IN this document few questions on transmission and distribution is shared.

1. Question bank on T&D for IA2
Q.1. Mention the properties which line supports should have. 3 Marks
Q.2. Classify supporting poles and explain each type. 8 Marks
Q.3. Mention the objections to the wooden poles. 3 Marks
Q.4. Why are insulators used in overhead lines? Discuss the desirable properties of insulators. 6
Marks
Q.5. Discuss the advantages and disadvantages of (i) pin-type insulators (ii)
Suspension type insulators 6 Marks
Q.6. Explain how the electrical breakdown can occur in an insulator. 5 marks
Q.7. What is a strain insulator and where is it used? 4 marks
Q.8. Give reasons for unequal potential distribution over a string of suspension insulators. 5
Marks
Q.9. Define and explain string efficiency. Can its value be equal to 100%? 8 marks
Q.10. Show that in a string of suspension insulators, the disc nearest to the conductor has the
highest voltage across it. 6 marks
Q.11. Explain various methods of improving string efficiency. 5 marks
Q.12. What is corona? What are the factors which affect corona? 6 marks
Q.13. Discuss the advantages and disadvantages of corona. 6 marks
Q.14. Explain the following terms with reference to corona: 12 Marks (4 marks each)
(i) Critical disruptive voltage
(ii) Visual critical voltage
(iii) Power loss due to corona
Q.15.Classify the tests of insulators and discuss each type in brief. 10 marks
Q.16. Discuss why:
(i) Insulators are often fitted with arcing horns 5 marks
(ii) Transmission lines in heavily polluted area are provided with greater number of insulator
discs. 5 marks

2. Q.17.What do you understand by electric potential? Derive an expression for electric potential
(i) at a charged single conductor
(ii) at a conductor in a group of charged conductors
Q.18. Derive an expression for the capacitance of a single phase overhead transmission line.
Q.19. Deduce an expression for line to neutral capacitance for a 3-phase overhead transmission
line when the conductors are (i) symmetrically placed (ii) un-symmetrically placed but
transposed
Numerical on String efficiency
Q.1. In a 33 KV overhead line, there are three units in the string of insulators. If the capacitance
between each insulator pin and earth is 11% of self capacitance of each insulator, find (i) the
distribution of voltage over 3 insulators and (ii) string efficiency
Q.2. A 3-phase transmission line is being supported by three disc insulators. The potentials
across top unit (near to the tower or cross arm) and middle unit are 8 KV and 11 KV
respectively. calculate (i) the ratio of capacitance between pin and earth to the self-capacitance of
each unit (ii) the line voltage and (iii) string efficiency
Q.3.The three bus-bar conductors in an outdoor substation are supported by units of post type
insulators. Each unit consists of a stack of 3 pin type insulators fixed one on the top of the other.
The voltage across the lowest insulator is 13.1 KV and that across the next unit is 11 KV. Find
the bus-bar voltage of the station.
Q.4. A string of 4 insulators has a self-capacitance equal to 10 times the pin to earth capacitance.
Find (i) the voltage distribution across various units expressed as a percentage of total voltage
across the string and (ii) string efficiency.
Q.5.Each conductor of a 3-phase high-voltage transmission line is suspended by a string of 4
suspension type disc insulators. If the potential difference across the second unit from top is 13·2
kV and across the third from top is 18 kV, determine the voltage between conductors.
Q.6.The self capacitance of each unit in a string of three suspension insulators is C. The shunting
capacitance of the connecting metal work of each insulator to earth is 0·15C while for line it is
0·1C. Calculate (i) the voltage across each insulator as a percentage of the line voltage to earth
and (ii) string efficiency.
Q.7. Each line of a 3-phase system is suspended by a string of 3 identical insulators of self-
capacitance C farad. The shunt capacitance of connecting metal work of each insulator is 0·2 C
to earth and 0·1C to line. Calculate the string efficiency of the system if a guard ring increases
the capacitance to the line of metal work of the lowest insulator to 0·3 C.

3. Q.8. Each line of a 3-phase system is suspended by a string of 3 similar insulators. If the voltage
across the line unit is 17·5 kV, calculate the line to neutral voltage. Assume that the shunt
capacitance between each insulator and earth is 1/8th of the capacitance of the insulator itself.
Also find the string efficiency.
Numerical on Corona
Q.1. A 3-phase line has conductors 2 cm in diameter spaced equilaterally 1 m apart. If the
dielectric strength of air is 30 kV (max) per cm, find the disruptive critical voltage for the line.
Take air density factor δ = 0·952 and irregularity factor mo =0·9
Q.2. A 132 kV line with 1·956 cm dia. Conductors is built so that corona takes place if the line
voltage exceeds 210 kV (r.m.s.). If the value of potential gradient at which ionization occurs can
be taken as 30 kV per cm, find the spacing between the conductors.
Q.3. A 3-phase, 220kV, 50 Hz transmission line consists of 1·5 cm radius conductor spaced 2
meters apart in equilateral triangular formation. If the temperature is 40ºC and atmospheric
pressure is 76 cm, calculate the corona loss per km of the line. Take mo =0·85
Q.4. A certain 3-phase equilateral transmission line has a total corona loss of 53 kW at 106 kV
and a loss of 98 kW at 110·9 kV. What is the disruptive critical voltage? What is the corona loss
at 113 kV?
Numerical on electric potential and capacitance
Q.1. A single-phase transmission line has two parallel conductors 3 meters apart, radius of each
conductor being 1 cm. Calculate the capacitance of the line per km. Given that ε0 =8·854 × 10− 12
F/m.
Q.2. A 3-phase overhead transmission line has its conductors arranged at the corners o f an
equilateral triangle of 2 m side. Calculate the capacitance of each line conductor per km. Given
that diameter of each conductor is 1·25 cm.
Q.3. A 3-phase, 50 Hz, 66 kV overhead line conductors are placed in a horizontal plane as shown
in Fig. The conductor diameter is 1·25 cm. If the line length is 100 km, calculate (i) capacitance
per phase, (ii) charging current per phase, assuming complete transposition of the line.

4. Q.4. Calculate the capacitance of a 100 km long 3-phase, 50 Hz overhead transmission line
consisting of 3 conductors, each of diameter 2 cm and spaced 2·5 m at the corners of an
equilateral triangle.
Q.5. A 3-phase, 50Hz, 132 kV overhead line has conductors placed in a horizontal plane 4m
apart. Conductor diameter is 2 cm. If the line length is 100 km, calculate the charging current per
phase assuming complete transposition.