator string from flashover or overvoltage
90. The use of a guard ring
(a) equalizes the voltage division between insulator discs.
(b) is an unnecessary complication.
(c) decreases string efficiency.
Answer: (a) equalizes the voltage division between insulator discs.
Explanation:
- A guard ring is a metallic ring connected to the conductor and surrounds the bottom disc. It is countered in such a way that the voltage distribution across the string is uniform.
- Due to the non-uniform distribution of voltage, the string efficiency is very less, the insulators are not utilized properly. Also, the insulator near the conductor is under more stress and is likely to be punctured. In order to avoid these drawbacks, a guard ring is used.
- A guard ring is a metallic ring connected to the conductor and surrounds the bottom disc. It is countered in such a way that the voltage distribution across the string is uniform. The basic idea behind using a guard ring is to create the effect of another capacitance such that the charging current of the earth capacitance and the ring capacitance are almost equal and opposite in direction.
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