Code No:310203

III-B-Tech I-Semester Supplementary Examinations

June-2003
Set No. 3

POWER SYSTEM – II

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks:80

Answer any FIVE questions

ALL questions carry equal marks

1. a) How can the inductance of a two sub-conductor bundled conductor single phase line be calculated? Derive expression for GMR and GMD for the arrangement.

b) A 3-phase transposed line has conductors of diameter 2cm and spaced at distance of 3, 5 and 8 meter between the centers. Calculate the inductance per phase per phase per km of line length.

2. Derive and expression for line to neutral capacitance for a 3-phase line when conductors are:

a) Symmetrically placed.
b) Draw the phasor diagram of a medium transmission line represented by a T model and derive the expression for voltage regulation.

3. a) Draw the phasor diagram of a medium transmission lines represented by a T model and derive the expression for voltage regulation.

b) Determine the efficiency and regulation of a 3 phase, 50 Hz transmission line having resistance, inductance and capacitance of 10ohms, 0.1H and 0.9 micro farads respectively. The line delivers a load of 35MW at 132KV and 0.8 p.f. lag. Use nominal T method.

4. a) How do you classify the transmission lines?

b) A 220KV, 3 phase transmission line is 40 km long . The resistance per phase is 0.15 ohms/Km and the inductance per phase is 1.3263 mH per km. The shunt capacitance is negligible. Use the short line model to find the voltage and power at the sending end and the voltage regulation and efficiency when the line is supplying a 3 phase load of :

(i) 381MVA at 0.8 power factor lagging at 220KV.
(ii) 381MVA at 0.8power factor leading at 220KV.

5. a) What is a stringing chart? Explain clearly the procedure adopted for stringing the power conductors on the supports.

b) A transmission line conductors having a dia of 19.5mm weighs 0.85kg/m. The span is 275 meters. The wind pressure is 40kg/m² of projected area with ice coating of 13mm. The ultimate strength of the conductors is 8000Kg. Calculate the maximum sag, if the factor of safety is 2 and ice weights 910kg/m³.

6. a) Classify the under ground cables according to various parameters.

b) A 3-core 11 KV cable supplies a load of 1500KW at 0.85 power factor lag for 280days in a year at an average of 9 hours per day. The capital cost per kilometer of the cable is Rs.8000a+20,000. The resistance per Km of a cable of cross-sectional area 1 cm² is 0.173 ohms. If the energy loss cost per unit is 2-paise and the rate of interest and depreciation is 12% calculate the most economical current density and the dia of the conductor.

7. a) What is the importance of power factor in the supply system? What is the effect of low 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) Explain the method of improving power factor by the method of using phase advancers.

b) A 3-phase, 50Hz, 400V Motor develops 80HP,the p.f. Being 0.75 (lag) and efficiency 93%. A bank of capacitors is connected in delta across the supply terminals and p.f.is raised to 0.95 (lag). Each of the capacitance units is built of 4 similar 100V capacitors. Determine the capacitance of each capacitor.