Tarbela power extension project

[LahoreEstate]

Tarbela power extension project



By Engr Hussain Ahmad Siddiqui

In the wake of looming power shortage, Water and

Power Development Authority (Wapda) is working on extension of Tarbela power station on a fast track basis, which would generate additional 960 MW of electricity. A delegation is scheduled to visit Washington this month to finalise an agreement with the World Bank for project financing.

The World Bank has agreed in principle, to provide technical and financial assistance for the construction of the fourth extension of power station at Tarbela Dam. Wapda has initiated action for the preparation of a detailed project study. The expression of interest (EOI) for short-listing of consultants for preparation of detailed engineering design and tender documents has been invited by Wapda by January 26, 2009. The study, being funded by the World Bank, is to be completed within nine months from the date of award of contract to the selected consultant.

Total cost of the project is estimated as Rs388 million including foreign exchange component of over $3 million. The project envisages installation of two turbines, each of 480 MW capacity, to be known as units 15 and 16 of Tarbela power station. Scope of powerhouse machinery will include turbines, generators, transformers and allied accessories. The main civil works for the proposed project will be limited to modification of intake structure, construction of powerhouse, penstock and extension of 500-kV switchyard. Commissioning of generating units could be possible in three years after award of engineering, procurement and construction (EPC) contract.

The Tarbela power station has a total installed capacity of 3,478 MW. The available energy supply however varies between high of 106 per cent of installed capacity in summer to low of 32 per cent in winter during water lean-flow period. Tarbela Dam, termed as the world’s largest earth and rock-filled dam is 148 meters high and 2,743 meters long, having gross storage capacity of 14,333 million cubic meters. Completed in 1976, it has been a challenge in global dam design and construction, given the complex geological conditions of the site.

There are five tunnels for irrigation supply releases and for power generation, which, in fact, is secondary function of the multipurpose project. According to the original design, three of the right- bank tunnels (tunnel nos. 1, 2 and 3) are used for power generation, whereas the remaining tunnels (tunnel no. 4 on the right-bank and tunnel no. 5, on the left-bank) are used exclusively for irrigation releases. Provision for an additional sixth tunnel has been made on the right-bank for subsequent development in future. In the first phase, ten generating units of 175 MW capacity each (units 1 to 4 on tunnel no. 1 and units 5 to 10 on tunnel no. 2) were installed. Four additional units, each of 432 MW capacity, were installed on tunnel no.3 under the third extension of Tarbela hydropower project.

Sometime in the 1980s, WAPDA carried out a power optimisation study at Tarbela project, and it was conceived that tunnel no. 4 could also be utilized for power generation without affecting irrigation releases. After extensive but initial studies Wapda commissioned its engineering consultants Chas. T. Main International Inc, USA and National Engineering Services Pakistan (Pvt) Ltd (NESPAK) to prepare an inception report on the techno-economic viability of the proposal. The report, finalised in October 1992, confirmed the project pre-feasibility, suggesting an additional power generation of 960 MW. Since then there had been no headway to conduct a detailed feasibility study, an essential component of developing any hydropower project.

Technically, it is the most complex project, given the peculiar characteristics of Tarbela Dam. Tarbela reservoir sedimentation is unprecedented in the global history of dam construction, and its rate is even higher than that envisaged at design stage. By the year 1999 more than 32 per cent of sedimentation had taken place, which has now reduced its storage capacity to a large extent. Wapda is now required to manage sedimentation deposition to increase the useful life of the reservoir. Accordingly, studies are being conducted for making some modifications in the same.

The international experts have suggested flushing of sediments through power tunnels from the right abutment of main dam, which is the tunnel no.4 to be utilised for the proposed power generation project. Wapda plans to carry out the flushing of sedimentation accordingly, in future. Water availability for the project may pose a problem, given the conditions of estimated 70 days a year. This however will be overcome with the construction of the upstream Diamer Basha Dam, as its reservoir will add to energy regulation of Tarbela power station.

Ten power generating units, out of total 14, were commissioned during 1977-1985, and require major maintenance and overhauling. Installation of the proposed new units will provide flexibility to Wapda to undertake systematic overhauling of existing units without disturbing peak power supplies in winter and base load in summer. Developing the 4th extension project will ensure power substitution, besides the power capacity addition.

The writer, a former Chairman of State Engineering Corporation, is currently on the Board of Directors, NESPAK

http://www.dawn.com/2009/02/02/ebr10.htm