Dagachhu plant shut for over two months

Project to incur a loss of about Nu 121M

Hydropower: More than a year after its commissioning, the Dagachhu Hydropower Plant has remained shut for the last two months.

It was shut down following a restriction in the flow of water from the tail race tunnel (TRT) since December 25. With the restriction in flow of water, only 90 megawatts (MW) of power could be generated.

Deputy CEO of the plant,  Sonam Wangdi, said they tried rectifying the problem without having to shut down the power plant by increasing aeration in the turbine pit by drilling an aeration hole. It did improve the flow and power generation increased to 108 MW.

However on the recommendation of the Electro-Mechanical Contractor, the generation was limited to 100MW.

“We had to find a permanent solution. After discussion with the designer of the civil contractor and design consultant from Austria we decided to lower the tunnel by 1.5 metres. Lowering the tunnel will increase the area to safely pass the design discharge of 50 cumecs (cubic metre per second) water,” he said.

The Dagachhu hydropower shareholders and board decided to completely shut down the plant during the lean season and rectify the problem.

“The rectification work is likely to complete by March 26, that is when the plant would come back to operation,” Sonam Wangdi said.

He explained that the problems occurred because of a technical hitch and a complex flow situation at the junction of the two tail race tunnels (TRT). Because of this, it restricted the flow of design discharge of 50 cumecs water to generate 126MW of energy.

The problem was noticed even during the load testing in February last year when the project was first commissioned. It was a peak season then so the plant waited until the lean season to rectify the restriction.

During the three month shutdown, the plant will incur a loss of about Nu 121M. Moreover it is a lean season when the river has minimum water flow, so it was prudent to execute a permanent solution. “The loss could have been significantly higher (almost 10 folds) if the plant was shut during the peak season,” the deputy CEO said. He added that the rectification works are totally at the risk and cost of the civil contractor since it was an EPC (engineering, procurement and construction) contract.

When the turbine is in operation, Sonam Wangdi said the water level at the turbine pit has to be  maintained at EL 534.60. If this maximum level is exceeded, it will automatically restrict power generation.

In order to get a full discharge or the installed capacity of power generation, the area of the tunnel is being increased by lowering the previously constructed tunnel by 1.5 metres. The floor level of the tunnel is being lowered so that the area increases.

To ensure that the project does not face a similar shut down henceforth, a hydraulic simulation was carried out before increasing the area of the tunnel.

During the shutdown, project employees are carrying out annual maintenance works.

Dagachhu plant is the first to be managed by a Bhutanese team with a review consultant from Austria. It is also the first public-private hydropower partnership. Druk Green Power Corporation holds a 59 percent equity, India’s Tata Power Company Ltd holds 26 percent and the National Pension and Provident Fund (NPPF) holds the remaining 15 percent.

The project has a 25-year sales agreement with the Tata Power Trading Company to supply it with 500,000 units of power annually. This is expected to generate Nu 15 billion or over USD 250 million in income in the next 25 years for the government.

Besides royalty energy, all the project’s power will be exported to India.

Nirmala Pokhrel, Tsirang

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