Code No:310203

III-B-Tech I-Semester Supplementary Examinations

November-2002
Set No. 3

POWER SYSTEM – II

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks:80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Derive the expression for capacitance of a 3-phase, uncomposed transmission line with an equal spacing section the conditions.

b) What do you understand by Transposition of lines. What is its effect on the performance of the lines.

2. a) What is a “Synchronous phase modifier”. clearly explain and derive the expression for th capacity of phase modifier to improve powerfactor of a system.

b) Derive symmetrical components transpermasion and obtain the symmetrical components of the current given I_a = 10 0⁰ Amps I_b = 10 180⁰ Amps I_C = 0.

3. a) Starting from first principals deduce expression for ABCD constant of a long line in terms of its parameters.

b) The generalized circuit constants of a transmission line are
A = 0.93 + jo.016 B = 20 + j140
The load at the receiving end is 60 MVA, 50Hz, 0.8 power factor lagging. The voltage at the supply end is 220KV. Calculate the load voltage.

4. a) Describe the phenomena of corona.

b) Find the disruptive critical voltage and visual corona voltage (local corona as well as general corona) for a 3 phase 220KV line consisting of 22.26mm diameter conductors spaced in a 6m delta configuration. The following data can be assumed.

Temperature 25⁰ pressure 73cm of mercury, surface factor 0.84 irregularity factor for local corona 0.72, irregularity factor for general corona 0.82.

5. An overhead line is erected across a span of 250m on level supports. The conductors has a diameter of 1.42cm and has dead weight if 1.09Kg per meter. The line is subjected to wind pressure of 37.8Kg per square metre of the projected area. The radial thickness of ice is 1.25cm. Calculate the sag.
a) In an inclined direction
b) In a vertical direction
Assume a maximum working stress 1050kg per sq.cm. One cubic meter of ice weight 913.5Kg.

6. Each conductors of a 33KV, 3-phase system is suspended by a string of three similar insulators, the capacitance of each disc is nine times the capacitance to ground. Calculate the voltage across each insulator. Determine the string efficiency.

7. a) Derive an expression for capacitance of a 3-core cable.

b) A cable has been insulted with insulting materials having permittivities of 6 and 4 respectively. The inner and outer dia of a cable are 3cm and 7cms. The dielectric stresses of 58 KV/cm calculate the radial thickness of each insulting layer and the safe working voltage of the cable.

8. a) Show that maximum stress in a single core cable is

      2V
-------------
d log. (D / d)

b) A 50 Km length of H-type cable has a capacitance per Km between any two conductors is 0.15 micro farad. Determine the charging current if the supply voltage is 3-phase 11KV at 50Hz.