Code No:310203

III-B-Tech I-Semester Supplementary Examinations

November-2002
Set No. 1

POWER SYSTEM – II

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks:80

Answer any FIVE questions

ALL questions carry equal marks

1. a) What are the advantages and disadvantages of suspension type insulators over pin type insulators.

b) Determine the voltage across each disc of suspension insulators as a percentage of the line voltage to earth. The self and capacitance to ground of each disc is C and 0.2C respectively. The capacitance between link pin and the guard ring is 0.1C. If the capacitance to the line of the lower link pin were increased to 0.3C by means of guard ring, determine the redistribution of voltage. Also determine the string efficiency in each case.

2. a) What are the factors which effect which effect the sag. What is string chart. Explain clearly the procedure adopted for stringing the power conductors on the supports.

b) A transmission line conductors consists of hard drawn copper conductors of diameter 14-77mm and a cross -sectional area of 120mm² having weight if 1118 kg/km and has a span of 200meters . The supporting structure being level. The conductor has an ultimate tensile stress of 42.2kg/mm²and allowable tension is not to exceed 1/4th of ultimate strength. Find (i) Sag in still air (ii) Sag with a wind pressure of 60kg/m² and an ice coating of 10mm. Also find the vertical sag under this condition. Assume density of ice as 0.915gm/c.c.

3. a) Describe with a neat sketch the construction of 3-core belted type cable. Discuss the limitations of such a cable.

b) A 66KV single core cable has a conductor diameter of 2.5cm and a sheate of insider diameter 6cm. Calculate the maximum stress. It is desired to reduce this stress by using the two intersheaths. Determine their best positions, the maximum stress and the voltage on each.

4. a) A 132kv line with 1.956cm. Diameter is built so that corona takes place if the line voltage is 210KV (N.m.s) If the value of potential gradient at which ionization occurs can be taken as 30KV per cm, find the spacing between the conductors.

b) In a three phase system the symmetrical components of voltage of phase A are as follows:
E_D=99 ∠264⁰48¹ ; E₁=87D ∠44⁰20¹ ; E₂=140 ∠284⁰20¹
Calculate the three phase voltages E_A, E_B and E_C.

5. Develop an expression for the inductance of a single phase transmission line taking into account the internal flues linkages.

6. What is method of images? Derive an expression for the capacitance per unit length of a 3-phase line completely transposed. What is the effect of earth on the capacitance of the line ?

7. A 3-phase transmission line has the following constants. Resistance lphaselkm = 0.16 ohm; Reactance lphaselkm=0.25 ohm. shunt admittance lphaselkm = 1.5 x 10_-6mho.

Calculate by rigorous method the sending end voltage and current when the lie is delivering a load Q-20MW at 0.8P.F. lagging. The receiving end voltage is kept constant at 110KV. Derive any formulas used.

8. a) Derive the equation for most economical P.F. For a constant KVA type loads.

b) A star connected 400H.P. (metric), 2000v, 50Hz motor works at a P.F. of 0.7 lagging. A bask of mesh connected condensers are used to rise the power factor to 0.93 lagging Calculate the capacitance of each unit and total number of units required if each is rated 500vm50Hz. The motor efficiency is 85^ob.