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Moving up

An underground power house constructed at Srisailam in Andhra Pradesh is the latest attraction of the State. M. MALLESWARA RAO on the project which has several firsts to its credit.

The 15-metre radius junction where three tunnels join, reportedly the biggest junction in the country.

THERE is a new addition from Andhra Pradesh to the list of wonders. A new, all-underground powerhouse constructed in an artificial space created within the bowels of hills, 740 feet below ground level at Srisailam where the Krishna river offers a breathtaking view, cutting the canyons in granite and meandering through the deep gorges of Nallamala forests.

From a distance, the hills which face the gargantuan Srisailam dam on the left do not indicate anything extraordinary. Only the usual sight — pilgrims taking a dip in the river flowing in a thin line at the foothills and vehicles that slowly descend and ascend the roads winding through the slopes and the baldy top. Not until the visitor enters one of the tunnels at the foothills does the subterranean spectacle unfold itself in a pleasant surprise.

On completion of civil works which involved excavation and the formation of a tunnel for a length of 12 km and bulging into gigantic caverns at some points to accommodate the powerhouse and surge shaft, the 900-MW capacity hydel station is a man-made wonder. Over a decade, hundreds of skilled workers had to drill and work through the rock to give the Rs.2,402-crore project the required four-acre ground space and thus its envied status. About 50 of them lost their lives in the process. When the first unit was commissioned recently, the Krishna, impounded by the dam upstream, tumbled down from a height of 100 metres and gushed into the tunnel, rotating the turbine in the powerhouse located precariously 65 feet below the riverbed. Waterfalls that become active at the press of a button in the control room!

The main features of the hydel station are the six units, each with 150 MW capacity, funded by the Japan Bank for International Cooperation. The biggest unit has a radius of 15 metres, and three caverns in which the powerhouse, surge shaft and transformer house are located, each separated by a wall of uncut rock. At some points, the tunnels are as high as a two-storey building and as wide as a two-lane road. The surge shaft cavern soars as high as a 22-storey apartment. To prevent landslips, the tunnel roofs have been rock-bolted and short-creted, and 7,000 steel liners used for pressure shaft and penstocks alone.

A record quantity of 25 lakh cubic metres of rock was removed from the womb of the hills while 2.1 lakh tonnes of cement, 39,000 tonnes of reinforcement and six lakh cubic metres of concrete have gone into the stupendous construction. The 900-MW capacity, coupled with the 770 MW available out of the already existing plant on the right side which was salvaged within a record period of four months after its submergence during the 1998 floods, makes the place a super hydel complex with a total availability of 1,670 MW, the largest capacity existing at any such single point in the country. The latest attraction for tourists, in addition to the medieval Shiva temple nestling in the summit of the mountain range, which was visited by two great warriors of the country — Sri Krishnadevaraya and Chatrapathi Shivaji. The thicket teems with wildlife, the mammoth multipurpose dam stands like a concrete mountain against the rapids of the Krishna which not only serve the two hydel stations but also projects like the Telugu Ganga that supplies drinking water to Chennai. From the View Point of the dam, one can see the reservoirs of Srisailam and Nagarjunasagar. The Srisailam reservoir is upstream, while the Nagarjunasagar, the largest man-made lake in the world, extends right up to the end of the dam.

The new project has several firsts to its credit. The turbines installed for all the six units are of reversible type and each account for a record capacity of 150 MW. Such turbines were provided for all the eight units of the Nagarjunasagar main powerhouse, but none of them was put to use for want of a pond from which water can be pumped back into the Nagarjunasagar. The Srisailam Underground Project scores in this respect as it is uniquely located in between two major reservoirs, one upstream with 308-tmcft capacity and the other downstream, 400-tmcft. The waters crashing down from Srisailam spin the turbines clockwise and when they move in the opposite direction acting as pumpsets, that of Nagarjunasagar reservoir climbs up the 100-metre height of the uphill path through the tunnel to fall back into the Srisailam reservoir. The power generated at the station joins the grid passing through pairs of gas-insulated cables laid through a tunnel serving as an approach road. These state-of-the-art cables imported from Japan have been used, as conventional cables burn while transmitting power at a voltage as high as 400 KV. Each turbine, while acting as a pumpset, requires 10 per cent more power — about 165 MW. At one stage, the Government wanted to give up the project because of "cost exceeding the prospective benefit" with turbines requiring big power output to do their reverse job and with workers facing faulty zones while tunnelling which was "costly and risky." But, it went ahead with the execution following the "green signal" from the all-party delegation which visited the site and considered abandonment, at that stage after spending Rs.1,400 crores.

The new project takes the grid-installed capacity to a total of 9,300 MW and that of the State-owned AP Genco, to about 6,500 MW, but it is being considered important more from its relative value. Mr. K. Venkataraman Reddy, Technical Director for AP Genco, who has been associated with the execution from the beginning, describes it as a "peaking station" in that it will be useful in meeting the demand during peak period.

Srisailam surpluses at least for about 40 days during the monsoon. Unlike thermal stations which require a minimum time to "come alive," hydel stations can be put into operation in minutes. During off-season, the project will pump backwaters into the Srisailam reservoir drawing power from thermal stations.

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