Overhead Line Insulator MCQ

1. In a 3 unit insulator string, the voltage across the lowest unit is 17.5 kV and string efficiency is 84.28%. The total voltage across the string is

  1. 58.27 kV
  2. 32.48 kV
  3. 40.87 kV
  4. 44.274 kV
Answer
Answer. d

2. Two insulators of identical capacitance C forms a string for a 22 kV, 50 Hz, single-phase overhead line insulation system. If pin to earth capacitance is also C, then string efficiency is

  1. 75 %
  2. 50 %
  3. 25 %
  4. 60 %
Answer
Answer. a

3. A string insulator has 5 units. The voltage across the bottommost unit is 25 % of the total voltage. The string efficiency is

  1. 60 %
  2. 80 %
  3. 45 %
  4. 55 %
Answer
Answer. b

4. There are 20 discs in the string insulators of a 3-phase, 400 kV transmission line. The string efficiency is 80%. What is the maximum voltage across any disc?

  1. 24 kV
  2. 12 kV
  3. 17 kV
  4. 14.43 kV
Answer
Answer. d

5. If the voltage across a string insulation assembly is 38 kV, number of insulator discs are 4 and voltage across lowermost disc is 12 kV the string efficiency is

  1. 80 %
  2. 79.16 %
  3. 75.27 %
  4. 85 %
Answer
Answer. b

6. Consider a 3-phase, 50 Hz, 11 kV distribution system. Each of conductors is suspended by an insulator string having two identical porcelain insulators. The self-capacitance of the insulator is 5 times the shunt capacitance between link and ground. The voltage across two insulators are

Answer
Answer. 2.89 kV and 3.46 kV

7. Two insulators of identical disc form a string. The self-capacitance is 8 times the pin to earth capacitance. The maximum voltage each unit can withstand should not exceed 17.5 kV. The line voltage of the complete string is

  1. 33 kV
  2. 44 kV
  3. 22 kV
  4. 34 kV
Answer
Answer. a

8. A string of suspension insulators consists of 4 units. The capacitance between each link pin and earth is (1/10)th of self-capacitance of a unit. The voltage between the line conductor and earth is 100 kV. Find string efficiency?

  1. 76.4 %
  2. 80 %
  3. 75 %
  4. 78.4 %
Answer
Answer. a

9. A string of 6 suspension insulators is to be graded to obtain uniform distribution of voltage across the string. If pin to earth capacitances are equal to ‘C’, self-capacitance of top insulator is ’10C’ then self-capacitance of remaing insulators are

Answer
Answer. C1 = 10C, C2 = 11C, C3 = 13C, C4 = 16C, C5 = 20C, and C6 = 25C

10. A string of 7 suspension insulators is to be fitted with a grading ring. If pin to earth capacitances are equal to ‘C’ the line to pin capacitances that gives a uniform voltage distribution over the string is

Answer
Answer. C1 = C/6, C2 = 2C/5, C3 = 3C/4, C4 = 4C/3, C5 = 5C/2, and C6 = 6C