# Element of Power System 2006-07

Note : Answer all the questions.

All questions carry equal marks.

1. Answer any four parts of the following :

(a) Explain why the voltage drop is of primary concern in the design of the distributors.

(b) Discuss the advantages of using over head lines compared to underground cables.

(c) Find the ratio of volume of copper required to transmit a given power over a given distance by over head system using (i) d.c. two wire system (ii) three phase three wire a.c. system.

(d) Discuss the effect of increase in voltage on the transmission line efficiency.

(e) Determine the best current density in Amps/mm2 for a three phase overhead line. The line is in use for 2600 hours per year and if the conductor costs Rs. 3.0/kg. It has a specific resistance of 1.73 x 10-8 fi-m and weighs 6200 kg/m3. Cost of energy is 10 paise/unit. Interest and depreciation is 12% of conductor costs.

(f)  Discuss the advantages of per unit system of calculations. Explain how would you convert the old per unit values to new base values.

2. Answer any three parts of the following :

(a) Prove that transposition of conductors of a three phase transmission line yields equal inductance per phase.

(b) Explain briefly ‘skin effect’ and ‘proximity effect’ in reference to over head lines.

(c) A 50Hz, three-phase transmission line has a total series impedance per phase of (40 xj 125) ohms and shunt admittance of 10-3 mho. The load is 50 MW at 220 kV, 0.8 pf lagging.

Using nominal II method, calculate the sending end voltage, current and power factor.

(d) Draw the phasor diagram of a short transmission line and derive an expression for voltage regulation.

3. Answer any two parts of the following :

(a) A three phase equilaterally spaced transmission line has a total corona loss of 53 kW at 106 kV and a loss of 98 kW at 110.9 kV. What is the disruptive critical voltage between lines?

What is corona loss at 113 kV?

(b) Explain corona and discuss its effects on performance of transmission lines.

(c) Why the voltage distribution across the insulators in an insulator string is not equal? Suggest methods to equalize it.

4. Answer any two parts of the following :

(a) An overhead line has the following data :

Span length – 160 m

Conductor diameter – 0.95 cm

Weight per unit length of the conductor = 0.65 kg/m

Ultimate stress – 4250 kg/cm2

Wind pressure – 40 kg/cm2 of the projected area

Factor of safety -5 Calculate the sag.

(b) Discuss the usefulness of various materials used in insulating cables.

(c) Derive relation between conductor radius ‘r’ and miner radius of sheath R for stable and satisfactory working of a cable. Discuss the effect of r/R on the stability of the cable.

5. Answer any two parts of the following :

(a) Give a comparison between solid grounding and resistance grounding of power systems neutrals. Justify your answer with the help of neat phasor diagrams.

(b) A 132 kv, 3-phase, 50 Hz transmission line 192 km long consists of three conductors of effective diameter 20 mm arranged in a vertical plane with 4 m spacing and regularly transposed. Find the inductance and kVA rating of the arc suppressor coil.

(c) Give a comparison of monopolar and bipolar HVDC links.