Power Systems Questions and Answers – Underground Cables – 2

This set of Power Systems Problems focuses on “Underground Cables – 2”.

1. For a string of five suspension insulators is to be fitted with a grading ring. If the pin to earth capacitances is equal to C, then the value of line to pin capacitance of first capacitance is
a) 0.25C
b) 0.667C
c) 1.5C
d) 4C
View Answer

Answer: a
Explanation: Ck= KC/(n-k) = 1*C/5-1 = C/4.

2. For a string of five suspension insulators is to be fitted with a grading ring. If the pin to earth capacitances are equal to C, then the value of line to pin capacitance of first and last capacitance(from top) is
a) 0.25C, 4C
b) 0.667C, 3C
c) 1.5C, 1.5C
d) 4C,0.25
View Answer

Answer: a
Explanation: Ck= KC/(n-k)
C1= 1*C/(5-1) = C/4
C4 = 4C/(5-4) = 4C.

3. How does grading neutralize the potential distribution across the units of the suspension insulators?
a) By forming capacitances to cancel the charging current from link pins
b) By forming capacitances with link pin to cancel the charging current from them
c) By increasing the capacitance of lower insulator units
d) By decreasing the capacitance of lower insulator units
View Answer

Answer: a
Explanation: The non uniform distribution of the voltage occurs due to the leakage currents at the pins. So grading is done to provide a capacitance which can neutralize these charging currents.
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4. The capacitance grading method is one of the most practical and most widely used method in Indian transmission system.
a) True
b) False
View Answer

Answer: b
Explanation: The capacitance grading method is uneconomical because it requires a large number of the different sized insulators.

5. A string of insulators consisting of 7 units. The voltage across the bottom most unit is 25% of the total voltage. The string efficiency of this system will be
a) 57.14 %
b) 50.6 %
c) 80.4 %
d) 67.7 %
View Answer

Answer: a
Explanation:The string efficiency will be 57.14 % for voltage across bottom most unit is 25%
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6. For a line-man while measuring the string efficiency of the insulator installation done, observes that it shows a reading of 100%. This means ___________________
a) equal potential across each insulator disc
b) one of the insulator disc is shorted
c) zero potential across each disc
d) none of the mentioned
View Answer

Answer: a
Explanation: As the equipotential exists between the plates there will not be any charging current and so 100% of efficiency.

7. The pin type insulators are generally not used for the voltage range exceeding ____________
a) 33kV
b) 66kV
c) 25kV
d) 11kV
View Answer

Answer: a
Explanation: The pin type insulators have the ceramic material and the string structure of it can not sustain the voltage above 33kV.
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8. If the frequency of the transmission line is changed from 50 Hz to 100 Hz, then the string efficiency ____________
a) remains unchanged
b) increases
c) decreases
d) May or may not increase depending on the parameters
View Answer

Answer: a
Explanation: The string efficiency is independent of system frequency.

9. For a 400 kV, 50 Hz five pin insulator setup provides the protection with an efficiency of 87.4%. The same setup is now meant to be used at the frequency of 60 Hz, the updated string efficiency of the system will be ____________
a) 87.4
b) 91
c) 82%
d) 86%
View Answer

Answer: a
Explanation: The string efficiency is independent of system frequency.
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10. How can one think of reducing the earth capacitance of line?
a) Using guard ring
b) Using special designed earth capacitances
c) Implementing parallel insulator lines
d) Any of the mentioned
View Answer

Answer: a
Explanation: The guard rings can provide an economical protection to the string and it is very simple method.

Sanfoundry Global Education & Learning Series – Power Systems.

To practice all areas of Power Systems Problems, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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