Code No:310203

III-B-Tech I-Semester Regular Examinations

November-2003
Set No. 3

POWER SYSTEM – II

(Electronics and Instrumentation Engineering)

[Time:3 hours]
Max. Marks:80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Derive the expression for calculating the internal and external flux linkages of a conductor carrying current. Use these expressions to derive for the inductance of a single phase line.

b) Calculate the loop inductance per km of a single phase Overhead line consisting of two conductors each 1cm dia 1.2meter apart.

2. What do you understand by electric potential? Derive an expression for electrical potential and hence deduce the formula for capacitance for the following cases:
a) Single phase 2-conductor line
b) 3-phase unsymmetrical spacing, but transformed.

3. a) Why transmission lines are of three phase three wire type and distribution lines are of three phase four wire type?

c) Input to a single phase short line is 2000KW at 0.8pf lagging. The line has a series impedance of 0.4+j0.4 ohms. If the load voltage is 3KV, find the load and receiving end power factor. Also find supply voltage and supply power factor

4. A string of suspension insulators consists of 8 units. If the maximum peak voltage per unit is 33KV.Calculate

a) the maximum voltage for which this string can be used
b) the string efficiency
Assume capacitance between each link pin and earth as 15 percent of the self-capacitance of each unit

5. Calculate maximum sag of a line unit copper conductor 7/0.295cm size, are 0.484 sq.cm, overall diameter 0.889 cm,weight 428kg/km and braking strength 1,975kg. Assume factor of safety 2. Span 200 meters. Level supports:

a) Due to weight if the conductors
b) Due to additional weight of iced loading of 1 cm thickness
c) Due to both a) and b) plus wind acting horizontally at a pressure of 39kg per sq. metre.

6 a) Derive the expression for the insulation resistance of a single core cable

b) A 11kv, 50Hz, single-phase cable has a diameter of 10mm and an internal sheath radius of 15mm. If the dielectric has a relative permittivity of 24, determine for a 2.5km length cable (i) the capacitance (ii) the charging current.

7. 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) Explain the need for per unit method in power system calculations.
b) Explain how base quantities can be selected and derive the formula for base impedance.
c) A 3-phase unbalanced system currents are read as I_R=150 A; I_Y=0 A; and I_B=80A. The phase sequence is RYB. Find all the three symmetrical components for the case.