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WorksheetsT&D Unit 3
Total questions: 30
Worksheet time: 30mins
Stringing chart is useful for
Finding sag in the conductor
Design of tower
Design of insulator string
Finding distance between towers
For an existing AC transmission line, the string efficiency is 80%. Now if DC voltage is applied for the same set up, the string efficiency will be
80%
90%
70%
100%
Effect of increase in temperature in overhead transmission lines is to
Increase the stress and length
Decrease the stress and length
Decrease the stress but increase the length
None of the above
A 66KV system has string insulators having 5 discs and the earth to disc capacitance ratio is 0.1. The string efficiency will be
0%
0-25%
50 to 75%
100%
The overhead conductor between two poles takes the shape of
V
STraight line
Catenary
None of the above
WHen overhead conductor tension is increased,
sag increases
sag decreases
No effect on sag
None of the above
Sag tension at any point in the conductor is
perpendicular to the point
parallel to ground
tangential to the point
perpendicular to ground
Sag for supports at equal levels does not depend on
Weight of conductor per unit length
Tension in the conductor
Line voltage
Span length
For supports at unequal levels, sag does not depend on
Distance between supports
DIfference in levels between supports
Distance between towers
Corona
Factor of safety for a conductor while calculating tension depends on
Ultimate strength
Allowable tension
both options 1 and 2
none of the above
Sag in a conductor does not depend on
Ice coating
Weight of conductor
Distance between phase conductors
Wind velocity
The effect of ice coating on conductors is to
Increase sag
Decrease sag
Non uniform sag
No effect on sag
Sag template is used to
compare sags at different locations
choosing conductor diameter
locating tower positions
none of the above
WHich of the following is not a type of tower
Wooden poles
Reinforced concrete tower
Plastic tower
Lattice steel tower
Classes of insulator testing are
Type tests
Routine tests
Sample tests
All of the above
Typical mechanical tests done on insulators are
Mechanical strength
Compression
Torsion
Bending
All of the above
WHich of the following electrical tests are not done on insulators?
50 cycle dry flashover
Short circuit
50 cycle wet test
power frequency withstand
impulse voltage withstand
WHich of the following tests is not done on insulators?
Porosity
Refraction coefficient
Proof load
Corrosion
Corona and radio interference
Which of the following is not a type of insulator?
Pin
Suspension
Foil
Strain
Which of the following is not a method for improving string efficiency?
Reducing line voltage
Longer cross arms
Grading of insulators
Using guard rings
The non-uniform distribution of voltage across the units in a string of suspension type insulators is due to
Unequal self capacitance of the units
Non-uniform distance of separation of the units from the tower body
Existence of stray capacitances between the metallic junctions of the units and the tower body
non-uniform distance between the cross-arms and the units
Whenever the conductors are dead-ended or there is a change in the direction of the transmission line, the insulators used are
Pin type
Suspension type
Strain type
Shackle type
In the case of suspension type insulators, string efficiency can be improved by
1) Using longer cross arms
2) Using a guard ring
3) Grading the insulator discs
4) Reducing the cross arm length
WHich of the above statements are correct?
1, 2, 3
2, 3, 4
2 and 4
1 and 3
A string insulator has 4 units. The voltage across the bottom unit is 33.33% of the total voltage. Its string efficiency is
25%
33.33%
66.67%
75%
In a string of suspension insulators, the voltage distribution across the different units of a string could be made uniform by the use of grading ring because, it
forms capacitances with link pins to cancel the charging current from link pins
forms capacitances which help to cancel the charging current from link pins
increases the capacitances of lower insulator units to cause equal voltage drop
decreases the capacitances of upper insulator units to cause equal voltage drop
A suspension type insulator has three units with self capacitance C and ground capacitance of 0.2C having a string efficiency of
78%
80%
82%
84%
The number of discs in a string of insulators for 220KV ac overhead transmission line lies in the range of
0 to 5
around 15
around 100
around 200
In a three unit insulator string, voltage across the lowest unit is 17.5KV and string efficiency is 84.28%. The total voltage across the string will be equal to
8.825KV
44.25KV
88.25KV
442.5KV
Two insulator discs of identical capacitance value C make up a string for a 22KV, 50Hz single phase overhead line insulation system. If the pin to earth capacitance is also C , then the string efficiency is
50%
75%
90%
86%
A string insulator has 5 units. The voltage across the bottom most unit is 25% of the total voltage. The string efficiency is
25%
50%
80%
75%
