Koyna: the engineering of the project

One of the main strengths of the Koyna project is its low cost. A friendly topography used to the best advantage by human ingenuity and engineering skills has contributed to the low cost. "A thermal power plant needs a boiler, and coal handling plant. It also requires a rail line up to the plant for the coal. In a hydel project, the prime mover being high pressure water, these costs are saved," explains K S Deshpande, superintending engineer at the Koyna project, who has been with the project ever since the first drawings were drafted.

/industry/power/images (6186 bytes)"Besides, stage 4 used an existing dam. Hence the cost of building a dam was also saved. This factor also kept the cost of land acquisition and rehabilitation to the minimum. Only eight families needed to be resettled for stage 4," he says.

Koyna's fourth stage uses a naturally occurring phenomenon to advantage. The difference in heights of the water heads of two lakes -- the Shivaji Sagar lake, at a height of around 600 metres above sea level, and the Kolkhewadi lake, about 130 metres above sea level -- provides a mean head advantage of 500 metres, which is being used to generate power.

The two lakes are connected through an underground tunnel, with the power plant situated in between. Water taken from the higher Shivaji Sagar lake provides the high-pressure water needed to power the generator, after which it is let back into the Kolkhewadi lake below.

The intake outlet connecting the two lakes was created by using the lake tapping process, for the first time in India. The bottom of the Shivaji Sagar lake was tapped earlier this year, and the water thus tapped is taken through the underground tunnel to the four 250 MW hydro turbines of Stage 4.

The hydro turbine itself is situated deep inside the belly of the Sahyadri, in a cavern that is 173 metres long, 18 metres wide and 51 metres high (about 14 stories high). The electricity generated is evacuated through 420KV XLPE cables, again a first for a hydel project. "Usually, this is done through bus ducts or oil filled cables," says Mr Deshpande.