# 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

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

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

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.

4. The capacitance grading method is one of the most practical and most widely used method in Indian transmission system.
a) True
b) False

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 %

Explanation:
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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

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

Explanation: The pin type insulators have the ceramic material and the string structure of it can not sustain the voltage above 33kV.

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

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%

Explanation: The string efficiency is independent of system frequency.

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

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

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