Question Bank for Power Transmission and Distribution.

  1. Define and explain skin effect ? Why is it absent in the D.C. system ?
  2. Give three reasons with proper mathematical expressions why electric power transmission is carried at high voltages.
  3. Derive an expression for the inductance per phase for a three-phase overhead transmission line when:
    a) Conductors are symmetrical placed.
    b) Conductors are unsymmetrical placed but the line is completely transposed.
  4. Show that in a string of suspension insulator, the disc nearest to the conductor has the highest voltage across it.
  5. What do you mean by string efficiency ? Also, explain various methods of improving string efficiency with proper circuit diagram.
  6. A string of five insulators is connected across a 100 KV line. If the capacitance of each disc to earth is 0.1 of the capacitance of the insulator, calculate the followings:
    a) The voltage distribution across various insulators of the string.
    b) String efficiency.
  7. Find the inductance per phase per km length of the system of conductors as shown in given figure: (Self G.M.D. of one conductor is 0.90 cm.) Assume transposition.

(Self G.M.D. of one conductor is 0.90 cm.) Assume transposition.

8. Deduce an expression with vector diagram for voltage regulation of a short transmission line.

9. An overhead 3-phase short transmission line delivers 5000 kW at 22 kV at 0-8 power factor lagging. The resistance and reactance of each conductor is 4 Ω and 6 Ω respectively.
Determine the followings:
(i) Sending end voltage
(ii) Percentage regulation
(iii) Transmission efficiency.

10. Show how regulation and transmission efficiency are determined for medium lines using
(i) nominal T method
(ii) nominal π method
Illustrate your answer with suitable vector diagrams.

11. Deduce an expression for line to neutral capacitance for a 3-phase overhead transmission line when the conductors are:
(i) symmetrically placed.
(ii) unsymmetrically placed but transposed.

12. What is corona ? What are the factors which affect corona ? Also, explain the following terms with reference to corona:
(i) Critical disruptive voltage
(ii) Visual critical voltage
(iii) Power loss due to corona

13. Deduce an approximate expression for sag in overhead lines when
(i) supports are at equal levels
(ii) supports are at unequal levels.

14. The towers of height 30 m and 90 m respectively support a transmission line conductor at water crossing. The horizontal distance between the towers is 500 m. If the tension in the conductor is 1600 kg, find the minimum clearance of the conductor and water and clearance mid-way between the supports. Weight of conductor is 1-5 kg/m. Bases of the towers can be considered to be at water level.

15. Find an expression for the most economical conductor size of a single core cable. Also, derive the expression for Capacitance grading of an underground cable with suitable diagram.

16. What is the purpose of using inter sheath (s) in an underground cable ? Show that in an underground cable using two inter sheaths, the maximum stress in the (homogeneous dielectric is reduced to

\[\color\green{\bigg[\dfrac{1}{\bigg\{\dfrac{1}{3(1+\alpha+\alpha^2)}\bigg\}}\bigg]}\]

of maximum stress without inters heaths,

\[\color\green{where}\;\;\color\red{\alpha\;=\;\dfrac{d_1}{d}\;=\;\dfrac{d_2}{d_1}\;=\;\dfrac{D}{d_2}}\]

(D and d are internal sheath diameter and core diameter. d1,d2 are the internal diameter of inter sheath 1 and inter sheath 2 respectively.

17. A Single core lead sheathed cable is graded by using two dielectrics of relative permittivity 3.6 (inner) and 2.5 (outer) the thickness of each being 01 cm. The core diameter is 01 cm. the system voltage is 66KV, 3 phase. Determine the maximum stress in the two dielectrics.

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