Code No:310203

III-B-Tech I-Semester Regular Examinations

May/June-2004
Set No. 1

POWER SYSTEM – II

(Electronics and Instrumentation Engineering)

[Time:3 hours]
Max. Marks:80

Answer any FIVE questions

ALL questions carry equal marks

1. (a) Develop an expression for the inductances of a single phase transmission line taking into account the internal flux linkage. Assume the conductors are solid.

(b) Calculate the inductance per km per phase of a 3-phase transposed line, with distance between any two conductors being 4m, 4m and 8 meter respectively.

2. (a) Derive an expression for the capacitance between conductors of a single phase line. Deduce the expression for line to neutral capacitance. Discuss the approximation involved in deriving the above expressions.

(b) A 400 KV, 3-phase single circuit bundled conductor line with two sub-conductors per phase has a flat configuration. The center to center distance between adjacent phase is 4m and distance between sub-conductors of phase is 45cm. The radius of each sub-conductor is 1.6cm. Find the capacitance per phase per km. (Given D₁₂=D₂₃=_D31=4m).

DIAGRAM

3. (a) What is skin effect?

(b) A 3 phase, 50Hz, 100km long transmission line delivers a load of 20000KW at 110KV at 0.9 power factor lagging. The copper conductors of the line are 1.2 cm in diameter and are spaced equilaterally, so that the distance between them is 2m. Using nominal π method, calculate the sending end voltage, current, power, factor, regulation and efficiency of the line. Neglect the leakage.

4. (a) Explain the various methods for equalizing the potential across the various units in an insulator string and discuss the methods for improving the string efficiency in a string of insulators.

(b) Explain the limitations of pin-type insulators.

5. An overhead line has a conductor of cross-section 2.5 cm² hard drawn copper and a span length of 150 metres. Determine the sag which must be allowed if the tension is not to exceed one-fifth of the ultimate strength of 4,175 kg/cm².

(a) in still air and

(b) with a wind pressure of 1.3 kg/ metre and an ice coating of 1.25 cms.

Determine also the vertical sag in the latter case.

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

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

7. a) What is the importance of power factor in the supply system ? What is the effect of the power factor on the generating stations ?

b) Why is unity power factor not the most economical p.f.? Suggest the most economical value of power factor for a particular customer by deriving appropriate derivation with suitable assumptions. KW of load is maintained constant.

8. (a) What is the symmetrical components? Why they are necessary?

(b) A 3-phase unbalanced system currents are read as I_R =150A, I_Y =130A and I_B =10A.