Code No:310203

III-B-Tech I-Semester Regular Examinations

May/June-2004
Set No. 4

POWER SYSTEM – II

(Electronics and Instrumentation Engineering)

[Time:3 hours]
Max. Marks:80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Find the inductance per phase per km of a 3-phase equally spaced double circuit line distance between any two adjacent conductors (example: between R and R' and R and Y) is 4 meters and dia of each conductor equal to 1.5cm. The conductors are transposed.

b) Also calculate line capacitance of the above double circuit line.

2. a) What is charging current? How con it be calculated? What is its effect?

b) A 3-phase, 50Hz,60KV over head line conductors are placed in a horizontal plane as shown in figure.

DIAGRAM

The conductor diameter is 1.25cm. If the line length is 100km. Calculate
i) Capacitance per phase per m ii) Charging current per phase.

3. a) Define critical disruptive and visual critical disruptive voltages and give their formulas.

b) A 3 phase 220KV, 50Hz transmission line consists of 1.2cm radius conductors spaced 2m at the corners of an equilateral triangle. Calculate the corona loss per km of the line. The condition of the wire is smooth and the weather is fair with temperature of 25^oC and barametric pressure is 76 cm. Assume breakdown strength of air is 21.2kr/cm (rms).Also determine critical disruptive and visual critical disruptive voltages.

4. A string of 6 suspension insulators is to be fitted with a guard ring. The pin-to -earth capacitances are all equal to C. What should be the values of line-to-pin capacitances so as to have uniform voltage distribution over the string?

5. An overhead line with stranded copper conductors is supported on two poles 200m apart having a difference in level of 10meters. The conductor diameter is 20cm and weighs 2.30kg. Per meter length. Calculate the sag at the lower support under the conditions if wind provides a pressure of 57.5 per square meter of the projected area and a fact of safety is 4. The maximum tensile strength of copper is 4,220Kg per sq.cm.

6. a) In which method a homogeneous dielectric is used for cable grading? Prove that dielectric stress in each layer is the same.

b) The inner and outer dia of a cable ore 4cms and 9cms.The cable is insulated with two materials having permittivtties of 5 and 3 respectively with corresponding stresses of 38KV/cm and 28KV/cm. Calculate the radial thickness of each insulating layer and the safe working voltage of the cable.

7. a) Write short notes on the following:

a) p.f. Improvement by synchronous condenser
b) Importance of p.f. improvement.
c) Economics of p.f. improvement.

8. A balanced star connected load takes 90A from a balanced 3-phase, 4-wire. If the fuses in the Y and B phase are removed, find the symmetrical components of the line currents

a) Before the fuses are removed
b) After the fuses are removed.