Work to be completed soon:

Renovation work on New Laxapana under way

By Gamini Warushamana

After 40 years of uninterrupted service, the New Laxapana hydro power station was stopped for a short break on January 20.

The contribution by this power plant to the national economy is huge and during the past 40 years it has generated over 20 trillion watt hours or 20,000 Gwh of electricity, and fed the national grid with the cheap electricity, at a unit cost of about Rs.1.50 against a thermal unit which costs over ten times that amount.

The Laxapana hydro power complex consists of five dams and five power stations constructed across Kehelgamu Oya and Maskeliya Oya, branches of the Kelani river.

Water level

These five hydro power stations that are in the wet zone generate power almost throughout the year. The water level of the reservoirs rarely declines but due to the prevailing drought the generation capacity of the power stations has reduced and the CEB is servicing and renovating the facilities.

Of the five hydro power stations in the Laxapana complex two are in Kehelgamu Oya and three in Maskeli Oya. The Castlereagh reservoir is at the top of the Kekelgamu Oya and is the main reservoir.

It feeds the 50 MW Wimalasurendra power plant which was commissioned in 1958.

The water which comes from the Wimalasurendra power plant is collected in the Norton Pond reservoir and it feeds the old 53.5 MW Laxapana power plant commissioned in 1950.

The Moussekelle reservoir is at the top of Maskeli Oya and it feeds 60 MW Canyon power plant commissioned in 1969.

The water passes to the Canyon pond and it feeds the 116MW New Laxapana power plant commissioned in 1974. Water released at the New Laxapana power plant is again collected at the Laxapana pond and it feeds the 75 MW Polpitiya power plant commissioned in 1969.

The system has been designed to use the water continuously to generate hydro power using the geographical advantages in the region.

The five power plants account for 338 MW or 28 percent of the total hydro capacity of the CEB. Major renovation work in the 18,014-foot long tunnel of the New Laxapana power plant is now under way.

According to the Chief Engineer of the Laxapana Generating Stations, H.M.A. Herath this is the first major renovation in 40 years, after its commissioning in 1974.

Significant loss

"The water leak in the tunnel was first observed in 1978, four years after commissioning and gradually the leak increased. In 1998 after studies it was confirmed that 100 percent of the water was from a leak in the tunnel and we estimated it as 230 litres per second. It was a significant loss and the estimated financial loss as a result of reduced power generation capacity was Rs.130 million a year," he said.

The civil engineer of the Laxapana Generating Stations, Erosh Fernando said that although the loss was significant renovation was difficult due to several reasons.

"First, the tunnel had to be shut down for inspection. But shutting down the tunnel was risky because it could lead to the collapse of weak rocks and take a long time to renovate," he said.

"According to engineering discipline this type of tunnel should be renovated every 10-15 years. From 2002-2004 we attempted to renovate the tunnel but as the demand for power was increasing and alternatives were limited it was difficult to get approval to shut down the tunnel," Fernando said.

"We waited for an opportunity after the Norochcholai coal power plant was commissioned but that too was not possible due to the frequent shut down of the coal power plant. Today with the drought and decline in water levels we got approval and found the fault and renovation is under way," he said.

Weak rock layers

Renovation work will be completed soon. After the tunnel was shut down a cavity and 56 metres of weak rock layers were found in the tunnel from where water was leaking.

A 3.2 metre diameter concrete structure of is being constructed to close the cavity and cover the weak rock layers.

Civil construction work is handled by ST and CC under the supervision of the CEB engineers and the renovation project is funded by the Dam Safety and Water Resources Management Project (DSWMP).

DSWMP has also provided a foreign expert knowledgeable in underground tunnels. The estimated cost of the renovation work is Rs.100 million and Herath said that the expenditure can be earned within a year by increased power generation.

Hydro power generation converts the potential energy of water in the upper stream to kinetic energy and finally to electric energy. This is a simple process but the civil engineering construction and investment is huge.

In New Laxapana's long tunnel, various simple technologies have been used to control the huge water flow. The surge shaft is used to reduce water pressure in the tunnel when the turbines are closed suddenly.

Water pressure

The surge shaft is similar to hummanaya or blowhole and when the water to the turbines is shut down the water pressure in the tunnel is released through the surge shaft. It is a vertical tunnel of over 300 feet. Rocks and hard material that come with the water are removed through rock traps in the tunnel.

Rock traps are deep pits in the tunnel and there are three rock traps in the New Laxapana tunnel. With soil erosion and deforestation in the catchment areas, rock traps have to be cleaned more frequently. Sri Lanka has now used almost all the thermal power potential in the country and only mini and micro hydro projects are left. The country now needs to maintain the sustainability of these mega hydro projects.

All the catchment areas of the reservoirs are under threat. Deforestation, encroachment for residential or agriculture purposes is taking place. Engineers said they face many difficulties in their bid to conserve forest land belonging to the CEB in the catchment areas.