This set of Basic Power Systems Questions and Answers focuses on “Insulator for Overhead Lines – 2”.
1. The unequal voltage distribution across individual unit of string of suspension insulator is __________
a) desirable and expressed by string efficiency
b) undesirable and expressed by string efficiency
c) desirable and expressed by impulse ratio
d) undesirable and expressed by impulse ratio
View Answer
Explanation: The unequal potential distribution is undesirable and is expressed in terms of string efficiency. Impulse ratio is calculated during Impulse Frequency Flash Over Test. Impulse ratio is the ratio of impulse park over voltage to Spark over voltage at power frequency.
2. Glass insulator cannot be used for voltage above __________
a) 25 KV
b) 11 KV
c) 33 KV
d) 50 KV
View Answer
Explanation: Glass is cheaper than porcelain in the simplest shapes and if properly toughened and annealed gives high resistivity and dielectric strength. The main disadvantage of Glass is that moisture more readily condenses on its surface and facilitates the accumulation of dust deposit thus giving a high surface leakage. Hence, glass insulators can be used upto 25 KV under ordinary atmosphere and well upto 50 KV in dry atmosphere.
3. Which of the following location is suitable to use shackle insulator?
a) Sharp Turn in transmission line
b) Dead end of low voltage distribution line
c) For bearing High Voltage transmission line conductor
d) Dead end of EHV transmission line
View Answer
Explanation: Shackle insulator are almost universally used for low voltage lines. They can be used at all positions either intermediate terminal or angle where the angle exceed 60 degree deviation. They are generally used in conjunction with shackle sharps.
4. Which of the following insulator is used for insulating stay wire from pole?
a) Pin type insulator
b) Shackle insulator
c) Suspension insulator
d) Stay insulator
View Answer
Explanation: Stay insulators are also called as guy insulators and are used in guy cables. Because it is necessary to insulate the lower part of the guy cable from pole for safety of people and animals on the ground.
5. What is the reason for unequal distribution of voltage among different unit of suspension insulator?
a) Unequal capacitance of different units
b) Unequal distribution of charging current caused by stray capacitance
c) Unequal resistivity of different units
d) Dirt deposited over the insulator disc
View Answer
Explanation: The capacitance due to two metal fittings on either side of insulator is known as the mutual capacitance and the capacitance between these metal fittings and earthed pole or tower is known as shunt capacitance. Due to shunt capacitance charging current is not the same through all the discs of strings. So voltage being directly proportional to the current flowing through them will be different.
6. Arcing horns are used for ______________
a) protecting insulators from birds
b) protecting insulators from cracking or breaking due to flash over
c) improving string efficiency
d) protecting insulator from deposition of dirt
View Answer
Explanation: During flash over insulators are cracked or broken up due to the heat of Arc. Grading ring in addition to equalization of voltage distribution across the insulators unit when used in conjunction with Arcing horns protects the insulator string from flashover. It does so by keeping the arc away from the insulator string until it is interrupted by protecting devices.
7. What is the effect of rain on string efficiency?
a) It becomes very low
b) It reduces slightly
c) It does not changes
d) It is improved
View Answer
Explanation: Due to rain insulators becomes wet and mutual capacitance increases while the shunt capacitance remains constant thereby decreasing the value of k. As k is inversely related to string efficiency so, string efficiency is improved. K is the ratio of stray capacitance to mutual capacitance.
8. A suspension string have 3 units. Voltage across topmost and lower most unit of string are 6.71 volts and 11 volts respectively. What is the voltage across middle string, if voltage across whole string is 25.76 volts?
a) 103.47 volts
b) 8.05 volts
c) 83.6 volts
d) 25.76 volts
View Answer
Explanation: Voltage across whole string is equal to the sum of voltage across each unit. From this the voltage across middle unit can be calculated.
9. If voltage across lowermost unit of a string is 34.8% of voltage across whole string and string efficiency is 57.46%. What is the voltage across whole string.
a) 100%of line voltage
b) 43.64% of line voltage
c) 75.2% of line voltage
d) 92.26% of line voltage
View Answer
Explanation: Voltage across string is equal to the line voltage of conductor. So it is 100% of line voltage.
10. By using which of the following method hundred percent string efficiency can be achieved?
a) Using long cross arms
b) Capacitance grading
c) Static shielding
d) 100% string efficiency cannot be achieved
View Answer
Explanation: 100% string efficiency can only be achieved by making voltage distribution across each unit of string equal. In practice it is impossible to attain such uniform voltage distribution. So 100% string efficiency cannot be achieved with any of the above methods.
Sanfoundry Global Education & Learning Series – Power Systems.
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