Determine the most economical cross-section for a 3-phase transmission line, 1 km long to supply at a constant voltage of 110 kV for the following daily load cycle :
6 hours 20 MW at p.f. 0.8 lagging
12 hours 5 MW at p.f. 0.8 lagging
6 hours 6 MW at p.f. 0.8 lagging
The line is used for 365 days yearly. The cost per km of line including erection is Rs (9000 + 6000 a) where 'a' is the area ofX-section of conductor in cm2. The annual rate of interest and depreciation is 10% and the energy costs 6P per kWh. The resistance per km of each conductor is 0.17 6/a.